Relative Humidity Calculator

Get relative humidity from air temp and dew point in F or C.

What it does

Relative humidity tells how full the air is with water vapor. At 100 percent the air is saturated and dew forms. At 50 percent it holds half what it could hold at that temperature. Warm air can hold far more than cold air, so the same dew point gives lower RH on a hot afternoon than on a cool morning.

The tool above takes air temperature and dew point and returns RH percent, vapor pressure, and depression. The defaults 77 F and 68 F match the worked example, so the page opens showing 73.8 percent. Switch to metric for Celsius.

The math uses the August Roche Magnus approximation for saturation vapor pressure: e = 6.112 x exp(17.625 x T / (243.04 + T)) with T in Celsius and e in hPa. Actual vapor uses dew point in the same form. RH is actual over saturation times 100. This form is the one used in EPA humidity processing and many weather calculators.

Depression, the gap T minus Td, gives a fast feel. A 9 F gap near 77 F is humid. A 30 F gap is dry. Vapor pressure in hPa suits formulas, while RH percent suits comfort, paint drying, and humidor checks.

This page shows the formula, one full hand checked example at 77 F with 68 F dew point, a reference table, and practice checks that run through the tool in both unit systems.

How to use

1. Pick Imperial (F) or Metric (C).
2. Type air temperature in the active unit.
3. Type dew point in the same unit. It must not exceed air temp.
4. Read Relative humidity as the main percent result.
5. Read Vapor pressure in hPa for formula work.
6. Read Depression as the temp minus dew gap in the active unit.

Formula

T

Air temperature in Celsius for the formula.

Td

Dew point in Celsius, at or below T.

e

Vapor pressure in hPa from the same Magnus form.

Fahrenheit inputs convert to Celsius first: C = (F minus 32) x 5/9. Results cap at 100 percent. The constants 17.625 and 243.04 suit water over liquid in the usual weather range.

Worked example

Reference table

Limits

• Magnus approximation, not a lab standard above extremes.
• Needs Td at or below T. Warmer dew than air triggers an error.
• Range minus 60 to 60 C. Beyond that use full steam tables.
• RH alone does not set comfort. Wind and sun shift feel a lot.

Common mistakes

• Entering dew above air temp, which is physically impossible at the surface.
• Mixing F with C in one run. Keep one system per calculation.
• Reading vapor hPa as RH percent.
• Assuming 50 percent RH means half the air is water. It means half of capacity.
• Using dew depression alone as RH without temperature.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• NOAA: Dew Point and Relative Humidity Calculations
• EPA: Humidity Data Algorithms with Magnus equation

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