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Guide

How to Calculate Dew Point From Temperature and Humidity

Dew point is the humidity number worth calculating, and the arithmetic is short. One formula covers indoor conditions to within about 0.3 °C; a table covers the common cases; and a rule of thumb gets within a degree in your head.

By the Akermin Editorial Team · 2 min read

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Dew point from temperature (T, °C) and relative humidity (RH, %) uses the Magnus formula: γ = ln(RH ÷ 100) + (17.62 × T) ÷ (243.12 + T), then dew point = (243.12 × γ) ÷ (17.62 − γ). For 22 °C and 55 percent RH: γ = ln(0.55) + (17.62 × 22) ÷ 265.12 = −0.598 + 1.462 = 0.864; dew point = (243.12 × 0.864) ÷ (17.62 − 0.864) = 210.1 ÷ 16.76 = 12.5 °C. For a quick estimate, dew point ≈ T − (100 − RH) ÷ 5, which gives 13 °C here. The humidity checker does the full calculation.

The Magnus formula

With T in °C and RH in percent:

  1. γ = ln(RH ÷ 100) + (b × T) ÷ (c + T)
  2. Dew point = (c × γ) ÷ (b − γ)

Constants b = 17.62 and c = 243.12 °C (Sonntag 1990) are accurate to about 0.3 °C across indoor and outdoor conditions. Older constants (b = 17.27, c = 237.7) appear in many references and give nearly identical results.

Worked example

Step Calculation Value
Inputs T = 22 °C, RH = 55%
ln(RH/100) ln(0.55) −0.598
b × T ÷ (c + T) 17.62 × 22 ÷ 265.12 1.462
γ −0.598 + 1.462 0.864
Dew point 243.12 × 0.864 ÷ (17.62 − 0.864) 12.5 °C

Rule of thumb

For RH above about 50 percent: dew point ≈ T − (100 − RH) ÷ 5. Each 5 points of RH below 100 lowers the dew point about 1 °C. At 20 °C and 70% RH: 20 − 6 = 14 °C (Magnus gives 14.4). At 24 °C and 40% RH: 24 − 12 = 12 °C (Magnus gives 9.6; the rule drifts at lower RH).

Lookup table (°C)

Temp RH 30% 40% 50% 60% 70% 80%
16 °C −1.5 2.4 5.6 8.2 10.5 12.5
18 °C 0.2 4.2 7.4 10.1 12.4 14.5
20 °C 1.9 6.0 9.3 12.0 14.4 16.4
22 °C 3.6 7.8 11.1 13.9 16.3 18.4
24 °C 5.4 9.6 12.9 15.8 18.2 20.3
26 °C 7.1 11.4 14.8 17.6 20.1 22.3

Fahrenheit

Convert °F to °C: (°F − 32) × 5 ÷ 9. Calculate. Convert back: °C × 9 ÷ 5 + 32. At 72 °F (22.2 °C) and 55% RH the dew point is 12.7 °C, or 54.9 °F.

Using the result

Any surface colder than the dew point will collect condensation; surfaces within 2 to 3 °C of it are at mold risk from high surface humidity. Measure a suspect surface with an infrared thermometer and compare. Because dew point ignores temperature, a rising dew point means water is being added to the air, which points to the source. See what dew point is and why condensation forms on windows. For measuring the inputs properly, see how to measure relative humidity correctly.

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Frequently Asked Questions

What is the simplest dew point formula?

Rule of thumb: dew point ≈ temperature − (100 − RH) ÷ 5, in °C. At 22 °C and 55% RH: 22 − 9 = 13 °C. Accurate to about a degree between 50 and 100% RH.

How accurate is the Magnus formula?

Within about 0.3 °C for temperatures from −40 to 50 °C and RH from 1 to 100 percent, which is far better than the hygrometer feeding it.

Can I calculate dew point in Fahrenheit?

Convert temperature to Celsius first, calculate, and convert the result back: °F = °C × 9/5 + 32.

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