TECHNICAL TOOL / 10

Relative Humidity Calculator

Find relative humidity from dry bulb and dew point or thermodynamic wet bulb. Check humidity ratio and absolute humidity for industrial rooms, greenhouses and HVAC air streams.

CALCULATED AIR STATE

Relative humidity: 60.00 %
Dew / frost point: 16.70 °C
Humidity ratio · dry-air basis: 11.90 g/kg
Absolute humidity · vapour density: 13.82 g/m³

Example: 25°C dry bulb, 60% RH, 101.325 kPa.

01 / CHOOSE YOUR INPUT

Start with the two temperatures you know.

Dry bulb + dew point

Use a measured air temperature and dew point from the same air sample. At fixed temperatures, calculated RH is independent of total pressure; humidity ratio still needs pressure.

Dry bulb + wet bulb

Enter thermodynamic wet bulb and local atmospheric pressure. For a ventilated psychrometer, first apply the instrument method and corrections; raw readings may differ.

Already know RH?

Use the moist-air property converter to expand a measured dry-bulb/RH state into enthalpy, specific humidity and other properties. This page solves for the missing RH.

Open the moist-air converter

02 / INTERPRET THE RESULT

RH changes with temperature. Water content needs its own unit.

Quantity	Use
RH · %	Vapour pressure divided by saturation pressure at dry bulb. Reheating can lower RH without removing water.
Humidity ratio · g/kg dry air	Compare moisture entering and leaving an air stream on the same dry-air mass basis. Multiply the difference by dry-air mass flow for a water-removal rate.
Absolute humidity · g/m³	Water-vapour mass per volume of air at the stated temperature. Volume changes with temperature, so this is not interchangeable with g/kg.

Calculation method and sources

RH = 100 × pws(Tdew/frost) / pws(Tdry)
Tdry + Twet + p → w → pᵥ → RH

The first mode inverts saturation pressure; the second uses the ASHRAE water/ice wet-bulb balance. Both reuse the same library as the psychrometric chart. Example: dry bulb 25°C and dew point 16.70113°C, or wet bulb 19.47105°C at 101.325 kPa, give approximately 60.00% RH, 11.90 g/kg and 13.82 g/m³.

ASHRAE Fundamentals (2017), Chapter 1 · PsychroLib

The saturation equations use ice at or below 0.01°C and water above; the wet-bulb energy balance switches at 0°C. A subzero input labelled “dew point” must match this frost-point convention. Check the sensor’s reference phase before using cold-room readings.

What this tool does not decide

It does not choose a crop or storage RH target, validate a sensor, determine condensation on an unmeasured surface, or size a dehumidifier. Define the material/process target and evaluate moisture sources separately.

NEXT / PROJECT REVIEW

Bring the operating conditions to an equipment review.

Share inlet and target air states, room volume, outdoor airflow, moisture sources and installation constraints.

Review project requirements