Dew Point Calculator
The temperature at which air becomes saturated and dew forms — the honest measure of how humid it feels.
Dew point is the temperature air must cool to before its water vapour starts condensing.
How the dew point calculator works
Dew point is the temperature air must cool to before its water vapour starts condensing. Unlike relative humidity it does not move when the temperature changes, which makes it the better comfort measure: 60% humidity at 10 °C and at 30 °C feel nothing alike, but their dew points differ by 18 degrees and tell you so.
The rule of thumb forecasters use: a dew point below 10 °C is dry, 16 °C is noticeable, 20 °C is uncomfortable and above 24 °C is oppressive — because at that point sweat evaporates too slowly to cool you.
This uses the Magnus–Tetens approximation with the Alduchov–Eskridge coefficients, accurate to about 0.1 °C between −40 °C and 50 °C.
Formula: γ = ln(RH/100) + bT/(c+T); Td = cγ/(b−γ), with b = 17.625, c = 243.04
Worked examples
| Inputs | Dew point | Note |
|---|---|---|
| 25 °C at 60% humidity | 16.7 °C | dew point 16.7 °C — becoming noticeable |
| Saturated air | 20.0 °C | dew point equals the air temperature |
| A dry winter day | -3.0 °C | dew point −2.6 °C |
FAQFrequently asked questions
What is the dew point?
The temperature air must cool to before water vapour condenses out of it as dew, fog or cloud.
Is dew point better than humidity?
For comfort, yes. Relative humidity depends on the temperature, so the same percentage feels completely different in winter and summer. Dew point does not move.
What dew point is uncomfortable?
Above about 18 °C feels sticky; above 24 °C is oppressive, because sweat can no longer evaporate fast enough to cool you.
Can the dew point be higher than the air temperature?
No. At 100% humidity they are equal, and that is the ceiling.
How does dew point predict fog?
When the air temperature falls to within a couple of degrees of the dew point, fog or dew becomes likely.
Where these figures come from
- UK DESNZ / DEFRA — Greenhouse gas reporting: conversion factors — the flight, fuel and waste emission factors used here
- Scarborough et al. (2014), Climatic Change — Dietary greenhouse gas emissions — the per-diet daily emission figures
- US EPA — Technical Assistance Document for the Reporting of Daily Air Quality (AQI) — the PM2.5 breakpoints the air quality calculator uses
- Australian Government — National Greenhouse Accounts Factors — the official Australian emission factors
Last checked: September 2026. Factors are taken from published government and peer-reviewed sources, each named on the page with its year.