What is relative humidity?

Relative humidity (RH) is the ratio of the actual water vapor pressure in the air to the saturation vapor pressure at the same temperature, expressed as a percentage. At 100% RH the air is fully saturated and condensation (dew, fog) can form.

What formula does this tool use?

Saturation vapor pressure is calculated with the Magnus formula: e(T) = 6.1078 × exp((17.625 × T) / (243.04 + T)), where T is in Celsius. Relative humidity is then RH = 100 × e(Td) / e(T), where Td is the dew point temperature.

Can relative humidity exceed 100%?

In theory, supersaturation above 100% can occur briefly in very clean air, but in practice RH is capped at 100% when the dew point equals the air temperature. This calculator flags any input where dew point exceeds air temperature as invalid.

What is a comfortable relative humidity level?

Most people feel comfortable indoors between 30% and 60% RH. Below 30% the air feels very dry and can irritate skin and mucous membranes. Above 60% the air feels muggy and promotes mold growth.

How does dew point relate to relative humidity?

The dew point is an absolute measure of moisture; relative humidity compares that moisture to how much the air could hold at the current temperature. Warm air can hold more moisture, so the same dew point gives a lower RH on a hot day than on a cool day.

What units are vapor pressure measured in?

This tool accepts hPa (hectopascals, equivalent to millibar) or kPa (kilopascals). Standard sea-level saturation vapor pressure at 20 °C is about 23.4 hPa (2.34 kPa).

Relative Humidity Calculator

Calculate relative humidity from temperature and dew point, or from actual and saturation vapor pressure. Supports both Celsius and Fahrenheit.

Input Method

Temperature Unit

Air Temperature (°F)

Dew Point (°F)

Enter values on the left, then click
Calculate Relative Humidity.

Relative Humidity

-- %

0% 50% 100%

Actual Vapor Pressure

Saturation Vapor Pressure

Humidity Comfort Scale

Very Dry

Below 25%

Dry

25–40%

Comfortable

40–60%

Humid

60–80%

Very Humid

Above 80%

Formula: RH = 100 × e(Td) / e(T) using the Magnus approximation (a = 17.625, b = 243.04 °C).

Summary

The Relative Humidity Calculator uses the Magnus formula to compute relative humidity — the ratio of the current amount of water vapor in the air to the maximum amount the air can hold at the same temperature. You can calculate RH either by entering the air temperature and dew point, or by entering actual and saturation vapor pressure directly. The tool supports both Celsius and Fahrenheit inputs and displays a humidity comfort level with a descriptive breakdown.

How it works

Choose your preferred input method: Temperature + Dew Point, or Vapor Pressure.
For Temperature + Dew Point: enter both values in Fahrenheit or Celsius.
The Magnus formula computes saturation vapor pressure at the air temperature and at the dew point.
Relative humidity is the ratio e(Td) / e(T) × 100, where e is saturation vapor pressure.
For Vapor Pressure input: enter actual and saturation vapor pressure in hPa or kPa.
Results include the RH percentage, vapor pressures, and a comfort level rating.

Use cases

Verify weather station readings by cross-checking temperature, dew point, and humidity.
Determine indoor air quality and comfort for HVAC system settings.
Assess mold and condensation risk in buildings and storage facilities.
Help meteorologists and students understand the relationship between temperature and moisture.
Plan outdoor activities by converting dew point readings to intuitive humidity percentages.
Support agricultural decisions about irrigation and crop moisture stress.

Frequently Asked Questions

Last updated: 2026-06-13 · Reviewed by Nham Vu