Humidity Calculator

Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer Last updated: August 19, 2026

📚 Confused about how this is calculated? Read the full The Humidity Family: Absolute, Relative, Specific, and Why RH Misleads →

Relative Humidity Is a Ratio, Not an Absolute Amount

Relative humidity expresses how much moisture the air is holding compared to the maximum it could hold at that temperature — not a fixed quantity of water vapor. That's why the same absolute moisture level reads as high humidity on a cool morning and moderate humidity once the air warms up: warmer air can hold more water vapor before reaching saturation. This calculator derives relative humidity from either a temperature/dew point pair or from measured vapor pressures directly.

The Formulas

From temperature and dew point (Magnus approximation):
RH% = 100 × exp((17.625×Td)/(243.04+Td)) ÷ exp((17.625×T)/(243.04+T))

From vapor pressures:
RH% = 100 × (Actual Vapor Pressure ÷ Saturation Vapor Pressure)

T and Td are air temperature and dew point in °C. The Magnus approximation estimates saturation vapor pressure from temperature; the ratio of actual vapor pressure (implied by the dew point) to saturation vapor pressure at the actual air temperature gives relative humidity directly.

Worked Examples

Relative humidity from temperature and dew point
Air temperature	Dew point	Relative humidity
25°C	15°C	53.83%
30°C	20°C	55.08%

A smaller gap between temperature and dew point always means higher relative humidity; when the two are equal, the air is fully saturated at 100% RH.

Where This Matters

Comfort and health — high relative humidity makes hot weather feel more oppressive because sweat evaporates more slowly, while very low humidity dries out skin, airways, and wood furniture.
Mold and condensation risk — indoor relative humidity above roughly 60% creates conditions favorable to mold growth and condensation on cold surfaces.
Agriculture and storage — greenhouse operators and grain storage facilities monitor relative humidity closely since it directly affects plant transpiration rates and spoilage risk.

How to Use This Calculator

Choose a mode: From Temperature & Dew Point, or From Vapor Pressures.
For temperature and dew point: enter Air Temperature and Dew Point, both in °C.
For vapor pressures: enter the Actual Vapor Pressure and Saturation Vapor Pressure, both in hPa.
Select Calculate to see the relative humidity percentage.

Related Calculations

Work in the opposite direction with the Dew Point Calculator, or see how humidity combines with temperature in the Heat Index Calculator.

Principles of Atmospheric Moisture and Psychrometric Humidity

A humidity calculator computes the thermodynamic state of moist air, converting between Relative Humidity (RH %), Dew Point Temperature (T_d), Absolute Humidity (AH), Specific Humidity (q), and Vapor Pressure (e). In HVAC building science, meteorological forecasting, agricultural greenhouse management, and cleanroom manufacturing, humidity control governs indoor comfort, static electrical discharge, and microbial mold growth.

The Magnus-Tetens Saturation Vapor Pressure Formula

The maximum water vapor pressure that air can sustain before condensation occurs (Saturation Vapor Pressure, e_s in hectoPascals hPa / millibars) is modeled via the Magnus-Tetens Empirical Equation:

e_s(T) = 6.112 × exp [ ( 17.67 × T ) / ( T + 243.5 ) ]

Where T is dry-bulb air temperature in degrees Celsius (°C).

Relative Humidity, Dew Point, and Absolute Humidity Definitions

Relative Humidity (RH %): The ratio of actual partial water vapor pressure (e) to saturation vapor pressure (e_s) at the current air temperature:
RH (%) = [ e / e_s(T) ] × 100%
Dew Point Temperature (T_d, °C): The temperature to which air must be cooled at constant pressure for water vapor to condense into liquid dew:
γ(T, RH) = [ ( 17.67 × T ) / ( T + 243.5 ) ] + ln(RH / 100)
Dew Point (T_d) = [ 243.5 × γ(T, RH) ] / [ 17.67 - γ(T, RH) ]
Absolute Humidity (AH, g/m³): The physical mass of water vapor dissolved per cubic meter of total moist air volume:
Absolute Humidity (AH) = [ 216.7 × e (hPa) ] / [ T (°C) + 273.15 ]

Step-by-Step Worked Calculation Example

Example: Calculating Dew Point and Absolute Humidity for Summer Indoor Air

Problem: A commercial server room has an ambient dry-bulb temperature T = 25.0°C (77.0°F) and a relative humidity RH = 60.0%. Calculate: (1) Saturation vapor pressure e_s; (2) Actual partial vapor pressure e; (3) Dew Point temperature T_d; and (4) Absolute Humidity AH in grams per cubic meter.

Step 1: Compute Saturation Vapor Pressure e_s:

Exponent = ( 17.67 × 25.0 ) / ( 25.0 + 243.5 ) = 441.75 / 268.5 = 1.64525

e_s(25°C) = 6.112 × exp(1.64525) = 6.112 × 5.1823 = 31.675 hPa

Step 2: Calculate actual vapor pressure (e = e_s × RH):

e = 31.675 hPa × 0.60 = 19.005 hPa

Step 3: Calculate Dew Point Temperature T_d:

γ = 1.64525 + ln(0.60) = 1.64525 - 0.51083 = 1.13442

T_d = ( 243.5 × 1.13442 ) / ( 17.67 - 1.13442 ) = 276.23 / 16.5356 = 16.70°C (62.07°F)

Step 4: Compute Absolute Humidity AH:

AH = ( 216.7 × 19.005 ) / ( 25.0 + 273.15 ) = 4,118.38 / 298.15 = 13.81 g/m³

Conclusion: Air at 25°C and 60% RH holds 13.81 g/m³ of water vapor with condensation occurring on surfaces cooled below 16.7°C.

ASHRAE Standard 55 Indoor Comfort Envelopes

Ideal Indoor Relative Humidity: Maintain between 30% and 50% RH (suppresses dust mite proliferation and respiratory viral droplet transmission while preventing dry mucous membranes).
Mold Colonization Risk: Surface relative humidity sustained above 70% RH triggers rapid fungal Stachybotrys mold spore germination within 48 hours.

Wet-Bulb Temperature and the Critical Human Survival Limit

In human biometeorology, the Wet-Bulb Temperature (T_w) represents the lowest temperature that can be reached purely through the evaporative cooling of water into air. Measured using a sling psychrometer with a moistened wick, wet-bulb depression (T_dry - T_wet) reflects atmospheric dryness.

The 35°C Wet-Bulb Physiological Threshold

The human body maintains an internal core temperature of 37°C (98.6°F) by sweating; when sweat evaporates from skin, it dissipates metabolic heat. However, if the ambient Wet-Bulb Temperature reaches or exceeds 35.0°C (95.0°F) at 100% relative humidity, evaporative cooling becomes thermodynamically impossible:

At T_w ≥ 35°C: Evaporative Heat Dissipation Rate = 0.0 W/m²

Even healthy young individuals resting in the shade with unlimited drinking water will experience fatal core hyperthermia and heat stroke within 4 to 6 hours when wet-bulb temperature reaches this physiological ceiling.

Sling Psychrometer Operation and Wet-Bulb Depression

Meteorologists measure ambient humidity manually using a Sling Psychrometer — an instrument pairing two matched mercury thermometers: a dry-bulb thermometer measuring ambient air temperature and a wet-bulb thermometer wrapped in a saturated muslin cloth sock.

Whirling the instrument through the air creates rapid forced evaporation; comparing dry-bulb temperature against the lower wet-bulb temperature yields the Wet-Bulb Depression, looked up on standardized psychrometric hygrometry tables to determine precise relative humidity and dew point.

Specific Humidity in Atmospheric Thermodynamics

In atmospheric meteorology and cloud physics, Specific Humidity (q, in g/kg) measures the mass ratio of water vapor to total moist air mass: q = 0.622 × e / ( P - 0.378 × e ), remaining strictly conserved during air parcel vertical adiabatic ascent before condensation occurs.

Enthalpy of Moist Air in Psychrometric Design

In air conditioning thermodynamics, moist air specific enthalpy combines sensible dry air heat and latent water vapor heat: h = 1.006×T + W×(2501 + 1.86×T) kJ/kg.