Relative humidity is the amount of water vapour in the air compared with the most it could hold at that temperature. This guide shows how to calculate relative humidity from the air temperature and the dew point.

The relative humidity formula

Relative humidity is the saturation vapour pressure at the dew point divided by the one at the air temperature:

RH = 100 × e(Td) ÷ e(T)

Each vapour pressure comes from the Magnus equation, e(T) = exp(17.27 × T ÷ (237.7 + T)), with T in degrees Celsius. You run it twice, once for the dew point Td and once for the air temperature T, then take the ratio. The relative humidity calculator does both passes at once.

Worked example

Air at 25°C with a dew point of 16.7°C gives about 60 percent relative humidity. The closer the dew point sits to the air temperature, the higher the humidity; when they are equal, the air is saturated and the reading is 100 percent.

What the number means

Comfortable indoor air sits between 40 and 60 percent. Below 30 percent the air feels dry and pulls moisture from skin and wood; above 70 percent it feels muggy and can grow mould. The reading tops out at 100 percent, which is saturation, where dew or fog forms. Ventilation is the first fix for a damp room, and the air changes per hour requirements guide lists the rate each room needs.

Dew point and cooling load

Relative humidity and dew point are two views of the same air. The dew point calculator finds the dew point from temperature and humidity, the reverse of this.

Removing that moisture adds to the cooling load, and the BTU calculator sizes the equipment that does it.

Common mistakes

• Dew point above air temperature. The dew point cannot exceed the air temperature; the reading caps at 100 percent.
• Mixing units. Enter both temperatures in the same scale.
• Confusing relative and absolute humidity. Relative humidity is a percentage; absolute humidity is grams per cubic metre.

Quick reference

RH = 100 × e(Td) / e(T). Aim for 40 to 60 percent indoors. Calculate it fast with an online calculator free to use on CalculateGuide.