This dew point calculator finds the dew point from air temperature and relative humidity using the Magnus formula. The dew point is the single best number for how muggy air actually feels — and for predicting condensation, fog and frost.
What Is the Dew Point?
The dew point is the temperature at which air becomes saturated and water vapor starts condensing into liquid — dew on grass, fog, or droplets on a cold glass. Unlike relative humidity, which swings wildly with temperature through the day, the dew point changes only when the actual moisture content of the air changes. That is why forecasters call it the true measure of mugginess. This calculator is part of our Physics Calculators collection.
Worked Example (Verified)
Air at 25 °C (77 °F) with 60% relative humidity: dew point = 16.7 °C (62 °F) — right at the "noticeably humid" threshold. Cool that air (or any surface in it) below 16.7 °C and condensation forms: this is exactly why bathroom mirrors fog and why cold water pipes "sweat" in summer.
How Dew Point Feels (Comfort Scale)
| Dew point | How it feels |
|---|---|
| Below 10 °C / 50 °F | Dry and comfortable |
| 10–13 °C / 50–55 °F | Comfortable |
| 13–16 °C / 55–60 °F | Pleasant, slightly humid |
| 16–18 °C / 60–65 °F | Humid, muggy edge |
| 18–21 °C / 65–70 °F | Very humid, uncomfortable |
| Above 21 °C / 70 °F | Oppressive, tropical |
Dew Point in Practice
- Weather: when the temperature falls to the dew point overnight, expect dew or fog; below freezing, frost instead (the frost point).
- Aviation: pilots watch the temperature–dew point spread; a spread under ~2 °C warns of fog at the airfield.
- Buildings: condensation inside walls happens where surfaces drop below the indoor dew point — the physics behind mold problems and why vapor barriers matter.
- Running: a high dew point caps performance because sweat stops evaporating efficiently — many runners adjust pace by the temperature + dew point sum.
Moist air is also slightly less dense than dry air at the same temperature — a small but real effect in the world of our Density Calculator.
Related Tools
- Density Calculator — mass, volume, density
- Physics Calculators — the full collection
Frequently Asked Questions
What is a good dew point?
For comfort, below about 13 °C (55 °F). For weather-watching, "good" depends on the goal — farmers welcome dew in a drought; pilots do not welcome the fog it signals.
Can the dew point be higher than the temperature?
No. The dew point is always at or below the air temperature; when they meet, humidity is 100% and condensation begins.
What is the difference between dew point and relative humidity?
Relative humidity is a percentage of the maximum moisture air can hold at its current temperature, so it changes as temperature changes. Dew point is an absolute moisture measure in degrees — stable until the air mass itself changes.
What formula does this calculator use?
The Magnus approximation (a = 17.27, b = 237.7 °C), accurate to about ±0.4 °C over typical weather ranges.
What does a dew point of 70 °F mean?
Very humid, tropical-feeling air (21 °C) — uncomfortable for most people and a sign of a moisture-laden air mass.
Is this dew point calculator free?
Yes — free, instant, no sign-up.