Dew Point Calculator

The dew point is the temperature at which air becomes saturated with water vapor, so moisture starts condensing out as dew, fog, or frost. It is a more reliable indicator of human discomfort than relative humidity alone.

This page answers the outdoor question: how the air feels, how close it is to fogging, and how high the cloud base would sit if that air lifted. If what you actually want to know is whether a particular surface will sweat, a window pane, a cold-water pipe, the inside face of an outside wall, use the dew point and condensation calculator instead. It takes the surface temperature as a third input and tells you what humidity you would have to hold to keep it dry. Both pages return the same dew point to a tenth of a degree and band comfort on the same scale, so nothing is lost by moving between them.

Magnus formula: Dew Point (°C) = (243.5 × γ) ÷ (17.67 − γ), where γ = ln(RH/100) + (17.67 × T)/(243.5 + T)

Where:

• T = air temperature in °C
• RH = relative humidity in percent
• ln = natural logarithm

The two constants come in several published pairs. This page uses 17.67 and 243.5, which is the pair the Bolton 1980 fit uses and the one the humidity and cloud base calculators on this site also use, so the three agree to the decimal place. You will also see 17.625 and 243.04 quoted, which is the Alduchov-Eskridge fit and what the condensation page uses. The two differ by about seven thousandths of a degree at room temperature, which is smaller than any thermometer you own.

Simplified approximation (accurate within 1°C for RH > 50%): Dew Point ≈ T − ((100 − RH) / 5)

Worked example: Temperature: 28°C | Relative Humidity: 75% Dew Point ≈ 28 − ((100 − 75) / 5) = 28 − 5 = 23°C

Human comfort scale by dew point. These are the bands the calculator reports against, so whatever you type in will be described by whichever row it lands in.

The boundaries are round numbers in Fahrenheit rather than Celsius, which is why they look untidy in metric. That is not an accident of this page: the 60/65/70/75°F scale is the one American forecasters have used for decades, and it is what the bands were built on.

Dew point vs. relative humidity: RH of 80% on a cold 5°C day feels dry and crisp. RH of 60% on a hot 35°C day feels muggy and uncomfortable. Dew point above 21°C (70°F) is uncomfortable regardless of temperature, which is why meteorologists prefer it as a humidity metric.

The dew point spread

The gap between air temperature and dew point is called the spread, and the calculator reports it alongside the dew point because it is the more actionable of the two numbers. These are the bands it uses:

The 4°F line is the one pilots watch, and it is the same threshold the cloud base and fog visibility calculators on this site use. It is not arbitrary: air cools about 5.4°F for every 1,000 ft it rises while its dew point falls only 1°F, so a 4°F spread puts the condensation level roughly 900 ft up. Take the spread to zero and the cloud is standing on the ground, which is all fog is.