Relative Humidity Calculator

Get RH from the dew point or a wet-bulb reading, see why heated rooms turn bone-dry in winter, and find how much water your humidifier needs.

By Manzoor Ahmad, pharmacist · Updated October 25, 2026

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How do you calculate relative humidity?

RH = 100 × eₛ(dew point) ÷ eₛ(air temperature), where eₛ(T) = 6.1094 × e^{17.625T/(243.04+T)} hPa is the saturation vapor pressure. Air at 25 °C with a 15 °C dew point has a relative humidity of 53.8%. The US EPA recommends keeping indoor RH between 30% and 50%.

Air temperature

Dew point

Dry-bulb temp

Wet-bulb temp

Altitude (m)Sets air pressure for the psychrometer equation (1013.25 hPa at sea level).

Outdoor air

Temperature

Relative humidity (%)

Indoor room

Heated to

Target RH (%)

Room volume (m³)Length × width × height. A 4 × 4 m room with a 2.5 m ceiling is 40 m³.

Relative humidity—

What relative humidity is healthy?

Indoor RH	Effect	What to do
Below 30%	Dry eyes, skin and airways; static shocks; wood shrinks	Humidify; reduce over-ventilation
30–50%	Comfortable and healthy (EPA range)	Maintain
50–60%	Acceptable; dust mites start to thrive	Ventilate kitchens and bathrooms
Above 60%	Mold, condensation, musty smells	Dehumidify; fix leaks; vent outdoors

Why winter air feels so dry

Outdoor air at 0 °C and 80% RH holds only 3.9 g of water per cubic meter. Heat it to 21 °C and the same moisture is just 20% of what the warmer air could hold. Bringing a 40 m³ room up to 45% RH takes about 185 g (a small glass) of water, replaced continuously as air leaks out.

A pharmacist’s note on storing medicines

Labels that say “store in a dry place” refer to the US Pharmacopeia definition: no more than 40% average relative humidity at 20 °C. Bathrooms regularly exceed 80% RH after a shower, which can soften tablets, clump powders and degrade moisture-sensitive drugs. A bedroom drawer or kitchen cupboard away from the kettle is usually better.

Relative humidity FAQs

Relative humidity (RH) is the amount of water vapor in the air compared with the maximum the air could hold at that temperature, expressed as a percentage: RH = 100 × e ÷ eₛ(T), where e is the actual vapor pressure and eₛ the saturation vapor pressure.

RH = 100 × eₛ(Td) ÷ eₛ(T), with eₛ(T) = 6.1094 × exp(17.625T ÷ (243.04 + T)) hPa. At 25 °C with a 15 °C dew point, RH = 53.8%.

First find the actual vapor pressure: e = eₛ(Tw) − 0.00066 × (1 + 0.00115Tw) × (T − Tw) × P, with P in hPa. Then RH = 100 × e ÷ eₛ(T). A 25 °C dry bulb and 20 °C wet bulb at sea level gives about 63% RH.

The US EPA recommends keeping indoor relative humidity between 30% and 50%, and below 60% to limit mold and dust mites. Below about 30%, dry eyes, skin, static and irritated airways become common.

Cold outdoor air holds little moisture. When it is heated indoors its moisture content stays the same but its capacity rises sharply, so RH collapses: 0 °C air at 80% RH becomes about 20% RH when warmed to 21 °C.

Multiply the room volume by the difference in absolute humidity. Raising a 40 m³ room at 21 °C from 20% to 45% RH needs about 40 × (8.23 − 3.66) ≈ 183 g of water, before air leakage replaces it.

Yes. If the moisture stays the same, RH falls as air warms and rises as it cools, by roughly 6–7% of its value per °C. That is why RH often peaks near dawn and dips in the afternoon.

Most medicines say 'store in a dry place'. The US Pharmacopeia defines a dry place as one not exceeding 40% average relative humidity at 20 °C, which is why bathrooms are a poor place for a medicine cabinet.

Absolute humidity is the actual mass of water vapor per volume of air, in g/m³. Relative humidity compares that with saturation at the current temperature, so it depends on temperature as well as moisture.

Only briefly and slightly (supersaturation) in very clean air, as inside a developing cloud. In practice, once RH reaches 100% the extra vapor condenses into dew, fog or cloud droplets.

A pair of thermometers, one with a wet wick, spun in the air. Evaporation cools the wet bulb; the drier the air, the bigger the gap between the two readings, which this calculator converts to RH.

With the Magnus formula the conversion itself is accurate to a fraction of a percent RH in normal conditions. Results are only as good as your inputs: a 0.5 °C error in dew point shifts RH by about 3%.

Sources

Calculations use the Magnus formula and the standard psychrometer equation for a ventilated wet bulb. Humidifier water estimates ignore air leakage and absorption by furnishings, so real needs are higher.