Relative Humidity Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Air-property calculations are tightly coupled: temperature, moisture, pressure, and density can all move together. Relative Humidity Calculator focuses that system on the page’s relative humidity, so the number can be traced back to the exact atmospheric inputs rather than treated as a generic weather value.

What this calculator does

The Relative Humidity Calculator brings together Air temperature, Dew-point temperature, Relative humidity around the page’s relative humidity. The formula section below identifies which values actually drive that result and which fields are supporting or derived quantities, so you can check the page without assuming every visible box is an independent input.

How to use it

The main fields on this page are Air temperature, Dew-point temperature, Relative humidity. Enter the quantities you actually know, keep their units consistent, and leave derived/output-style fields blank unless the formula explicitly allows solving in the opposite direction. For a clean check of relative humidity, change one driving quantity at a time and confirm that the result moves in the direction predicted by the equation.

How the calculation works

The page estimates vapor pressure at the dew point and saturation vapor pressure at the air temperature, then calculates RH = 100·e(Td)/es(T). A dew point close to the air temperature therefore produces a relative humidity close to 100%.

Example

For example, start with the page’s populated scenario: Air temperature = 25; Dew-point temperature = 15. Apply the equation above using the units shown on the page, then compare the calculated relative humidity with the displayed result. As a second check, change one physical input while holding the others fixed and confirm that the direction of change makes sense for this formula.

How to interpret the result

Interpret relative humidity as an estimate for the entered air state and the specific approximation described above. Compare it with the related supporting values, and remember that a numerically precise result can still be wrong if pressure type, temperature scale, or humidity basis is inconsistent.

Limitations and notes

Atmospheric formulas simplify a variable real environment. Sensor error, nonuniform air, altitude-dependent pressure, radiation, wind, and local microclimate can dominate small calculated differences. Recheck units and use measured/local data when an operational decision depends on the result. For Relative Humidity Calculator, keep that general caution tied to the exact fields and equation shown here rather than carrying assumptions over from a different calculator.