Relative Humidity from Dew Point

Understanding the relative humidity from dew point calculator — concept and principle

Weather reports, aviation observations and many data loggers give the dew point rather than relative humidity. This calculator converts air temperature and dew point into relative humidity (RH), so you can judge comfort, drying conditions, static risk or the humidity a museum or warehouse is actually experiencing.

The dew point tells how much vapour is there; the air temperature tells how much could be: 53.85%.

How it is calculated

RH = e_s(T_d) / e_s(T) × 100%, where e_s is the saturation vapour pressure from the Magnus formula:

e_s(T) = 6.112 · exp(17.62·T / (243.12 + T)) hPa.

T: air (dry-bulb) temperature in °C.
T_d: dew point in °C, which must not exceed T.

The constant 6.112 cancels in the ratio, so the calculator only needs the two exponential terms.

The principle

The dew point fixes the amount of water vapour actually present: at the dew point that vapour would just saturate the air, so the actual vapour pressure equals e_s(T_d). Relative humidity is the actual vapour pressure divided by the maximum the air could hold at its current temperature, e_s(T). Because saturation pressure grows roughly exponentially with temperature, a fixed gap between temperature and dew point gives a lower RH in warm air than in cold air.

Worked example

Inputs: T = 25 °C, T_d = 15 °C.
e_s(15) = 6.112 · exp(17.62 × 15 / 258.12) = 6.112 · e^1.02394 ≈ 17.017 hPa.
e_s(25) = 6.112 · exp(17.62 × 25 / 268.12) = 6.112 · e^1.64292 ≈ 31.601 hPa.
RH = 17.017 / 31.601 × 100 ≈ 53.85%.

Each point is the calculator re-run with only “Air temperature (°C)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Air temperature (°C)	Relative humidity
6.25	179.02%
12.5	117.62%
18.8	78.85%
25	53.85%
31.2	37.42%
37.5	26.43%
50	13.78%

How to read the result

Where does your result fall?

Indoors, about 40–60% RH is a common comfort target; persistently above about 60–70% raises the risk of mould and dust mites, while below about 30% can feel dry and increase static. As a rule of thumb at room temperature, each 1 °C of spread between temperature and dew point lowers RH by roughly 5–6 percentage points near saturation, though the relation is not linear. If T equals T_d, RH is 100%.

Common mistakes

Swapping the two temperatures, which gives RH above 100%.
Using dew points below 0 °C that are actually frost points, measured over ice.
Expecting RH to describe absolute moisture; use the dew point for that.
Mixing °F and °C.

Keep in mind: The result is only as accurate as the measured dew point and temperature, and the Magnus constants used here are for saturation over liquid water. Wet-bulb readings must first be converted to dew point using a psychrometric method; this calculator takes the dew point directly.

Frequently asked questions

Can I enter a wet-bulb temperature instead of the dew point?

Not directly. The wet-bulb temperature lies between the dew point and the air temperature, so using it as the dew point overestimates RH.

Why does the same dew point feel worse in summer?

A high dew point means sweat evaporates poorly, so the body struggles to shed heat even when RH looks moderate.

Is the result affected by altitude?

RH from this ratio is essentially independent of pressure; the humidity ratio, however, does depend on pressure.

Sources: WMO Guide to Instruments and Methods of Observation (WMO-No. 8), Annex on humidity formulae · ASHRAE Handbook — Fundamentals, Psychrometrics chapter

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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