Thermodynamics

Dew Point Calculator

Calculate dew point temperature from air temperature and relative humidity.

Formula Td = (b × γ) / (a − γ)Reviewed Aug 20, 2026

Dew point is the temperature air would need to cool to for it to become fully saturated with the water vapor it's already carrying - condensation, dew, or fog forms below it. Unlike relative humidity, which changes with air temperature even when the actual moisture content doesn't, dew point tracks moisture directly, which is why meteorologists treat it as the better indicator of how humid a day will actually feel. Enter the air temperature and relative humidity to get an instant dew point.

Calculation Bench

01Air Temperature

T · Dry-bulb (regular) air temperature

02Relative Humidity

RH% · As a percentage, e.g. 50 for 50%

Solution

Enter air temperature and relative humidity to calculate dew point.

Td = (b × γ) / (a − γ)

Formula Sheet

\gamma = \dfrac{a \times T_c}{b + T_c} + \ln\left(\dfrac{RH}{100}\right)

T_d = \dfrac{b \times \gamma}{a - \gamma}

TdDew Point
TAir Temperature
TcAir Temperature (°C)
RH%Relative Humidity
γGamma
a, bMagnus Constants

Variables & Units

Symbol	Variable	Description	Common Units
Td	Dew Point	The temperature at which the air becomes saturated with its current moisture content.
T	Air Temperature	The dry-bulb (regular) air temperature.	°F, °C
Tc	Air Temperature (°C)	Air temperature converted to Celsius - the Magnus formula's constants are defined in this unit.
RH%	Relative Humidity	Relative humidity as a percentage, from just above 0% to 100%.
γ	Gamma	An intermediate value used by the Magnus formula, with no direct physical meaning of its own.
a, b	Magnus Constants	Empirically fitted constants (a = 17.625, b = 243.04°C) from Alduchov & Eskridge (1996).

How to Use This Calculator

01Enter the air (dry-bulb) temperature - the regular temperature reading you'd see on a thermometer or forecast.
02Enter the relative humidity as a percentage.
03The calculator instantly shows the dew point temperature and the dew point depression (how far the air temperature is above the dew point).

How the Formula Works

This calculator uses the Alduchov–Eskridge (1996) refinement of the Magnus (August-Roche-Magnus) approximation - the formula used by the National Weather Service and virtually every consumer weather station. It's accurate to within about ±0.35°C across the -40°C to 60°C range most weather falls into.

The formula first computes an intermediate value, γ (gamma), from the air temperature and the natural log of the relative humidity fraction, then solves for dew point from γ using the same two empirical constants.

Dew point depression - air temperature minus dew point - is a useful secondary number on its own: a small depression (a few degrees or less) means the air is close to saturation, which is the condition that produces fog, frost, or condensation.

At 100% relative humidity, dew point and air temperature are mathematically identical - the air is already fully saturated, so no further cooling is needed to reach the dew point.

Worked Example 01

75°F, 50% relative humidity

Known

Air Temperature (T): 75°F (23.9°C)
Relative Humidity (RH%): 50%

Formula

Td = (b × γ) / (a − γ)

Substitution

γ = (17.625×23.9)/(243.04+23.9) + ln(0.50) = 0.884, Td = (243.04×0.884)/(17.625−0.884)

Result

≈55.1°F (12.8°C)

A comfortable 75°F day at 50% relative humidity has a dew point around 55°F - solidly in the "comfortable/dry" range most comfort scales use, with a large 20°F dew point depression indicating dry air well short of saturation.

Worked Example 02

90°F, 80% relative humidity (muggy summer day)

Known

Air Temperature (T): 90°F (32.2°C)
Relative Humidity (RH%): 80%

Formula

Td = (b × γ) / (a − γ)

Substitution

γ = (17.625×32.2)/(243.04+32.2) + ln(0.80) = 1.840, Td = (243.04×1.840)/(17.625−1.840)

Result

≈83.0°F (28.3°C)

The same relative humidity reading means very different things at different temperatures - 80% RH at 90°F gives a dew point around 83°F, deep in the oppressive/very uncomfortable range on most comfort scales, with only a 7°F depression showing the air is close to fully saturated.

Applications

01Forecasting fog or frost risk from how close the dew point is to the air temperature (a small dew point depression)
02Judging how humid and uncomfortable a summer day will actually feel, which dew point tracks more directly than relative humidity does
03HVAC and building-envelope work, where dew point indicates the surface temperature at which condensation will start forming on a cool surface (e.g. a duct, window, or cold pipe)

General Dew Point Comfort Scale (approximate - sources vary by a few degrees)

Dew Point	General Comfort Level
Below 50°F (10°C)	Dry, comfortable
50–60°F (10–16°C)	Comfortable
60–65°F (16–18°C)	Somewhat humid
65–70°F (18–21°C)	Humid, sticky
70–75°F (21–24°C)	Very humid, oppressive
Above 75°F (24°C)	Extremely humid, uncomfortable for nearly everyone

Assumptions

01Air temperature is entered as the dry-bulb (ordinary) temperature, not a wet-bulb or "feels like" reading.
02The Alduchov–Eskridge constants used here are optimized for the -40°C to 60°C range that covers virtually all real-world weather; extreme lab conditions outside that range are less accurate.
03Relative humidity is assumed accurate as entered - sensor drift or a stale reading will carry directly into the dew point result, the same as with any dew point calculation.

Where This Model Stops

01Does not compute the reverse direction (relative humidity from temperature and dew point) - this calculator solves for dew point only.
02Does not compute heat index or a "feels like" temperature, which combines dew point/humidity with air temperature using a separate formula.
03The comfort-level reference table below is a general guide, not a precise physical boundary - perceived humidity varies by individual, acclimatization, wind, and sun exposure, and different meteorological sources draw the band edges a few degrees apart from each other.

References

[1]
Improved Magnus Form Approximation of Saturation Vapor Pressure
Alduchov & Eskridge, Journal of Applied Meteorology (1996), via American Meteorological Society
Primary source for the a = 17.625, b = 243.04°C constants used in this calculator's Magnus-formula approximation.
[2]
Temperature, Dewpoint, and Relative Humidity Calculator
University of Miami (Brian McNoldy)
Cross-reference for the same Magnus-formula structure and constants, and for the ±0.35°C accuracy figure.

Frequently Asked Questions

Why is dew point a better humidity indicator than relative humidity?

Relative humidity changes with air temperature even when the actual amount of moisture in the air stays exactly the same - the same air mass can read 80% RH at night and 40% RH on the same afternoon as it warms, with no water added or removed. Dew point tracks the actual moisture content directly, so a given dew point reading feels the same whether the air temperature is 75°F or 95°F.

What does it mean when the dew point is close to the air temperature?

A small dew point depression (air temperature minus dew point) means the air is close to saturation - this is the condition that produces fog, dew, or frost. A depression of just a couple of degrees is a strong signal that fog is likely, especially overnight as temperatures drop toward the dew point.

Can the dew point ever be higher than the air temperature?

No - physically, dew point can only equal or be lower than the air temperature. They're equal exactly at 100% relative humidity (full saturation); any measurement showing a dew point above air temperature indicates a sensor or reading error.

What formula does this calculator use?

The Alduchov–Eskridge (1996) refinement of the Magnus (August-Roche-Magnus) approximation, the same formula used by the National Weather Service and most consumer weather stations. It's accurate to within about ±0.35°C across the -40°C to 60°C range that covers essentially all real-world weather conditions.

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