How this dew point calculator works
The dew point is the temperature at which air can no longer hold all of its water vapour, so the vapour begins to condense into dew, fog or droplets. Because it reflects the actual amount of moisture in the air rather than a percentage that swings with temperature, the dew point is the most reliable single number for judging how humid and comfortable conditions are.
This calculator uses the well-known Magnus formula. With the air temperature T in °C and the relative humidity RH as a percentage, it first computes an intermediate term γ = ln(RH/100) + (a·T)/(b+T) using the constants a = 17.625 and b = 243.04. The dew point is then (b·γ)/(a−γ). If you enter the temperature in °F it is converted to °C before the calculation, and the result is converted back so you see both units.
Dew point comfort ranges
| Dew point | Dew point (°F) | How it feels |
|---|---|---|
| Below 10°C | Below 50°F | Dry and very comfortable |
| 10–16°C | 50–60°F | Comfortable |
| 16–18°C | 60–65°F | A bit humid |
| 18–21°C | 65–70°F | Humid and sticky |
| Above 21°C | Above 70°F | Oppressive |
Reference note: the Magnus coefficients used here are valid for typical surface conditions (roughly −40°C to 50°C). A simple heat index is shown when the air is warm (about 27°C/80°F and up); below that it is not meaningful, so it is left blank.
Frequently asked questions
- What is the dew point?
- The dew point is the temperature to which air must be cooled for it to become saturated and for dew to start forming. It is an absolute measure of moisture, so unlike relative humidity it does not change just because the temperature changes.
- How is dew point calculated?
- This tool uses the Magnus formula: γ = ln(RH/100) + (a·T)/(b+T) with a = 17.625 and b = 243.04 and T in °C, then dew point = (b·γ)/(a−γ). Fahrenheit values are converted before and after.
- What dew point feels humid or comfortable?
- Below about 10°C (50°F) feels dry, 10–16°C (50–60°F) is comfortable, 16–18°C (60–65°F) is a bit humid, 18–21°C (65–70°F) is noticeably humid, and above 21°C (70°F) feels oppressive.
- What is the difference between dew point and humidity?
- Relative humidity is a percentage that rises and falls with temperature, while dew point is the actual moisture content expressed as a temperature — a more reliable guide to how muggy the air feels.
- Why does dew point matter?
- It predicts condensation, fog and comfort. When a surface is colder than the dew point, water condenses on it, fogging windows and sweating pipes, and a high dew point makes warm days feel far more uncomfortable.
- What is a comfortable indoor humidity?
- Roughly 30–50% relative humidity is comfortable and healthy indoors. Drier air irritates skin and airways; wetter air risks condensation, mould and dust mites, where a humidifier or dehumidifier helps.