Relative Humidity Calculator

How Humidity Is Calculated

Humidity has several related but distinct measurements. Relative humidity is the most commonly reported one, and it describes how much water vapor is in the air as a percentage of the maximum the air could hold at that temperature.

Relative Humidity formula:

RH (%) = (Actual Vapor Pressure ÷ Saturation Vapor Pressure) × 100

This calculator gets the actual vapour pressure from a wet bulb reading, which is what a psychrometer gives you:

e = e_s(Tw) − k × P × (T − Tw)

where e_s is the saturation vapour pressure at the wet bulb temperature, P is station pressure in hPa, and k is the psychrometric constant. This page uses k = 0.000799, the value for a wet bulb sitting in still air, which is what a thermometer in a screen or a wick with no fan actually measures. A ventilated sling psychrometer whirled at 3 m/s uses about 0.000666, and reads 2 to 3 percentage points higher RH from the same pair of temperatures. If you are spinning a sling, add roughly two points to the answer here.

Worked example:

• Dry bulb: 30°C
• Wet bulb: 22°C

e_s(30) = 42.5 hPa, e_s(22) = 26.4 hPa e = 26.4 − 0.000799 × 1013.25 × 8 = 19.9 hPa RH = 19.9 ÷ 42.5 = 47%

Type those two temperatures in above and you get the same answer back.

There is also a rough shortcut from the dew point, worth knowing but not worth trusting:

RH ≈ 100 − 5 × (Temperature − Dew Point) [both in °C]

At 25°C with a 15°C dew point that gives 50%. The exact answer is 54%, so the shortcut is four points low here, and it drifts further as the gap widens. Use it to sanity-check a reading, not to report one.

Key humidity measurements:

• Relative Humidity (RH): Water vapor as % of maximum possible at current temperature
• Absolute Humidity: Actual mass of water per cubic meter of air (g/m³)
• Specific Humidity: Mass of water vapor per mass of moist air (g/kg)
• Dew Point: Temperature at which air becomes saturated (RH = 100%)

Absolute humidity formula:

AH (g/m³) = (6.112 × e^(17.67 × T/(T+243.5)) × RH × 2.1674) ÷ (273.15 + T)

Where T = temperature in °C and RH is entered as a whole number of percent, not a fraction. If you already have the actual vapour pressure in hPa, the same thing is AH = 216.7 × e ÷ (273.15 + T), which is what the calculator uses because it has computed e on the way to the relative humidity anyway.

Comfort ranges:

Dew point as a comfort indicator: Dew point above 60°F (16°C) feels humid, and above 70°F (21°C) it feels oppressive. Relative humidity moves every time the temperature does, even when the amount of water in the air has not changed at all. The dew point sits still, which is why forecasters quote it. The calculator gives you both.