Mini Split Sizing Calculator
If you just want a quick answer without running the calculator, use this master sizing chart. Find your room's square footage in the left column and read across to the recommended mini split size.
| Room Size (sq ft) | Calculated BTU Need | Recommended Mini Split Size | Tonnage | Best For |
|---|
| 150–250 sq ft | 3,000–6,250 BTU | 9,000 BTU | 0.75 ton | Small bedroom, home office |
| 250–350 sq ft | 5,000–8,750 BTU | 9,000 BTU | 0.75 ton | Standard bedroom, nursery |
| 350–500 sq ft | 7,000–12,500 BTU | 12,000 BTU | 1 ton | Large bedroom, living room |
| 500–700 sq ft | 10,000–17,500 BTU | 18,000 BTU | 1.5 ton | Open living area, large room |
| 700–1,000 sq ft | 14,000–25,000 BTU | 24,000 BTU | 2 ton | Open floor plan, large basement |
| 1,000–1,400 sq ft | 20,000–35,000 BTU | 30,000–36,000 BTU | 2.5–3 ton | Multi-zone, whole floor |
| 1,400–1,700 sq ft | 28,000–42,500 BTU | 36,000 BTU | 3 ton | Large open plan, whole house |
| 1,700–2,100 sq ft | 34,000–52,500 BTU | 48,000 BTU | 4 ton | Whole house, commercial |
Assumes standard 8-ft ceilings, average insulation, and moderate climate. Adjust up for poor insulation, high ceilings, or hot climates. Sources: HVACDirect.com, Trane.
Here's the deal: for most homeowners, a 12,000 BTU (1-ton) mini split is the sweet spot. It covers 450–600 sq ft — enough for a typical bedroom, living room, or home office — and it's the single most popular mini split size sold in the U.S.
If your room falls between two sizes, round up to the next standard size. Unlike central AC, mini splits use inverter compressors that modulate output from roughly 25% to over 100% of rated capacity, so slight oversizing won't cause the short-cycling problems you'd see with a conventional system.
That said, don't oversize by more than one step. Trane recommends staying within ½ ton (6,000 BTU) of your calculated load to maintain proper humidity control.
What Size Mini Split Do I Need? (Room-by-Room Guide)
Every room has different sizing considerations. Let's walk through the most common scenarios.
Mini Split for Bedroom (150–350 sq ft)
The average bedroom in the U.S. ranges from 150 to 350 sq ft. For most bedrooms, a 9,000 BTU mini split is the right choice — it handles up to 350–450 sq ft with ease.
For a small bedroom under 200 sq ft, 9,000 BTU is technically oversized, but since mini splits modulate down to as low as 1,500–3,000 BTU via inverter technology, efficiency stays high even in a small space. Master bedrooms in the 300–500 sq ft range may need a 12,000 BTU unit, especially if the room has vaulted ceilings or large south-facing windows.
Noise matters in bedrooms. Mini split indoor units operate at 19–26 dB(A) on low fan — quieter than a whisper — making them ideal for sleeping spaces. If your bedroom is part of a multi-zone system, pair a 9K indoor head with an appropriately sized outdoor unit (we cover multi-zone sizing below).
Mini Split for Living Room and Open Floor Plan (400–1,000 sq ft)
Open-concept living areas are where mini split sizing gets interesting. A standard living room of 400–600 sq ft needs a 12,000–18,000 BTU mini split.
For open floor plans above 800 sq ft, consider two options: a single 24,000 BTU unit or two smaller heads (like a pair of 12K units) connected to one outdoor condenser. Two heads distribute air more evenly across a large space — one 24K unit blasting from one wall can leave the far end of an open plan undertreated.
If your open living area flows into a kitchen, add 4,000 BTU to your calculation for appliance heat from the fridge, oven, and cooktop. A 700 sq ft combined living/kitchen space that calculates to 14,000 BTU should be bumped to 18,000 BTU.
Mini Split for Basement (400–1,500 sq ft)
Basements are one of the most popular mini split applications, and they have a unique sizing advantage. If your basement sits below a conditioned first floor, apply a 0.7 multiplier to your calculated BTU — the heated floor above reduces your cooling load significantly.
Here's what that looks like in practice: a 1,000 sq ft basement that calculates to 20,000 BTU only needs 14,000 BTU (20,000 × 0.7), which rounds to a 12,000 or 18,000 BTU unit. This factor comes directly from Mitsubishi's sizing recommendations for rooms below conditioned space.
However, if your basement is a walkout with exposed walls, or if it's poorly insulated with moisture issues, skip the 0.7 factor and size it like any other room — then add 20% for below-grade dampness. Basements with server equipment or home gyms need additional BTU for the equipment heat load. For related insulation considerations, check our R-value guide.
Mini Split for Garage and Workshop (400–1,500 sq ft)
Garages are demanding environments for any HVAC system. A standard 2-car garage (400–576 sq ft) needs an 18,000 BTU mini split — not 12,000.
Why larger than the chart suggests? Garages are almost always uninsulated, have large uninsulated doors, and get blasted with direct sunlight on at least one face — add 20% to your base calculation.
A 576 sq ft garage (24×24) calculates to 14,400 BTU base, plus 20% = 17,280 BTU, which rounds to an 18,000 BTU unit.
| Garage Size | Dimensions | Base BTU | +20% Adjustment | Recommended Unit |
|---|
| 1-car garage | 12×24 ft (288 sq ft) | 7,200 BTU | 8,640 BTU | 9,000 BTU |
| 2-car garage | 24×24 ft (576 sq ft) | 14,400 BTU | 17,280 BTU | 18,000 BTU |
| 2.5-car garage | 24×30 ft (720 sq ft) | 18,000 BTU | 21,600 BTU | 24,000 BTU |
| 3-car garage | 24×36 ft (864 sq ft) | 21,600 BTU | 25,920 BTU | 24,000–30,000 BTU |
If you use your garage as a workshop with heat-generating tools (welders, compressors), bump up another size. For cooling cost estimates on running your garage unit, check our calculator.
Mini Split for Sunroom (100–300 sq ft)
Sunrooms are BTU sinks. They're surrounded by glass, poorly insulated compared to standard walls, and absorb massive solar heat gain. A 200 sq ft sunroom that would need 5,000 BTU as a standard room needs 7,500–10,000 BTU as a sunroom.
The rule for sunrooms: add 30% to your base BTU calculation for heavy sun exposure, then add another 10–20% if the sunroom has a non-insulated roof or single-pane glass. Most sunrooms in the 150–300 sq ft range end up needing a 9,000 or 12,000 BTU mini split.
Good news: mini splits excel in sunrooms because the indoor unit mounts on the shared wall between the sunroom and the house, keeping the installation simple with a short lineset run to the outdoor unit. For ideal temperature settings in sunrooms, we have a separate guide.
Mini Split for Server Room and Special Use Cases
Server rooms generate constant, significant heat loads — a single server rack can produce 5,000–15,000 BTU/h depending on equipment density. Size your mini split to handle the total equipment heat load plus the room's ambient load.
A 200 sq ft server room with two racks producing 8,000 BTU each needs: 5,000 BTU (room) + 16,000 BTU (equipment) = 21,000 BTU, rounded to a 24,000 BTU mini split. For operating cost savings on 24/7 server cooling, inverter efficiency is a major advantage.
Mini Split Sizes Available: 9,000 to 48,000 BTU
Mini splits come in standard sizes. You can't buy a 13,000 or 20,000 BTU unit — you pick the nearest standard size above your calculated need. Here's every size available on the market with its coverage range.
| Mini Split Size | Tonnage | Coverage Area (Cooling) | Coverage Area (Heating) | Standard Voltage | Common Brands |
|---|
| 6,000 BTU | 0.5 ton | 200–300 sq ft | 200–250 sq ft | 115V/208-230V | Mitsubishi, Fujitsu |
| 9,000 BTU | 0.75 ton | 300–450 sq ft | 250–400 sq ft | 115V/208-230V | All major brands |
| 12,000 BTU | 1 ton | 450–600 sq ft | 400–550 sq ft | 208-230V | All major brands |
| 15,000 BTU | 1.25 ton | 600–750 sq ft | 550–700 sq ft | 208-230V | Fujitsu, Mitsubishi |
| 18,000 BTU | 1.5 ton | 650–900 sq ft | 600–800 sq ft | 208-230V | All major brands |
| 24,000 BTU | 2 ton | 850–1,200 sq ft | 800–1,100 sq ft | 208-230V | All major brands |
| 30,000 BTU | 2.5 ton | 1,100–1,450 sq ft | 1,000–1,350 sq ft | 208-230V | Mitsubishi, Daikin |
| 36,000 BTU | 3 ton | 1,500–1,700 sq ft | 1,400–1,600 sq ft | 208-230V | All major brands |
| 48,000 BTU | 4 ton | 1,750–2,100 sq ft | 1,650–2,000 sq ft | 208-230V | Mitsubishi, Daikin |
Indoor air handlers are most commonly available in 9K, 12K, 18K, and 24K sizes. The 6K and 15K sizes are available from Mitsubishi and Fujitsu. 30K+ sizes are typically outdoor condensers for multi-zone systems. Sources: HVACDirect.com, PICKHVAC.
Now let's reverse the question. Instead of "what size mini split for my room," let's answer "what size room will my mini split cover?"
What Size Room Will a 9,000 BTU Mini Split Cool?
A 9,000 BTU mini split covers 300–450 sq ft under standard conditions. That's a room roughly 15×20 ft to 18×25 ft. It's the right size for bedrooms, home offices, nurseries, and small living rooms.
This is the most searched mini split sizing question, and the answer is: a 12,000 BTU (1-ton) mini split covers 450–600 sq ft. That's a room about 20×25 ft to 20×30 ft. It's the most popular single-zone mini split size for good reason — it handles the majority of room types.
In a well-insulated room with moderate sun exposure, a 12K unit can stretch to 650 sq ft. In a poorly insulated space or a hot climate, cap it at 450 sq ft. The Daikin Aurora 12K, for example, is rated for rooms up to 550 sq ft.
18,000 BTU Mini Split Room Size
An 18,000 BTU mini split covers 650–900 sq ft. This is the go-to for large living rooms, combined kitchen-dining areas, and 2-car garages.
At 1.5 tons of cooling capacity, the 18K unit sits in a sweet spot between single-room and multi-room coverage. If your open floor plan is 800+ sq ft, the 18K handles it. If you're heating too, check the performance data in our cold climate section.
A 24,000 BTU (2-ton) mini split covers 850–1,200 sq ft. That's large enough for a big open-concept living area, a finished basement, or a substantial workshop.
For reference, a 24×24 ft garage (576 sq ft) is often sized with a 24K unit due to poor insulation, even though the raw square footage suggests 18K. Always account for the adjustment factors — 24K is the safe bet for any garage over 500 sq ft.
36,000 and 48,000 BTU Mini Split Coverage
These are whole-floor and whole-house sizes. A 36,000 BTU (3-ton) system covers 1,500–1,700 sq ft, and a 48,000 BTU (4-ton) system handles 1,750–2,100 sq ft.
At these capacities, you're almost always looking at a multi-zone system with one outdoor condenser and 3–5 indoor heads. Mitsubishi's MXZ-SM36NAM supports up to 4 indoor zones with connectable capacity of 46,800 BTU — more than the rated 36K, because not all zones run at peak simultaneously.
The baseline rule of thumb is 20 BTU per sq ft for standard conditions (8-ft ceilings, average insulation, moderate climate). Many sources round up to 25 BTU per sq ft as a safety margin.
Base Formula: Room Sq Ft × 20 to 25 = BTU Needed
Here's why there's a range instead of a single number:
| Condition | BTU Per Sq Ft | When to Use |
|---|
| Well-insulated, shaded room | 18–20 BTU/sq ft | Newer construction, dual-pane windows, north-facing |
| Average room (baseline) | 20–25 BTU/sq ft | Standard home, 8-ft ceilings, moderate climate |
| Poorly insulated room | 25–30 BTU/sq ft | Old home, single-pane windows, minimal insulation |
| Kitchen | 25 BTU/sq ft + 4,000 BTU | Add for appliance heat |
| Sunroom / solarium | 30–35 BTU/sq ft | Excessive glass, direct sun exposure |
| Uninsulated garage | 28–32 BTU/sq ft | Add 20% for poor insulation and drafts |
Sources: The Trade Table (20 BTU/sq ft baseline), Call Clean Air (25 BTU/sq ft general rule), Total Home Supply (adjustment factors).
For the most accurate sizing, use a Manual J load calculation — it's the ACCA-certified method that professional HVAC contractors use. Our calculator above is based on simplified Manual J principles. For a comparison of how this relates to central AC sizing, see our AC tonnage calculator.
How Many Mini Splits Do I Need? (Single-Zone vs Multi-Zone)
This is one of the most common questions we get. The short answer: one indoor head per room you want to independently temperature-control.
Single Zone vs Multi Zone Mini Split
A single-zone system has one outdoor unit and one indoor unit. It controls one room. A multi-zone system has one outdoor unit connected to 2–8 indoor units, each with independent temperature control.
| Feature | Single-Zone | Multi-Zone |
|---|
| Indoor units | 1 | 2–8 |
| Independent temp control | 1 room | Each room separate |
| Outdoor units needed | 1 per room | 1 for all rooms |
| Installation complexity | Simple | More complex (branch box, longer linesets) |
| Efficiency at partial load | Excellent (single compressor modulates) | Good, but less efficient than multiple single-zones |
| Cost per zone | Lower per unit, higher per zone | Higher upfront, lower per added zone |
| Best for | Single room additions, garages, sunrooms | Whole-house coverage, 3+ rooms |
If you're adding a mini split to one room (garage, bedroom addition, sunroom), go single-zone. If you need to cover your whole house or 3+ rooms, a multi-zone system is more practical — one outdoor unit instead of 3–5 separate ones cluttering your exterior walls.
How Many Mini Split Zones Do I Need?
Count the rooms you want individually controlled. Each room gets one indoor head. Common configurations:
- 2-zone system: Master bedroom + living room
- 3-zone system: 3 bedrooms, or 2 bedrooms + living room
- 4-zone system: 3 bedrooms + living room (whole-house replacement)
- 5-zone system: Full house coverage including home office
How Many Indoor Heads Per Outdoor Unit?
This depends on the outdoor condenser's total BTU capacity and its maximum zone count. Here's a guide for Mitsubishi's MXZ multi-zone lineup:
| Outdoor Unit Capacity | Max Zones | Max Connectable BTU | Example Head Combo |
|---|
| 20,000 BTU (MXZ-2C20) | 2 zones | ~26,000 BTU | 9K + 12K |
| 24,000 BTU (MXZ-3C24) | 3 zones | ~31,200 BTU | 6K + 9K + 12K |
| 30,000 BTU (MXZ-3C30) | 3 zones | ~39,000 BTU | 12K + 12K + 12K |
| 36,000 BTU (MXZ-SM36NAM) | 4 zones | 46,800 BTU | 9K + 9K + 12K + 12K |
| 48,000 BTU (MXZ-SM48NAM) | 5 zones | ~62,400 BTU | 9K + 9K + 12K + 12K + 12K |
| 60,000 BTU (MXZ-SM60NAM) | Up to 8 zones | ~78,000 BTU | Various combinations |
Notice that connectable capacity exceeds rated outdoor capacity by 110–130%. This works because not all rooms demand full BTU simultaneously — the inverter compressor allocates capacity in real-time. Source: Mitsubishi MXZ-SM product page.
The key sizing rule for multi-zone systems: add up each room's BTU need, then select an outdoor unit with rated capacity equal to or slightly above that total. Don't oversize the outdoor unit — an oversized condenser wastes energy when only one or two zones are active.
For a full comparison of ducted alternatives, see our heat pump sizing guide.
Why Mini Splits Need Fewer BTU Than Central AC
If you've used our central AC tonnage calculator, you may have noticed that a mini split recommendation for the same room is 15–25% lower in BTU. This isn't an error. There are three reasons mini splits are inherently more efficient at delivering conditioned air.
1. Zero Duct Loss
This is the biggest factor. According to ENERGY STAR, a typical forced-air duct system loses 20–30% of conditioned air through leaks, holes, and poor connections.
The U.S. Department of Energy puts the number even higher for ducts in attics and crawl spaces: 25–40% energy loss.
Mini splits have zero duct loss. The conditioned air goes directly from the indoor unit into your room — no ducts to leak through.
A central AC system that needs 24,000 BTU at the register might need 30,000–32,000 BTU at the air handler to account for duct losses. A mini split just needs 24,000 BTU, period.
2. Inverter Variable-Speed Compressor
Central AC systems (most of them) use single-speed compressors that cycle on at 100% and off at 0%. This is like driving by flooring the gas pedal, then coasting, then flooring it again.
Mini splits use inverter-driven compressors that modulate continuously — typically from 25% to over 100% of rated capacity. A 12,000 BTU mini split might deliver as little as 3,000 BTU when the room is near setpoint, or boost to 15,000+ BTU during initial cooldown. This means the system matches output to load in real-time, wasting far less energy.
3. Zone Control Eliminates Waste
A central AC system conditions your entire house, including rooms nobody is in. Mini splits only condition the rooms you're actively using. This alone can reduce total BTU demand by 20–40% compared to a whole-house central system.
Combined, these three advantages mean you can typically size a mini split 15–25% smaller than the central AC equivalent for the same space and still get equal or better comfort. For a side-by-side look at how many BTU window and portable ACs need for the same rooms, that comparison is striking.
Cold Climate Mini Split Sizing (Heating Mode)
If you're using your mini split for heating — not just cooling — sizing gets more critical. You need enough capacity to handle your home's heating BTU load at the coldest outdoor temperatures your area experiences.
All heat pumps lose heating capacity as outdoor temperatures drop. The question is: how much capacity do they lose, and at what temperature do they stop being useful?
| Brand / Series | Min Operating Temp | 100% Capacity Down To | Capacity at 5°F | Capacity at -13°F | SEER Rating |
|---|
| Mitsubishi H2i® (Hyper-Heat) | -13°F | 5°F | 100% rated | ~67–90% rated | Up to 33.1 |
| Fujitsu XLTH | -15°F | ~20°F | ~100% rated | Reduced but operating | Up to 33.1 |
| Fujitsu XLTH+ (Orion) | -22°F | -15°F | 100%+ | 100% rated | Up to 33.5 SEER2 |
| Mitsubishi Standard | ~0°F | ~32°F | ~79% rated | N/A (shuts down) | Up to 22 |
| Daikin Aurora | -4°F to -13°F | Varies | Reduced | Reduced | Up to 21 SEER2 |
| MrCool DIY 4th Gen | -22°F (some) | Varies | Reduced | Reduced | Up to 22 |
Sources: Mitsubishi Electric HVAC, Fujitsu General XLTH, HVAC Insider (XLTH+ specs), Got Ductless (Hyper-Heat comparison).
The standout here is the Fujitsu XLTH+ (Orion), which maintains 100% rated capacity at -15°F and 90% at -22°F — the most extreme cold-weather performance in any residential mini split available today. Mitsubishi's H2i® Hyper-Heat is the most proven and widely-installed cold-climate option, delivering 100% capacity down to 5°F.
For heat pump COP at various temperatures, see our efficiency guide. The NEEP (Northeast Energy Efficiency Partnerships) maintains a searchable database of over 2,000 cold-climate certified heat pumps with verified performance data — an invaluable resource if you're comparing models.
Sizing for Heating-Dominant Climates
In cold climates (IECC zones 5–7), size your mini split for the heating load, not the cooling load. Heating loads are almost always larger than cooling loads in these regions.
Here's the adjustment process:
- Calculate your cooling BTU need using the standard 20–25 BTU/sq ft method
- For heating, add 10–30% above the cooling number (depending on climate severity)
- Check that your chosen model delivers enough BTU at your area's 99% design temperature (the coldest temperature your area hits in all but 1% of winter hours)
- If you're in climate zones 5–7, choose a cold-climate rated model (Mitsubishi H2i, Fujitsu XLTH/XLTH+, or similar)
A good rule of thumb from Total Home Supply: for heating mode, mini splits are so efficient that they need roughly 1,000 fewer BTU than the cooling calculation at moderate temps. But at extreme cold (below 0°F), budget 30–50% extra for capacity loss at your design temperature.
For HSPF rating comparisons that measure seasonal heating efficiency, we have a dedicated guide.
Worked Examples
Let's put the sizing methodology into practice with 6 real-world scenarios.
Example 1: Single Bedroom Mini Split Sizing
Scenario: 12×14 ft master bedroom in Atlanta, Georgia. Standard 8-ft ceilings, average insulation, two east-facing windows, moderate sun exposure.
Calculation:
- Room area: 12 × 14 = 168 sq ft
- Base BTU: 168 × 25 = 4,200 BTU
- Adjustments: none needed (average insulation, moderate sun, moderate climate)
- Calculated need: 4,200 BTU
- Nearest standard size: 9,000 BTU
Result: Install a 9,000 BTU mini split. The inverter will modulate down to ~2,500 BTU at steady-state, keeping the bedroom comfortable without overcooling. Estimated annual cost: $120–$180 at Atlanta electricity rates.
Example 2: Open Plan Living Area (600 sq ft)
Scenario: Combined living room and dining room in Phoenix, Arizona. 20×30 ft open plan, 9-ft ceilings, good insulation (newer build), large west-facing sliding glass doors.
Calculation:
- Room area: 20 × 30 = 600 sq ft
- Base BTU: 600 × 25 = 15,000 BTU
- Ceiling adjustment: 9-ft ceiling (+12.5% over 8-ft standard) = 15,000 × 1.125 = 16,875 BTU
- Sun exposure adjustment (west-facing glass in Phoenix): +15% = 16,875 × 1.15 = 19,406 BTU
- Hot climate adjustment (Phoenix): +10% = 19,406 × 1.10 = 21,347 BTU
- Calculated need: ~21,350 BTU
- Nearest standard size: 24,000 BTU
Result: Install a 24,000 BTU (2-ton) mini split. Phoenix's extreme heat and west-facing glass pushed this 600 sq ft room well past the 18K range. For ideal thermostat settings in hot climates, check our comfort guide.
Example 3: 2-Car Garage Workshop
Scenario: 24×24 ft detached garage in Nashville, Tennessee. Uninsulated walls, standard garage doors, used as a woodworking shop with dust collection and power tools. 10-ft ceiling.
Calculation:
- Room area: 24 × 24 = 576 sq ft
- Base BTU: 576 × 25 = 14,400 BTU
- Ceiling adjustment (10-ft): +25% = 14,400 × 1.25 = 18,000 BTU
- Poor insulation adjustment: +20% = 18,000 × 1.20 = 21,600 BTU
- Equipment heat: +2,000 BTU for tools and dust collector = 23,600 BTU
- Calculated need: ~23,600 BTU
- Nearest standard size: 24,000 BTU
Result: Install a 24,000 BTU mini split. The uninsulated garage, high ceilings, and equipment heat pushed this well beyond the 18K unit that raw square footage might suggest. For an uninsulated garage, never undersize.
Example 4: Whole-House Multi-Zone System (1,500 sq ft)
Scenario: 1,500 sq ft ranch home in Charlotte, North Carolina. 3 bedrooms, living room, finished basement. Average insulation, 8-ft ceilings throughout. Need heating and cooling.
Room-by-room calculation:
| Room | Sq Ft | BTU Need | Recommended Head |
|---|
| Master bedroom | 250 sq ft | 6,250 BTU | 9,000 BTU head |
| Bedroom 2 | 180 sq ft | 4,500 BTU | 9,000 BTU head |
| Bedroom 3 | 150 sq ft | 3,750 BTU | 9,000 BTU head (or 6K if available) |
| Living room/kitchen | 500 sq ft | 12,500 BTU | 12,000 BTU head |
| Finished basement | 420 sq ft | 10,500 × 0.7 = 7,350 BTU | 9,000 BTU head |
| Total | 1,500 sq ft | ~34,350 BTU | — |
System recommendation: A 36,000 BTU multi-zone outdoor unit (like the Mitsubishi MXZ-SM36NAM) with 5 indoor heads (three 9K + one 12K + one 9K for the basement). The outdoor unit's 46,800 BTU connectable capacity easily handles the total head combination.
Estimated total annual cost: $900–$1,400 for heating and cooling, depending on usage patterns. Compare this to a central ducted heat pump system that would need to condition all rooms simultaneously.
Example 5: Cold Climate Heating-Dominant Sizing
Scenario: 400 sq ft addition (sunroom + office) in Minneapolis, Minnesota. 99% design temperature: -11°F. Well-insulated addition with newer windows, but heavy heating load 7 months per year.
Calculation:
- Room area: 400 sq ft
- Cooling BTU: 400 × 25 = 10,000 BTU (summer)
- Heating BTU at 47°F: ~10,000 BTU
- Heating BTU at design temp (-11°F): need to account for 30–40% capacity derating in standard units
- With Mitsubishi H2i or Fujitsu XLTH+, capacity at -11°F ≈ 80–100% of rated
- Required heating capacity: 12,000–15,000 BTU at design temp
Result: Install a 15,000 or 18,000 BTU cold-climate mini split (Mitsubishi H2i or Fujitsu XLTH series). The 15K Fujitsu XLTH+ can deliver up to 28,000 BTU max at moderate temps and still output well over 15,000 BTU at -15°F. This gives you massive headroom.
Do NOT install a standard (non-cold-climate) mini split in Minneapolis. A standard 18K unit would deliver only ~14,000 BTU at 5°F and shut down below 0°F. For heat pump efficiency at low temperatures, see our data guide.
Example 6: Sunroom Addition
Scenario: 12×15 ft sunroom in Richmond, Virginia. Three walls of floor-to-ceiling windows, cathedral ceiling (12 ft), south-facing, lightweight roof with minimal insulation.
Calculation:
- Room area: 12 × 15 = 180 sq ft
- Base BTU: 180 × 25 = 4,500 BTU
- Cathedral ceiling adjustment (12 ft = +50%): 4,500 × 1.50 = 6,750 BTU
- Heavy sun exposure: +30% = 6,750 × 1.30 = 8,775 BTU
- Poor roof insulation: +15% = 8,775 × 1.15 = 10,091 BTU
- Calculated need: ~10,100 BTU
- Nearest standard size: 12,000 BTU
Result: Install a 12,000 BTU mini split. A 180 sq ft room needing a 12K unit might seem aggressive, but sunrooms with cathedral ceilings, south-facing glass, and poor insulation are BTU-hungry spaces. The mini split also handles dehumidification — critical in humid climates where sunroom glass causes condensation.
Mini Split Sizing FAQ
What size mini split do I need for a 12×12 room?
A 12×12 room is 144 sq ft. At 25 BTU/sq ft, that's 3,600 BTU — well within a 9,000 BTU mini split's range. The 9K is the smallest standard single-zone size from most brands, and its inverter will modulate down to match the modest load.
The baseline is 20–25 BTU per sq ft for rooms with average insulation and 8-ft ceilings. Adjust up for poor insulation (+20%), high ceilings (+20% per extra 2 ft), heavy sun exposure (+10–15%), hot climates (+20–30%), or kitchens (+4,000 BTU flat).
Can one mini split cool an entire house?
A single indoor unit can effectively condition one room or one open area, not an entire house with separate rooms. For whole-house coverage, you need a multi-zone system with one indoor head per room — a single outdoor condenser can support 2–8 indoor heads depending on capacity.
Is it better to oversize or undersize a mini split?
Neither is ideal, but slight oversizing (one size up) is more forgivable with mini splits than with central AC. Inverter compressors modulate down, so a 12K unit in a room that only needs 8K will simply run at low output.
However, Trane recommends staying within ½ ton (6,000 BTU) of your calculated load — gross oversizing still causes humidity problems and wastes money on a larger unit. Undersizing is worse: the system runs constantly, can't reach setpoint, and wears out faster.
Do mini splits use less energy than central AC?
Yes, significantly. Mini splits with SEER ratings of 18–33 are far more efficient than typical central AC systems at 14–21 SEER.
Combined with zero duct losses (which waste 20–30% in central systems) and zone control, mini splits typically cut cooling costs by 20–40% compared to central air. See our cost to run air conditioner calculator for specific numbers.
What size mini split do I need for a 24×24 garage?
A 24×24 garage is 576 sq ft, which calculates to 14,400 BTU base. After adding the +20% garage adjustment for poor insulation and drafts, you need ~17,280 BTU — which rounds to an 18,000 BTU mini split. If the garage is completely uninsulated or in a hot climate, bump to 24,000 BTU.