Humidity from Wet & Dry Bulb Calculator
Turn two thermometer readings from a sling or whirling psychrometer into relative humidity. Enter the dry-bulb and wet-bulb temperatures (and the air pressure if you have it) and the tool returns the relative humidity plus the actual and saturation vapour pressures behind it.
Enter Values
How to use this calculator
- Read the dry-bulb temperature (a normal thermometer) and enter it in °C.
- Read the wet-bulb temperature (the thermometer with the wetted wick, after spinning or ventilating) and enter it — it must be at or below the dry-bulb.
- Optionally enter the local atmospheric pressure in hPa (leave blank to use the 1013 hPa standard), then read the relative humidity.
How it works
Saturation vapour pressure is found from the Magnus formula es(T) = 6.112 × exp(17.62·T / (243.12 + T)) in hPa, evaluated at both the wet-bulb and dry-bulb temperatures. The actual vapour pressure is e = es(Twet) − A·P·(Tdry − Twet), where A = 0.000662 /°C is the psychrometric constant and P is atmospheric pressure. Relative humidity is then RH = e / es(Tdry) × 100. The bigger the gap between the dry and wet bulbs, the lower the humidity.
Worked example
Worked example. With a dry-bulb of 25 °C and wet-bulb of 20 °C at 1013 hPa: es(25) ≈ 31.601 hPa and es(20) ≈ 23.326 hPa, so e = 23.326 − 0.000662×1013×(25−20) ≈ 19.973 hPa and RH = 19.973 / 31.601 × 100 ≈ 63.2%.
Common mistakes
- Swapping the bulbs — the wet-bulb reading is always the lower of the two; entering a wet-bulb above the dry-bulb is physically impossible and is rejected.
- Letting the wick dry out or using tap water with mineral deposits, which lifts the wet-bulb reading and makes the humidity look higher than it is.
- Not ventilating the wet bulb (spinning the sling or using a fan) before reading, so evaporative cooling is incomplete.
Frequently asked questions
What is the wet-bulb depression?
It is the difference between the dry-bulb and wet-bulb temperatures (Tdry − Twet). A large depression means dry air; a depression of zero means the air is saturated and humidity is 100%.
Do I need to enter the air pressure?
It is optional. Pressure only fine-tunes the result through the psychrometric term, so the 1013 hPa default is fine for most conditions. Enter your actual pressure for greater accuracy at altitude.
Related tools
- Apparent Temperature Calculator
- Frost Risk Temperature Calculator
- Vapour Pressure Deficit (VPD) Calculator
- Air Density Calculator
- Absolute Humidity Calculator
- Dew Point Calculator
Explore more in Weather & Environmental Monitoring.
Tip: Enter any known values to calculate the remaining results.
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