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Relative Humidity From Dew Point Calculator

Work out relative humidity from dew point instantly with clear inputs, formula shown and shareable results.

About Relative Humidity From Dew Point Calculator

Dew point is the temperature at which the air's existing moisture would saturate, so the actual vapour pressure equals the saturation pressure at the dew point. Relative humidity is that value as a percentage of saturation at the real air temperature, computed here with the Magnus-Tetens expression. Air at 30 C with a 20 C dew point sits near 55% relative humidity.

Formulas

Magnus saturation pressure

es(T) = 6.112 x exp(17.62T / (243.12 + T)) hPa

Relative humidity

RH = 100 x es(dew point) / es(air temperature)

Worked example

With the default values below, Relative Humidity From Dew Point Calculator returns Relative Humidity of 55.10%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Air temperature
30 C
Dew point temperature
20 C

Results

Relative Humidity
55.10%
Saturation Vapour Pressure At Air Temperature
42.337 hPa
Actual Vapour Pressure
23.326 hPa
Vapour Pressure Shortfall
19.011 hPa

Step by step

  1. 1Values used
  2. 2Magnus saturation pressure
  3. 3Relative humidity
  4. 4Relative Humidity= 55.10
  5. 5Saturation Vapour Pressure At Air Temperature= 42.337 hPa
  6. 6Actual Vapour Pressure= 23.326 hPa
  7. 7Vapour Pressure Shortfall= 19.011 hPa

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Can the dew point exceed the air temperature?

No - it would mean supersaturation. If entered that way the calculator clamps the dew point to the air temperature, giving 100%.

Why is dew point a better comfort measure than humidity?

Because it tracks the absolute moisture content. A 24 C dew point feels oppressive whatever the relative humidity reads.

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