Dew Point Calculator formula

This calculator uses the Magnus approximation, a widely used relationship for ordinary weather conditions. Temperature is converted to Celsius before calculation.

Here, T is air temperature in °C and RH is relative humidity as a percentage. Results are estimates; sensor accuracy and local conditions affect real observations.

Dew point comfort guide

Dew point describes the actual amount of moisture in the air more consistently than relative humidity alone. A dew point below about 50°F (10°C) often feels dry, 50–60°F (10–16°C) is generally comfortable, 61–65°F (16–18°C) can feel humid, 66–70°F (19–21°C) feels muggy, and values above 70°F (21°C) often feel oppressive.

When surfaces cool below the dew point, water vapor can condense on them. If the dew point is below freezing and surfaces are cold enough, frost may form. For official observations and forecasts, use your national weather service; NOAA explains why dew point is useful for judging moisture at weather.gov.

Worked example

At 75°F with 60% relative humidity, the estimated dew point is about 60°F. The roughly 15°F spread indicates moderately moist air that is not yet saturated.

How the Dew Point Calculator helps

The Dew Point Calculator is useful when planning outdoor work, exercise, travel, ventilation or moisture control. Unlike relative humidity, which rises and falls as temperature changes, dew point gives a steadier picture of how much water vapor is actually present. Comparing both values helps explain why the same humidity percentage may feel comfortable on a cool day but unpleasant during hot weather.

For indoor spaces, compare the calculated dew point with the temperature of windows, pipes, walls and air-conditioning surfaces. Condensation becomes possible when a surface is colder than the dew point. Persistent moisture can damage finishes and support mold growth, so a calculator result should be followed by direct temperature and humidity measurements when building conditions matter.

Tips for more reliable results

Use a shaded, well-ventilated temperature reading rather than a thermometer heated by direct sunlight. Place a humidity sensor away from showers, cooking steam, open windows and supply vents unless that exact location is what you want to evaluate. Allow the sensor time to stabilize, and repeat the calculation if conditions are changing quickly.

This Dew Point Calculator rounds results to one decimal place. Small differences between this estimate, another calculator and a weather station can result from sensor precision, rounding or use of a slightly different Magnus formula constant. For important agricultural, industrial or building-envelope decisions, use calibrated instruments and professional guidance.

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Frequently asked questions

What is dew point?

Dew point is the temperature to which air must cool, at constant pressure and moisture content, to become saturated.

Is a higher dew point more humid?

Yes. A higher dew point means more water vapor is present and usually feels more humid to people.

Can dew point exceed air temperature?

Under normal atmospheric conditions, no. At 100% relative humidity, dew point is approximately equal to air temperature.

Is dew point the same as relative humidity?

No. Relative humidity changes when air temperature changes, while dew point is a more direct indicator of the air’s moisture content.

How accurate is this calculator?

The Magnus approximation is suitable for common weather planning. It is not a substitute for calibrated instruments or official meteorological observations.

Important: This calculator provides an educational estimate. Use official weather alerts and professional guidance for safety-critical decisions.