Relative Humidity Calculator
Find relative humidity from the air temperature and the dew point. Enter both in Fahrenheit or Celsius and get the RH percentage, a plain-language comfort read, and the vapor-pressure deficit that drives evaporation.
Example: with Air temperature 77 · Dew point 59 · Temperature unit Fahrenheit (°F) → Relative humidity: 53.8%.
- Comfort readComfortable, on the dry side
- Vapor-pressure deficit1.46 kPa
Computed by the calculator below using its default values. Change any input to see your own numbers.
Relative humidity is the ratio of the water vapor now in the air to the most it could hold at this temperature. The dew point fixes the numerator, the air temperature the denominator.
How dew point sets relative humidity
Warm air can hold more water vapor than cold air, and the dew point is the temperature at which the air would be fully saturated with the vapor it already carries. Relative humidity is just how close you are to that ceiling: the saturation vapor pressure at the dew point divided by the saturation vapor pressure at the current air temperature, times 100. When the dew point equals the air temperature, the air is saturated and RH is 100%.
This tool uses the Magnus approximation, es = 6.112·exp(17.625·T / (243.04 + T)), the same form the National Weather Service relies on. Because the two temperatures enter through the same curve, RH falls fast as the air warms above the dew point — which is why an afternoon can feel dry even though the actual moisture content has not changed since morning.
How it’s calculated
RH = 100 · es(Td) / es(T), with the August-Roche-Magnus saturation vapor pressure es(T) = 6.112 · exp(17.625·T / (243.04 + T)) in hPa, T and Td in °C. Fahrenheit is converted with (°F − 32)·5/9. Vapor-pressure deficit is (es(T) − es(Td)) reported in kPa.
The Magnus coefficients (17.625, 243.04 °C) are accurate to a few tenths of a percent from about −40 °C to 50 °C; outside that band the fit drifts. Assumes the dew point is measured at the same pressure as the air.
Relative humidity at 77 °F (25 °C) by dew point
| Dew point | Relative humidity | How it feels |
|---|---|---|
| 45 °F (7 °C) | 31% | Dry |
| 55 °F (13 °C) | 47% | Comfortable |
| 59 °F (15 °C) | 54% | Comfortable |
| 65 °F (18 °C) | 66% | Sticky |
| 70 °F (21 °C) | 78% | Humid |
| 75 °F (24 °C) | 92% | Oppressive |
Computed with es = 6.112·exp(17.625·T/(243.04+T)) at a fixed 77 °F air temperature; rounded.
Common mistakes
- Swapping the inputs — the dew point must be the lower number; if it exceeds the air temperature the air would be supersaturated.
- Mixing units: entering the air temperature in °F and the dew point in °C throws the ratio off badly.
- Confusing dew point with wet-bulb temperature — they are close only when the air is already near saturation.
- Reading a dropping RH through the day as drying air; usually only the temperature rose while the dew point held steady.
Frequently asked questions
What is the relative humidity formula?
RH = 100 × es(dew point) / es(air temperature), where es is the saturation vapor pressure from the Magnus formula es(T) = 6.112·exp(17.625·T/(243.04+T)). Both temperatures go in as °C.
Can relative humidity be over 100 percent?
Briefly, in supersaturated air such as fog or a cloud, but at ground level it is capped at 100% when the dew point equals the air temperature. This calculator clamps the result at 100%.
Why does humidity drop when the day warms up?
The amount of water vapor often stays constant, but warmer air can hold more, so the same moisture fills a smaller share of the ceiling. The dew point barely moves while RH falls.
Is dew point or relative humidity a better comfort gauge?
Dew point is more direct: above about 65 °F it feels sticky regardless of RH, because it reflects the actual moisture in the air rather than a ratio that shifts with temperature.