Quick answer
The dew point is the temperature at which air becomes saturated. Using the Magnus formula, γ = ln(RH/100) + aT/(b + T) and Td = bγ/(a − γ), with a = 17.62 and b = 243.12 °C. Air at 25 °C and 60% humidity has a dew point of 16.7 °C.
Enter the air temperature and the relative humidity. The calculator returns the dew point.
Related tools: Heat Index Calculator: Apparent temperature from air temperature and relative humidity (°F). Wind Chill Calculator: Wind chill temperature (°F, mph).
How to use the Dew Point Calculator
- Enter air temperature (°C) (between -45 and 60).
- Enter relative humidity (%) (between 0 and 100).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Air temperature | A number, in °C | ≥ -45, ≤ 60 |
| Relative humidity | A number, in % | > 0, ≤ 100 |
Formula and method
Formula
γ = ln(RH/100) + aT/(b + T); Td = bγ/(a − γ), a = 17.62, b = 243.12 °C
IEEE-754 double precision; shown to at most 6 decimal places.
Magnus–Tetens approximation with Sonntag (1990) constants; accurate to about ±0.4 °C between −45 and 60 °C.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Air temperature = 25; Relative humidity = 60.
| Result | Value |
|---|---|
| Dew point | 16.7 °C |
Example 2
Inputs: Air temperature = 20; Relative humidity = 100.
| Result | Value |
|---|---|
| Dew point | 20 °C |
The first line is the headline result (dew point). IEEE-754 double precision; shown to at most 6 decimal places.
What it tells you
When air cools to its dew point, water vapour condenses as dew, fog or condensation on a cold glass. A dew point near the air temperature means high humidity. As a rough comfort guide, dew points below about 13 °C (55 °F) feel dry and above about 21 °C (70 °F) feel oppressive.
Accuracy
The Magnus–Tetens approximation with Sonntag constants is accurate to about ±0.4 °C between −45 and 60 °C. Relative humidity must be above 0 and at most 100%; at 100% the dew point equals the air temperature.
How the result changes with air temperature
Holding every other input at the first worked example, this table changes only air temperature. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Air temperature | Dew point |
|---|---|
| 12.5 | 4.9 °C |
| 25 | 16.7 °C |
| 50 | 40.1 °C |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Relative humidity: entering "0" shows "Relative humidity must be greater than 0." beside that field and no result.
Choosing between the Dew Point Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Heat Index Calculator](/science/heat-index/) | air temperature and relative humidity | Heat index |
| [Wind Chill Calculator](/science/wind-chill/) | air temperature and wind speed | Wind chill |
| [Basis Point Calculator](/finance/basis-point/) | value and convert | Percent |
| [D&D Point Buy Calculator](/other/point-buy/) | sTR score, dEX score and cON score | Points spent |
| [Point Estimate Calculator](/statistics/point-estimate/) | successes and trials | Sample proportion |
Background: environment and acoustics
Decibels are a logarithmic scale, so sound levels cannot be added arithmetically: two equal sources raise the level by about 3 dB, not double it. Environmental and emissions results depend on the factors behind them (for example the US EPA's equivalency factors), which are averages for a country, fuel and year.
Use the result to compare scenarios rather than as a measured value.
Assumptions and limitations
- RH above 0 and up to 100%
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for relative humidity to confirm this.
Frequently asked questions
Is dew point the same as humidity?
No: relative humidity depends on temperature, the dew point does not.
What dew point causes mold?
Sustained indoor humidity above about 60% promotes mold.
Does it work in Fahrenheit?
No. Enter the air temperature in °C (from −45 to 60); convert from °F with (°F − 32) × 5 ÷ 9.
What does the Dew Point Calculator calculate?
Dew point from temperature and relative humidity. It returns Dew point.
What formula does the Dew Point Calculator use?
γ = ln(RH/100) + aT/(b + T); Td = bγ/(a − γ), a = 17.62, b = 243.12 °C. Magnus–Tetens approximation with Sonntag (1990) constants; accurate to about ±0.4 °C between −45 and 60 °C.
What do I need to enter in the dew Point Calculator?
You enter Air temperature (°C) and Relative humidity (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. RH above 0 and up to 100%.
What is an example of a dew Point result?
With air temperature 25 and relative humidity 60, the dew point is 16.7 °C.
Where does the method come from?
The method follows: Sonntag D, Zeitschrift für Meteorologie 1990;40:340–344.
Related calculators
- Heat Index Calculator — Apparent temperature from air temperature and relative humidity (°F).
- Wind Chill Calculator — Wind chill temperature (°F, mph).
- Point Estimate Calculator — Point estimates of a proportion (MLE, Laplace and Wilson).
- Point-Slope Form Calculator — Point-slope and slope-intercept equations of a line.
Sources
- Sonntag D, Zeitschrift für Meteorologie 1990;40:340–344
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.