What this calculator does
Relative humidity is the amount of water vapour actually in the air, expressed as a percentage of the maximum the air could hold at that temperature before it starts to condense. Warmer air can hold more moisture before reaching that limit, which is why the same absolute amount of water vapour can read as a high relative humidity on a cool morning and a low one by mid-afternoon.
This calculator works from two figures that are commonly reported together: the dry-bulb temperature, which is the ordinary air temperature a thermometer reads, and the dew point, the temperature at which the air would need to cool for that moisture to start condensing. The relative humidity formula used here is the Magnus approximation, a well-established empirical fit that is accurate enough for everyday and most technical purposes across ordinary outdoor temperature ranges.
The formula
Both the dry-bulb temperature (T) and the dew point (Td) are converted through the same exponential term, using the constants b = 17.625 and c = 243.04°C: RH = 100 × exp[(b·Td)/(c+Td) − (b·T)/(c+T)]. When the dew point equals the air temperature the two terms cancel and RH comes out at exactly 100%, since the air is already holding as much moisture as it can at that temperature.
| Term | Meaning |
|---|---|
| Dry-bulb temperature (T) | The ordinary air temperature, as read from a standard thermometer. |
| Dew point (Td) | The temperature the air would need to cool to for water vapour to start condensing, at a fixed moisture content. |
| Relative humidity | Current water vapour content as a percentage of the maximum the air could hold at the dry-bulb temperature. |
The inputs explained
| Field | What to enter |
|---|---|
| Dry-bulb air temperature (°C) | The current air temperature, as reported by a standard thermometer. |
| Dew point (°C) | The dew point for the same air, often reported alongside temperature in weather data. |
When to use it
Reading a weather report
Forecasts often list temperature and dew point separately rather than relative humidity directly, since dew point stays more stable through the day. Converting the two into relative humidity gives the figure most people actually think in.
Judging indoor comfort or mould risk
Relative humidity above roughly 60% indoors makes mould growth more likely and can encourage condensation on cold surfaces, so working it out from a temperature and dew point reading (or a hygrometer’s dew point mode) flags a problem before it becomes visible.
Checking whether fog or frost is likely
As the dew point depression, the gap between air temperature and dew point, narrows toward zero overnight, relative humidity climbs toward 100% and fog or ground frost becomes far more likely.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How relative humidity changes with dew point at a fixed air temperature
A fixed 25°C dry-bulb temperature, against a range of dew points.
How relative humidity changes with air temperature at a fixed dew point
A fixed 10°C dew point, against a range of dry-bulb temperatures.
Questions
Can dew point be higher than air temperature?
No. Dew point is always at or below the dry-bulb temperature, since it is the temperature the air would need to cool to for its current moisture to condense. If dew point briefly exceeds a temperature reading, one of the two measurements is wrong.
Why use dew point instead of a hygrometer reading directly?
Weather stations and forecasts often report dew point because it changes more slowly through the day than relative humidity, making it a steadier indicator of the actual moisture in the air. This calculator converts that steadier figure into the percentage most people are used to thinking in.
How accurate is the Magnus formula?
It is a well-established empirical approximation, accurate to within about 0.1 to 0.3% relative humidity across the ordinary range of outdoor temperatures. It is not intended for extreme conditions well outside typical weather, such as far below freezing or near-boiling temperatures.
What relative humidity is considered comfortable indoors?
Most guidance places comfortable indoor relative humidity somewhere between 30% and 50%. Levels much above that raise the risk of condensation and mould, and levels much below it can dry out skin, airways and timber fittings.
To work out dew point from temperature and relative humidity instead, see the dew point calculator. For the related psychrometric wet-bulb temperature, see the wet-bulb calculator.