MasterTool

Dew Point Calculator

Find the dew point from air temperature and relative humidity with the Magnus formula — plus vapour pressure, absolute humidity and a comfort reading.

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What this tool does

Find the dew point from air temperature and relative humidity with the Magnus formula — plus vapour pressure, absolute humidity and a comfort reading. Magnus coefficients a = 17.625 and b = 243.04 °C (valid roughly −40 °C to +50 °C over water). At 100% RH the dew point equals the air temperature. Comfort bands are the commonly published dew-point comfort scale.

How to use the Dew Point Calculator

  1. Enter or select air temperature (°c).
  2. Enter or select relative humidity (%).
  3. Read the calculated result; change any measurement to compare alternatives.

Formula

Magnus formula: α = ln(RH/100) + 17.625·T/(243.04 + T); dew point = 243.04·α / (17.625 − α), with T in °C (Alduchov & Eskridge coefficients).
temp
Air temperature (°C)
rh
Relative humidity (%)

Magnus coefficients a = 17.625 and b = 243.04 °C (valid roughly −40 °C to +50 °C over water). At 100% RH the dew point equals the air temperature. Comfort bands are the commonly published dew-point comfort scale.

Worked example

For dew point calculator, the following measurements illustrate the exact method: Air temperature (°C): 30; Relative humidity (%): 50.

Inputs

  • Air temperature (°C)30
  • Relative humidity (%)50

Result

  • Dew point (°C)18.45
  • Dew point (°F)65.2
  • Temperature spread (T − dew point) (°C)11.55
  • Actual vapour pressure (hPa)21.23
  • Absolute humidity (g/m³)15.17
  • Comfort levelHumid — muggy for many people

Results explained

Dew point (°C)
Dew point (°C) from the formula above. Magnus coefficients a = 17.625 and b = 243.04 °C (valid roughly −40 °C to +50 °C over water). At 100% RH the dew point equals the air temperature. Comfort bands are the commonly published dew-point comfort scale.
Dew point (°F)
Dew point (°F) from the formula above. Magnus coefficients a = 17.625 and b = 243.04 °C (valid roughly −40 °C to +50 °C over water). At 100% RH the dew point equals the air temperature. Comfort bands are the commonly published dew-point comfort scale.
Temperature spread (T − dew point) (°C)
Temperature spread (T − dew point) (°C) from the formula above. Magnus coefficients a = 17.625 and b = 243.04 °C (valid roughly −40 °C to +50 °C over water). At 100% RH the dew point equals the air temperature. Comfort bands are the commonly published dew-point comfort scale.
Actual vapour pressure (hPa)
Actual vapour pressure (hPa) from the formula above. Magnus coefficients a = 17.625 and b = 243.04 °C (valid roughly −40 °C to +50 °C over water). At 100% RH the dew point equals the air temperature. Comfort bands are the commonly published dew-point comfort scale.
Absolute humidity (g/m³)
Absolute humidity (g/m³) from the formula above. Magnus coefficients a = 17.625 and b = 243.04 °C (valid roughly −40 °C to +50 °C over water). At 100% RH the dew point equals the air temperature. Comfort bands are the commonly published dew-point comfort scale.
Comfort level
Comfort level from the formula above. Magnus coefficients a = 17.625 and b = 243.04 °C (valid roughly −40 °C to +50 °C over water). At 100% RH the dew point equals the air temperature. Comfort bands are the commonly published dew-point comfort scale.

Frequently asked questions

Magnus formula: α = ln(RH/100) + 17.625·T/(243.04 + T); dew point = 243.04·α / (17.625 − α), with T in °C (Alduchov & Eskridge coefficients).

Magnus coefficients a = 17.625 and b = 243.04 °C (valid roughly −40 °C to +50 °C over water). At 100% RH the dew point equals the air temperature. Comfort bands are the commonly published dew-point comfort scale.

Relative humidity changes with temperature even when the air's moisture content stays the same; the dew point is an absolute measure of moisture. A dew point of 18 °C feels muggy whether the air is 24 °C or 35 °C — at 35 °C the RH would look deceptively low.

When a surface (or the air itself) cools to the dew point, water vapour condenses. If the overnight low is forecast to reach the evening dew point, expect dew; if the whole air mass cools to it, expect fog.

Every field is labelled with its unit — enter values in exactly the labelled unit and read the result in the labelled output unit. Fields with a unit symbol also support the site-wide imperial/metric switch, which converts before the formula runs.

It uses double-precision arithmetic and the published constants and formulas named on this page. The last displayed digit may be rounded, and real conditions can differ from the idealized model described in the assumptions.

Yes. Every calculation runs entirely in your browser; nothing you enter is uploaded, stored or shared.