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Dew Point Calculator

Find the dew point from temperature and humidity, or the humidity from the dew point.

By Nitin Raturi · Last reviewed

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How to use the Dew Point Calculator

Convert between relative humidity and dew point for a given air temperature.

  1. Choose what to find: the dew point or the relative humidity.
  2. Choose °C or °F and enter the air temperature.
  3. Enter the relative humidity, or the dew point.
  4. Read the answer, the vapour pressure and the gap between air and dew point.

Good to know

  • Use readings taken at the same time and place.
  • A small gap between air temperature and dew point means fog or dew is likely.
Find

Find

Whether to work out the dew point or the relative humidity.

How to fill it in

  • Choose Dew point if you know the humidity; choose Humidity if you know the dew point.
Temperature unit

Temperature unit

Celsius or Fahrenheit, for both the air temperature and the dew point.

How to fill it in

  • Switching converts the values you entered.

Air temperature

The dry-bulb air temperature, as a thermometer or weather report gives it.

How to fill it in

  • Enter −50 to 60 °C (−58 to 140 °F).

Relative humidity

How much water vapour the air holds, as a percentage of the most it could hold at that temperature.

How to fill it in

  • Enter 1% to 100%.

Example

At 20 °C, 50% humidity gives a dew point of 9.3 °C; at 40 °C, 20% gives 12.8 °C.

Dew point

23.9 °C

Air at 30 °C and 70% relative humidity would form dew if cooled to 23.9 °C, 6.1 °C below the air.

NWS (Tetens) vapour pressure over water. Vapour pressure 29.71 hPa of a possible 42.44 hPa.

ChangelogHow we test this calculatorEmbed this calculator

How this calculator works

  1. Step 1SaturationesThe most vapour the air could hold at its temperature.
  2. Step 2Actual vapoure = es × rh ÷ 100Relative humidity scales it down.
  3. Step 3Dew pointes(Td) = eThe temperature at which e would saturate.

Warm air can hold more water vapour than cold air. The dew point is the temperature at which the vapour already in the air would be enough to saturate it.

The calculator uses the vapour pressure formula from the US National Weather Service weather calculator sheets.

Worked example: 30 °C at 70% humidity

Dew point on a humid afternoon
Saturation
es = 6.11 × 10^(7.5T ÷ (237.3 + T))
Dew point
Td = 237.3L ÷ (7.5 ln 10 − L), L = ln(e ÷ 6.11)
  • TAir temperature 30 °C
  • rhRelative humidity 70%
  1. Saturationes at 30 °C42.44 hPa
  2. Actual vapoures × 70 ÷ 10029.71 hPa
  3. Dew pointsolve for Td23.9 °C

Cool this air by 6.1 °C and dew forms. Going the other way, 30 °C with a 24 °C dew point gives 70.3% humidity.

Same humidity, different dew points

  • 20 °C, 50%9.3 °C
  • 40 °C, 20%12.8 °C
  • 30 °C, 70%23.9 °C
Dew point for three kinds of air

The hot desert afternoon has only 20% humidity, yet more vapour in the air than the comfortable room at 50%.

Read the result

Relative humidity changes as the air warms or cools; the dew point stays the same until vapour is added or removed.

That is why forecasters often quote the dew point: it measures how much moisture the air holds. At 25 °C, a 10 °C dew point means 38.8% humidity.

Assumptions and limits

  • Over waterVapour pressure over liquid water, also used below 0 °C.
  • ApproximationThe NWS formula is a fitted curve, close but not exact.
  • PressurePressure does not enter this formula.
  • Frost pointBelow freezing, frost forms slightly above the dew point shown.

Dew point measures moisture. How the air feels also depends on heat and wind.

For how hot it feels, use the heat index calculator. In winter, the wind chill calculator gives the felt temperature in wind.

Common mistakes

  • Mistake: Treating high humidity as lots of moisture.

    Instead: Compare dew points; humidity depends on the temperature.

  • Mistake: Entering a dew point above the air temperature.

    Instead: The dew point can at most equal the air temperature.

  • Mistake: Mixing readings from different times.

    Instead: Take temperature and humidity together.

Checking the method

Every golden value is re-worked by a separate script that finds the dew point by bisection in 40-digit decimal.

The script never uses the closed-form inverse, so a slip in that formula would show up as a mismatch.

Read how we test our calculators, or browse more maths calculators.

Frequently asked questions

What is the dew point?

The temperature air must cool to for dew to form. 30 °C air at 70% humidity has a dew point of 23.9 °C.

How do I find relative humidity from the dew point?

Divide the vapour pressure at the dew point by the one at the air temperature. 30 °C with a 24 °C dew point gives 70.3%.

Which formula does this use?

The NWS weather calculator formula: es = 6.11 × 10^(7.5T ÷ (237.3 + T)) hPa, solved for the dew point.

Can dry air have a high dew point?

Yes. 40 °C at only 20% humidity has a dew point of 12.8 °C, because hot air can hold a lot of vapour.

Can the dew point be higher than the temperature?

No. At 100% humidity they are equal; the calculator rejects a higher dew point.

Sources and methodology

The formulas are standard time-value-of-money results. We check every figure against golden values worked out independently, by closed form and by month-by-month simulation; see the methodology.

Written and maintained by Nitin Raturi, a software engineer (about the author). Last reviewed 4 Nov 2026; next review due by . Found an error? Tell us.

Changelog

  • : Initial release.

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