Calculator

Relative humidity calculator

Air temperature 75°F

Dew point 60°F

59.6% Relative humidity · Humid
Muggy. Sustained readings above 60% let mould and dust mites take hold.

Dew point: 60°F
Water in the air: 12.94 g/m³

The figures above are the answer for the values this page loaded with. Changing the controls needs JavaScript — without it the calculator cannot recalculate, so read them as one worked example rather than a live result.

How do you calculate relative humidity?

Divide the actual vapour pressure of the air by the saturation vapour pressure at that temperature, then multiply by 100. What changes between methods is only how you get the actual vapour pressure — from a dew point, from a density in g/m³, or from a mixing ratio. All three routes are above and all three give the same answer.

One definition, three doors

Relative humidity is a ratio of pressures: how much water vapour pressure the air actually has, against how much it could have at that temperature before saturating.

RH = 100 × e / e_s(T) e is actual vapour pressure, e_s(T) the saturation value at the air temperature.

Everything else is bookkeeping. Whichever figure you start with, the job is to turn it into e:

From a dew point — the dew point is the temperature at which today's vapour pressure would be saturation, so e = eₛ(dew point). This is the most common case, because forecasts publish dew points.
From g/m³ — invert the ideal gas law: e = ρᵥ × Rᵥ × T, with Rᵥ = 461.5 J/(kg·K).
From g/kg — e = w × p / (621.97 + w), where p is the station pressure. This is the only route that needs a pressure.

Worked example, the dew point route

Air at 75°F (23.9°C) with a dew point of 60°F (15.6°C).

eₛ at the dew point, 15.6°C: 6.112 × exp(17.62 × 15.6 / (243.12 + 15.6)) = 17.7 hPa. That is the actual vapour pressure.
eₛ at the air temperature, 23.9°C: 29.6 hPa.
RH = 100 × 17.7 / 29.6 = 60%.

Which is the dew point calculator run backwards — feed 75°F and 60% into that one and it returns a 60°F dew point. The two agree because they are the same code.

Why the answer moves when nothing was added

The single most confusing thing about relative humidity is that it changes all day while the water in the air does not. Only the denominator moved: eₛ(T) roughly doubles for every 10°C of warming, so warming the same air halves its relative humidity.

Fix the dew point at 60°F and watch the percentage swing:

The same air — the same water, the same dew point — warmed through one ordinary day.
Air temperature	Relative humidity	Mixing ratio	Density
60°F / 15.6°C	100%	11.02 g/kg	13.2 g/m³
70°F / 21.1°C	71%	11.02 g/kg	13.0 g/m³
80°F / 26.7°C	51%	11.02 g/kg	12.7 g/m³
90°F / 32.2°C	37%	11.02 g/kg	12.5 g/m³

Four wildly different percentages, and not one molecule of water added or removed — the mixing ratio is identical in every row. This is the whole argument for reading the dew point instead, and it is why humidity is higher at night without anything having been added to the air.

Note that the density column does drift slightly, from 13.2 down to 12.5 g/m³. That is not water leaving: it is the air expanding as it warms, so the same water is spread through more cubic metres. Grams per cubic metre is a density and inherits that; grams per kilogram of dry air does not, which is precisely why forecasters reach for it.

What the percentage is good for

None of this makes relative humidity useless. It is the right number whenever the question is about a rate rather than a quantity: how fast something evaporates, whether mould can establish on a surface, how much static builds up. Those all depend on how close the air is to full, which is exactly what the percentage measures.

It is the wrong number when you want to compare two airs, or one air on two days. That needs grams per cubic metre.

Common questions

What is the formula for relative humidity?

RH = 100 × e / eₛ(T), where e is the actual vapour pressure and eₛ(T) is the saturation vapour pressure at the air temperature. Using the Magnus approximation, eₛ(T) = 6.112 × exp(17.62T / (243.12 + T)) in hectopascals with T in Celsius.

How do I get relative humidity from temperature and dew point?

Take the saturation vapour pressure at the dew point and divide it by the saturation vapour pressure at the air temperature. The dew point is by definition the temperature at which the current vapour pressure would be saturation, so eₛ(dew point) is the actual vapour pressure.

Why do I need the temperature if I already have the humidity in g/m³?

Because relative humidity is relative to the saturation point, and that is set by temperature. 12 g/m³ is about 70% at 20°C and about 40% at 30°C — the same water, two very different percentages. Without a temperature the question has no answer.

Can relative humidity be over 100%?

In everyday conditions, no. Supersaturation exists in clean air in laboratories and briefly in rising cloud parcels, but wherever there are dust and salt particles to condense on — which is everywhere near the ground — the surplus becomes droplets almost immediately. This calculator caps at 100% and flags the input that caused it.

Is relative humidity the same as the humidity in a weather forecast?

Usually yes — a bare "humidity" percentage means relative humidity. It is worth knowing that a forecast percentage refers to outdoor air at a station or model grid point, which is why it can differ from a reading taken indoors or a few streets away.

Sources

Published 31 July 2026