Zerdly

Relative Humidity Calculator

The Relative Humidity Calculator finds relative humidity (RH) from the air (dry-bulb) temperature and the dew point using the Magnus formula. It also shows the saturation vapour pressure at the air temperature and the actual vapour pressure, both in hectopascals — the two quantities whose ratio defines RH.

100% FreeRuns in your browserBasic
How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the air (dry-bulb) temperature in °C — use the ± toggle for below-zero values.
  2. Enter the dew point in °C; it must be equal to or lower than the air temperature.
  3. Read the relative humidity, along with the saturation and actual vapour pressures.

How it works

Saturation vapour pressure is estimated with the Magnus formula eₛ(T) = 6.112·exp(17.62·T / (243.12 + T)) in hPa. The actual vapour pressure equals the saturation pressure at the dew point, e = eₛ(Td). Relative humidity is then RH = 100 × e / eₛ(T). Since the dew point is where the air becomes saturated, Td can never exceed T, and Td = T gives 100% RH.

Worked example

Worked example. For an air temperature of 25 °C and a dew point of 15 °C: eₛ(25) = 31.601 hPa and e = eₛ(15) = 17.017 hPa, so RH = 100 × 17.017 / 31.601 = 53.8%.

Common mistakes

  • Entering a dew point higher than the air temperature — physically impossible, so the calculator returns an error.
  • Confusing dew point with wet-bulb temperature; they are different measurements and only the dew point is used here.
  • Expecting exact agreement with a psychrometric chart — the Magnus formula is an accurate approximation but not identical to every published table, especially at temperature extremes.

Frequently asked questions

Why does the dew point have to be at or below the air temperature?

The dew point is the temperature at which the current amount of water vapour would saturate the air. Air can never hold more than its saturation amount, so the dew point equals the air temperature at 100% RH and is lower for any drier air — a dew point above the air temperature is impossible.

How accurate is the Magnus formula?

Very good for everyday near-surface temperatures, typically within a fraction of a percent of reference tables. Accuracy falls off at temperature extremes and over ice, where slightly different Magnus coefficients are used.

Related tools

Explore more in HVAC, Plumbing, Water & Building Services.

Tip: Enter any known values to calculate the remaining results.

All calculations run in your browser. Your inputs are never saved or transmitted.