Dew Point Calculator

What is dew point?

The dew point is the temperature air must be cooled to, at constant pressure, to reach 100% relative humidity. At that point the air can't hold any more water as a gas, so vapor starts to condense into dew, fog, or cloud. The dew point temperature measures how much moisture is actually in the air: the higher it is, the more moisture there is, whatever the air temperature.

Relative humidity is different. It compares the moisture in the air with the most the air could hold at its current temperature, so it changes as the air warms and cools even when the moisture doesn't. That is why dew point is usually the better guide to how muggy it will feel.

How to calculate dew point

This dew point calculator also works as a relative humidity calculator: enter any two of air temperature, relative humidity, and dew point to find the third. It uses the Buck equation for the saturation vapor pressure of liquid water, with T in °C and pressure in pascals:

e_s(T) = 611.21 × exp((18.678 − T ÷ 234.5) × (T ÷ (257.14 + T)))

1. Find the saturation vapor pressure at the air temperature.
2. Multiply it by the relative humidity to get the actual vapor pressure.
3. Find the temperature whose saturation vapor pressure equals that value. That is the dew point.

For example, air at 75°F (23.9°C) has a saturation vapor pressure of about 2,965 Pa. At 55% relative humidity the vapor pressure is about 1,631 Pa, which is the saturation pressure at 57.8°F (14.3°C), so the dew point is 57.8°F.

Below freezing, the results still use the equation for liquid water. Dew points below 32°F are where frost forms instead of dew, and a frost point measured over ice would read slightly higher.

Dew point and humidity: how it feels

The National Weather Service uses these general summer comfort ranges:

Dew point chart

Dew points in °F for common air temperatures and relative humidity levels, calculated with the same equation and rounded to the nearest degree.

What the other results mean

• Water vapor pressure: the pressure from the water vapor actually in the air.
• Saturation vapor pressure: the most water vapor pressure the air could have at its temperature.
• Absolute humidity: grams of water vapor in a cubic meter of air.
• Moisture by volume and by weight: water vapor as a share of the air, in parts per million, assuming sea-level pressure.

Frequently asked questions

Can the dew point be higher than the air temperature?

Not in normal conditions. The dew point can equal the air temperature, which means 100% relative humidity, but the air can't stay cooler than its dew point without moisture condensing out. The calculator rejects a dew point above the air temperature.

What dew point feels comfortable in summer?

Comfort is personal, but a summer dew point of 55°F or lower generally feels dry and comfortable, while 65°F or higher starts to feel oppressive.

Why does the humidity rise at night when no moisture is added?

Cooler air can hold less water vapor, so the same moisture becomes a larger share of what the air can hold. The dew point stays about the same while relative humidity climbs toward 100%, which is why dew often forms overnight.

What temperatures can this calculator handle?

Air temperature and dew point from −80°C to 100°C (−112°F to 212°F), and relative humidity above 0% up to 100%.