Dew Point Calculator — Temperature, Humidity, and Condensation

"There's condensation dripping from my AC vents every morning. My HVAC tech said the dew point in my house is too high. What does that mean?"

If moisture is forming on your vents, windows, or ductwork, the culprit is almost always the same thing: the dew point. In plain terms, the dew point is the temperature at which moisture condenses out of the air. If any surface in your home is colder than the dew point, you get condensation — on vents, windows, pipes, walls, everywhere.

The good news? You can calculate your exact dew point in seconds. All you need are two numbers:

1. Air temperature (°F) — your thermostat reading or a thermometer.
2. Relative humidity (% RH) — from a hygrometer or your thermostat's humidity readout.

Plug those two values into our calculator below, and you'll get your dew point, a comfort rating, a condensation risk assessment, and the wet bulb temperature.

Let's put it to work with a quick example.

Worked Example 1: What's the Dew Point in My Living Room?

1. Air temperature: 75°F
2. Relative humidity: 55%
3. Plug into the Magnus formula (the calculator does this automatically).
4. Result: Dew point = 58°F

That 58°F dew point falls in the "slightly humid" range on the NWS comfort scale. Any surface in your home colder than 58°F — cold water pipes, AC vents, single-pane windows — will develop condensation.

What Is Dew Point? (And Why It Matters More Than Humidity)

The dew point is the temperature at which air becomes 100% saturated with water vapor. Cool the air to that temperature, and moisture starts condensing into liquid water — forming dew on grass, fog in the air, or water droplets on your cold AC vents.

Here's the key insight: dew point tells you exactly how much moisture is in the air. A dew point of 65°F means there's a lot of moisture present, period — regardless of the air temperature. A dew point of 40°F means the air is dry. Simple.

What Is Dew Point Temperature?

Dew point temperature is a fixed measure of the actual water vapor content of a specific air mass. Unlike relative humidity, which shifts up and down as air temperature changes throughout the day, the dew point stays constant unless moisture is actually added to or removed from the air.

This makes dew point far more useful for HVAC applications. When your AC system is dehumidifying, it's literally cooling air below its dew point to squeeze out moisture.

What Does Dew Point Measure?

Dew point measures the absolute moisture content of the air, expressed as a temperature. The higher the dew point, the more water vapor the air contains. The lower the dew point, the drier the air.

Think of it this way: dew point is the honest answer to "how humid is it really?" — while relative humidity can be misleading.

Dew Point vs. Relative Humidity: Why Dew Point Is the Better Metric

Here's a scenario the National Weather Service uses to illustrate the problem with relying on relative humidity alone:

The winter day has double the relative humidity of the summer day — yet the summer day feels far more humid. Why? Because the dew point is 60°F vs. 30°F. The summer air contains vastly more water vapor, even at only 50% RH.

Source: NWS La Crosse (weather.gov/arx/why_dewpoint_vs_humidity)

This is exactly why HVAC professionals, meteorologists, and building scientists all prefer dew point over relative humidity. Relative humidity is a ratio that depends on temperature. Dew point is an absolute measure of moisture content.

Dew Point vs. Humidity Chart

Notice: a 60°F dew point gives you 61% RH when it's 75°F — but 100% RH (fog, condensation everywhere) when it's only 60°F. The dew point stays at 60°F in both cases. That's the whole point.

Can You Have Low Dew Point and High Humidity?

Yes. Every cold winter morning proves it. When the air temperature is 35°F and the dew point is 33°F, relative humidity is about 93% — but the air feels dry and crisp because the actual moisture content (the dew point) is extremely low.

This is why your skin cracks in winter despite "high humidity" readings. The dew point — and thus the actual moisture in the air — is very low.

Dew Point Comfort Chart: What Dew Point Feels Humid?

The National Weather Service publishes a dew point comfort scale that HVAC professionals and meteorologists use daily. Here's the expanded version:

Source: NWS La Crosse (weather.gov/arx/why_dewpoint_vs_humidity) and NWS Heat Index page (weather.gov/arx/heat_index)

Comfortable Dew Point Range

For most people, the sweet spot is a dew point between 50°F and 60°F. Below 50°F starts to feel dry (and can dry out skin and nasal passages). Above 60°F starts to feel muggy.

For ideal indoor AC temperature settings, aim for a dew point in the low to mid-50s°F range. This typically corresponds to indoor conditions around 72–75°F at 40–50% RH — right in line with the ASHRAE Standard 55 comfort recommendation of 30–60% RH.

What Is a High Dew Point?

Any dew point above 65°F is considered high by the NWS. At a 70°F dew point, nearly everyone feels uncomfortable — sweat can't evaporate efficiently, and your body struggles to cool itself.

Indoors, a dew point above 60°F signals potential problems: condensation on AC vents, sweating ductwork, and elevated mold risk on cold surfaces. If your indoor dew point is consistently above 60°F, your HVAC system may not be removing enough humidity.

Dew Point Chart: Temperature × Humidity Lookup Table

This is the master reference table. Find your air temperature in the left column, your relative humidity across the top, and read the dew point at the intersection. All values in °F.

Dew Point Temperature Chart (°F)

How to use this table: If your home is 75°F at 50% RH, find the intersection: your dew point is 56°F. That means any surface below 56°F — your AC supply vents, cold water pipes, basement walls — will develop condensation.

Source: Values calculated using the Magnus-Tetens formula with Alduchov & Eskridge (1996) constants (a = 17.625, b = 243.04°C).

How to Calculate Dew Point: The Magnus Formula

The standard method for calculating dew point is the August-Roche-Magnus approximation, published by Alduchov and Eskridge in the Journal of Applied Meteorology (1996). It's accurate to within ±0.35°C for temperatures between −45°C and 60°C.

Here's the formula:

α = (a × T) / (b + T) + ln(RH / 100)

Td = (b × α) / (a − α)

Where:

• Td = dew point temperature (°C)
• T = air temperature (°C)
• RH = relative humidity (%)
• a = 17.625
• b = 243.04°C
• ln = natural logarithm

Source: Alduchov, O.A. and Eskridge, R.E., 1996: "Improved Magnus form approximation of saturation vapor pressure." J. Appl. Meteor., 35, 601–609. Also referenced by the UK National Physical Laboratory (npl.co.uk) and the University of Miami (bmcnoldy.earth.miami.edu).

Dew Point Temperature Formula (Step-by-Step)

Here's how to calculate dew point manually:

1. Convert your air temperature to Celsius: T(°C) = (T(°F) − 32) × 5/9
2. Plug T and RH into the alpha formula: α = (17.625 × T) / (243.04 + T) + ln(RH / 100)
3. Calculate the dew point: Td = (243.04 × α) / (17.625 − α)
4. Convert back to Fahrenheit: Td(°F) = Td(°C) × 9/5 + 32

Worked Example 2: Manual Dew Point Calculation

Let's calculate the dew point for 78°F air at 62% RH:

1. Convert to Celsius: (78 − 32) × 5/9 = 25.56°C
2. Calculate α: (17.625 × 25.56) / (243.04 + 25.56) + ln(62/100) = 16.78 / 268.60 + (−0.478) = 6.249 + (−0.478) = 5.771

Wait — let's be precise:

• (17.625 × 25.56) / (243.04 + 25.56) = 450.50 / 268.60 = 1.6773

Corrected:

• α = 1.6773 + ln(0.62) = 1.6773 + (−0.4780) = 1.1993

3. Calculate Td: (243.04 × 1.1993) / (17.625 − 1.1993) = 291.48 / 16.426 = 17.75°C
4. Convert: 17.75 × 9/5 + 32 = 63.9°F

Result: The dew point is approximately 64°F — right at the border of "sticky" and "uncomfortable" on the NWS comfort scale. Surfaces below 64°F will see condensation.

Or, you know, just use our calculator above. It does all of this instantly.

How Dew Point Affects Your HVAC System

Dew point isn't just a weather concept — it's the fundamental principle behind how your AC removes humidity, why your vents sweat, and why your basement walls are wet in July.

Why Your AC Vents Are Sweating (Condensation on Vents)

Your AC's evaporator coil typically operates at 40–50°F — well below the dew point of most indoor air. This is by design. As warm, humid air passes over the cold coil, its temperature drops below the dew point, and moisture condenses out of the air and drips into the condensate drip pan.

That's exactly how dehumidification works in every air conditioning system.

But here's the problem: if the air in your ducts has a high dew point (say, 65°F) and your AC supply vents are blowing 55°F air into a warm room, the vent register itself can cool below the surrounding air's dew point. Result: condensation dripping from your vents.

Source: ACHR News — "To Remove More Moisture, Lower Airflow Speed" (achrnews.com); HVAC School — "Impacts of Decreasing Evaporator Airflow" (hvacrschool.com)

Worked Example 3: Why AC Vents Sweat

1. Room conditions: 76°F at 60% RH
2. Dew point (from our table): ~61°F
3. AC supply air temperature: 55°F
4. Vent surface temperature: approximately 58°F (warmed slightly by surrounding air)
5. 58°F is below the 61°F dew point → condensation forms on the vent

The fix? Lower your indoor dew point by improving dehumidification, or insulate your ductwork. Check our guide on condensation on AC vents for step-by-step solutions.

Why Your Basement Walls Are Wet in Summer

This one surprises many homeowners. In summer, outdoor dew points routinely hit 65–75°F in humid climates. Your below-grade basement walls stay a relatively cool 58–65°F year-round (stabilized by ground temperature).

When warm, humid outdoor air enters your basement through open windows, leaky ducts, or natural infiltration, it hits those cool basement walls — and the walls are below the dew point.

Worked Example 4: Summer Basement Condensation

1. Outdoor air dew point in July: 72°F (oppressive)
2. Basement wall surface temperature: 62°F
3. 62°F is well below the 72°F dew point → heavy condensation on basement walls
4. Result: wet walls, musty smell, potential mold growth

The solution isn't opening windows (that brings in more humid air). The solution is a dehumidifier set to the right level to bring the indoor dew point below your wall surface temperature.

Window Condensation in Winter

Winter window condensation works in reverse. Your heated indoor air might be 70°F at 40% RH — giving a dew point of about 44°F. Single-pane windows in cold weather can have interior surface temperatures of 35–40°F.

Since 35–40°F is below the 44°F dew point, moisture condenses on the glass. Better-insulated windows with higher R-values stay warmer on the interior surface, reducing condensation risk.

HVAC Coil Temperature vs. Room Dew Point

For your AC to dehumidify at all, the evaporator coil must operate below the room's dew point temperature. If the coil is warmer than the dew point, zero moisture will be removed — regardless of how long the system runs.

A typical residential evaporator coil operates at about 40°F saturated suction temperature. For a room at 75°F and 50% RH, the dew point is 55°F. Since 40°F is well below 55°F, the system dehumidifies effectively.

But if an oversized AC cools the room too quickly and short-cycles, the coil may not run long enough to remove adequate moisture — even though its temperature is below the dew point. This is why oversized AC units often create humidity problems. For more on this relationship, see our superheat and subcooling guide.

Source: ACHR News (achrnews.com) — "the evaporator coil temperature must be 20–26 degrees below room temperature to properly control humidity."

Dew Point and Mold Growth

Mold doesn't need standing water to grow. According to ASHRAE research, mold can begin growing on surfaces where the local relative humidity exceeds 80% — even without visible condensation.

Here's where dew point connects to mold risk: when a surface temperature is close to (but not quite at) the air's dew point, the relative humidity right at that surface can exceed 80% — creating conditions for mold growth even before you see water droplets.

Source: ASHRAE Journal, September 2016 — "Minimum Conditions for Visible Mold Growth" (via Portland State University); EPA recommends indoor RH below 60%; NC DHHS/ASHRAE recommends below 60% to control mold, below 50% to control dust mites.

The practical takeaway: keep your indoor dew point below 55°F whenever possible. This keeps surface RH below 80% on most surfaces in a normally conditioned home, and well out of the mold danger zone. A dehumidifier is often the most cost-effective way to achieve this.

Dew Point in Building Science: Vapor Barriers, Wall Cavities, and Attic Condensation

In building science, the dew point determines where condensation forms inside your walls. This is a critical concept for insulation and vapor barrier placement.

Wall Cavity Condensation

As warm, moist indoor air migrates outward through a wall in winter, it passes through progressively colder layers of insulation. At some point, the temperature within the wall drops below the dew point of the indoor air — and moisture condenses.

Where does this condensation end up? According to building science expert Joe Lstiburek (Building Science Corporation), condensation collects on the first cold surface — typically the backside of the exterior sheathing — not at the theoretical "dew point plane" in the middle of the insulation, as older textbook calculations might suggest.

Source: Building Science Corporation — BSD-049 "Confusion About Diffusion" (buildingscience.com); BSD-163 "Controlling Cold-Weather Condensation Using Insulation" (buildingscience.com)

Vapor Barriers and the Dew Point

The role of a vapor barrier (more accurately, a vapor retarder) is to prevent warm, moist air from reaching cold surfaces where it would condense. Proper placement depends entirely on your climate:

• Cold climates: Vapor retarder on the interior (warm side) of the wall. This stops indoor moisture from reaching the cold sheathing.
• Hot-humid climates: Vapor retarder on the exterior (warm side) of the wall. This stops humid outdoor air from reaching the cool, air-conditioned interior surfaces.
• Mixed climates: This is where it gets tricky — and where proper insulation R-value planning is critical.

Source: Building Science Corporation — BSD-106 "Understanding Vapor Barriers" (buildingscience.com); Whole Building Design Guide — "Moisture Management" (wbdg.org)

Attic Condensation

Attic condensation follows the same principle. In winter, warm moist air leaking into a cold attic hits the underside of the roof sheathing — which is at or near outdoor temperature. If the sheathing temperature is below the dew point of the leaking air, condensation (or frost) forms on the sheathing, leading to wood rot and mold.

The solution: air-seal the attic floor to prevent warm moist air from entering, and maintain proper ventilation so any moisture that does enter can escape.

Wet Bulb Temperature vs. Dew Point: What's the Difference?

Both wet bulb temperature and dew point relate to moisture in the air, but they measure different things:

In short: dew point tells you when condensation starts; wet bulb tells you how much evaporative cooling is possible. For most HVAC homeowner applications — troubleshooting condensation, understanding humidity, setting dehumidifier levels — the dew point is the number you need.

For HVAC technicians working with refrigerant pressure-temperature charts, both values matter: the dew point determines latent load (moisture removal), while the wet bulb influences total heat energy in the air.

Source: Engineering Toolbox (engineeringtoolbox.com) — "Dry Bulb, Wet Bulb and Dew Point Temperatures"; Wikipedia — "Wet-bulb temperature" (citing standard psychrometric relationships)

Worked Example 5: HVAC Coil Temperature vs. Room Dew Point

An HVAC technician is troubleshooting a system that isn't removing enough humidity. Here's the diagnostic:

1. Room conditions: 76°F at 55% RH
2. Dew point (from calculator): 59°F
3. Measured evaporator coil (saturated suction) temperature: 52°F
4. Supply air temperature: 58°F
5. 58°F is just below the 59°F dew point — barely dehumidifying

The coil is cold enough, but the supply air leaving the coil has barely dropped below the dew point. The fix: reduce airflow (lower blower speed) to increase contact time between air and coil, dropping the supply air temperature further below the dew point for better moisture removal.

Source: ACHR News (achrnews.com) — "the supply air wet-bulb temperature must be 21–26 degrees below room temperature."

Frequently Asked Questions

What Is a Comfortable Dew Point?

For most people, a dew point between 50°F and 60°F feels comfortable. Below 50°F feels dry and crisp. Above 60°F starts feeling sticky. The National Weather Service classifies dew points at or below 55°F as "dry and comfortable."

What Is the Difference Between Dew Point and Humidity?

Dew point is an absolute measure of moisture content (expressed as a temperature). Relative humidity is a percentage that changes with air temperature — the same air can show 90% RH in the cool morning and 40% RH on a warm afternoon, while the dew point stays the same.

What Dew Point Is Uncomfortable?

According to the NWS, dew points in the 61–65°F range start to feel "sticky," and anything above 65°F is uncomfortable to oppressive for most people. In the 70s°F, it's genuinely oppressive — most residents of temperate climates find it hard to function comfortably outdoors.

At What Dew Point Does Mold Grow?

Mold doesn't grow at a specific dew point. It grows when the surface relative humidity exceeds about 80% for a sustained period. This happens when any surface drops to within a few degrees of the air's dew point. As a rule of thumb, keep your indoor dew point below 55°F to prevent mold on typical household surfaces.

How Do You Calculate Dew Point From Temperature and Humidity?

Use the Magnus formula: calculate α = (17.625 × T) / (243.04 + T) + ln(RH/100), then Td = (243.04 × α) / (17.625 − α). T is in °C, RH is in %. Or use our calculator at the top of this page — it does the math instantly.

Does Your AC Lower the Dew Point?

Yes. Your AC's evaporator coil operates at 40–50°F, well below the indoor dew point. As air passes over the coil, moisture condenses out and drains away, physically removing water vapor from the air and lowering the dew point. That's why your AC drip pan collects water — that's the humidity being pulled from your indoor air.