Relative Humidity Calculator
At 25°C air temperature with a 15°C dew point, the relative humidity is about 53.8%. This relative humidity calculator turns the two temperatures a weather report gives you, the air temperature and the dew point, into the humidity percentage most people actually want to read. It also runs the reverse calculation, taking a known relative humidity and air temperature and returning the dew point in Celsius. Both directions use the Magnus approximation, a well established curve fit for the saturation vapor pressure of water that meteorologists rely on for quick estimates. Pick a mode, type in two numbers, and read the result. Whether you are checking whether the air outside feels muggy, sizing a dehumidifier for a damp room, or trying to understand why a cold window fogs up, the relationship between temperature and dew point is the core of it, and this tool makes that relationship easy to see without any manual logarithms.
Quick answer
Relative humidity is the ratio of the water vapor currently in the air to the maximum the air could hold at that temperature, expressed as a percentage.
What this tells you
- •Relative humidity is the ratio of the water vapor currently in the air to the maximum the air could hold at that temperature, expressed as a percentage.
- •The dew point is the temperature the air would need to cool to for that same water vapor to reach saturation, meaning 100% relative humidity.
- •When the dew point equals the air temperature, the air is fully saturated and relative humidity is 100%.
- •The bigger the gap between air temperature and dew point, the lower the relative humidity, because warmer air can hold much more moisture.
- •This tool uses the Magnus approximation, e(T) = exp((17.625 * T) / (243.04 + T)), to model saturation vapor pressure in Celsius.
- •Switch modes to go either way, from dew point to humidity percent, or from humidity percent to dew point.
How to Use
- 11. Choose the calculation direction. Select 'Dew point to humidity' to find the relative humidity from known temperatures, or 'Humidity to dew point' to work backward from a humidity percentage.
- 22. Enter the air temperature in degrees Celsius. This is the temperature measured by a standard thermometer in the shade.
- 33. In dew-point mode, enter the dew point temperature in degrees Celsius. The dew point is always equal to or below the air temperature.
- 44. In humidity mode, enter the relative humidity as a percentage between 0 and 100.
- 55. Read the primary result and check the secondary values for the complementary temperature or humidity figure.
How It Works
Formula
RH = 100 * e(dewPoint) / e(airTemp), where e(T) = exp((17.625 * T) / (243.04 + T))Relative humidity is the ratio of the actual vapor pressure to the saturation vapor pressure at the air temperature, multiplied by 100 to give a percent. The Magnus approximation supplies both vapor pressures from temperature alone. Because the actual vapor pressure equals the saturation pressure at the dew point, the full expression reduces to e(dewPoint) divided by e(airTemp). To reverse the calculation and find the dew point from a known humidity, the tool solves the same relation for temperature. It computes gamma = ln(RH / 100) + (17.625 * T) / (243.04 + T), then returns dew point = 243.04 * gamma / (17.625 - gamma). The natural logarithm requires humidity to be greater than zero, which is why a zero or negative humidity input returns no result.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Comfortable indoor air
With a room at 22°C and a dew point of 10°C, the twelve degree gap means the air holds under half of the moisture it could at that temperature. The result, roughly 46.9%, sits in the comfortable indoor band that many people find neither dry nor muggy.
Fully saturated air on a foggy morning
When the dew point matches the air temperature, the air cannot hold any more water vapor, so relative humidity reads 100%. This is the condition under which fog, dew, and mist form, because further cooling would force water to condense out.
Muggy summer afternoon
A dew point of 24°C is high enough to feel oppressive. Paired with a 30°C air temperature, the relative humidity comes out near 69.4%, the kind of sticky air that makes sweat evaporate slowly and heat feel worse than the thermometer suggests.
Finding the dew point from a forecast humidity
Given a 25°C air temperature and a reported 50% humidity, the reverse calculation solves the Magnus relation for the dew point, returning about 13.9°C. That is the temperature a surface would need to drop to for condensation to begin.
Air Temperature, Dew Point, and Relative Humidity
Sample combinations showing how the gap between air temperature and dew point maps to relative humidity.
| Air Temp (°C) | Dew Point (°C) | Relative Humidity | Feel |
|---|---|---|---|
| 20 | 20 | 100% | Saturated, fog likely |
| 20 | 15 | 72.8% | Humid |
| 20 | 10 | 52.5% | Comfortable |
| 20 | 5 | 37.3% | Dry side |
| 30 | 24 | 69.4% | Muggy |
| 30 | 10 | 28.9% | Arid |
Values are computed with the Magnus approximation and rounded for display. Comfort labels are general guidance, not fixed thresholds.
Why dew point tells you more than humidity alone
Relative humidity is the number most weather apps show, but it can mislead you because it depends on temperature. A relative humidity of 60% on a cold winter morning and 60% on a hot summer afternoon describe very different amounts of actual moisture in the air. As temperature rises, the air can hold far more water vapor, so the same percentage represents a much larger absolute quantity of water.
The dew point sidesteps that problem. It is a direct measure of how much water vapor is in the air, independent of the current temperature. A dew point of 20°C feels muggy whether the air temperature is 24°C or 34°C, because the moisture content is the same. That is why meteorologists and pilots often watch the dew point rather than the humidity percentage when they want to judge comfort, fog risk, or the chance of condensation.
This calculator lets you move freely between the two views. Start from the air temperature and dew point that most forecasts publish to get the familiar humidity percentage, or start from a humidity reading and your room temperature to find the dew point, which tells you the surface temperature at which condensation will begin to form on windows, walls, or cold drinks.
Common mistakes
- Confusing dew point with relative humidity. The dew point is a temperature in degrees, while relative humidity is a percentage that changes as the air temperature changes even when the moisture content stays fixed.
- Entering the dew point higher than the air temperature. That is physically unusual, and this tool clamps the result to 100% rather than reporting an impossible value above saturation.
- Assuming 50% humidity always feels the same. It feels dry in a cool room and sticky in a hot one, because the underlying water vapor differs a lot with temperature.
- Mixing temperature units. This calculator works entirely in Celsius, so convert any Fahrenheit readings before entering them or the result will be wrong.
- Reading a computed dew point as an exact measurement. The Magnus approximation is a curve fit, so results are close estimates rather than laboratory-grade values.
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