Dew point calculator from temperature and relative humidity
Enter air temperature and relative humidity. HitEquals returns dew point in Celsius and Fahrenheit, the air-to-dew-point spread, saturation vapor pressure and actual vapor pressure.
Inputs used by this calculator
Dew Point Calculator uses the fields below. The starting values reproduce the page example; replace them with values from the same units and time period before using the result.
| Input | Starting example | Accepted value | How it is used |
|---|---|---|---|
| Temperature unit | Degrees Celsius | Degrees Celsius · Degrees Fahrenheit | Selects the unit or conversion path used by Dew Point Calculator. |
| Air temperature | 25 | -150 to 200 | Supplies the air temperature quantity used by the published equation or result breakdown. |
| Relative humidity (%) | 60 | 0.01 to 100 | Supplies the relative humidity (%) quantity used by the published equation or result breakdown. |
What dew point means
Dew point is the temperature to which air must cool, at roughly constant pressure and vapor content, to reach saturation.
Temperature and relative humidity
Relative humidity compares actual vapor pressure with saturation vapor pressure at the current temperature; saturation capacity changes with temperature.
Magnus-type equation
The engine uses the NWS constants 17.67 and 243.5°C, then algebraically inverts the relationship.
γ=ln(RH/100)+17.67T/(T+243.5)Td=243.5γ/(17.67−γ)Worked 25°C example
At 25°C and 60% RH, γ is about 1.11 and dew point is about 16.7°C, leaving an 8.3°C spread.
Dew point versus humidity
Dew point tracks actual moisture more directly, while relative humidity changes when temperature changes even if vapor content does not.
Condensation and model limits
A surface below local dew point can collect condensation, but surface properties, airflow and pressure matter. Weather sensors and forecasts carry uncertainty.
Dew-point interpretation examples
| Dew point | Typical moisture description |
|---|---|
| Below 10°C / 50°F | Dry |
| 10–16°C / 50–61°F | Moderate |
| 16–21°C / 61–70°F | Humid |
| Above 21°C / 70°F | Very humid |
Frequently asked questions
Can dew point exceed air temperature?
Under ordinary unsaturated observations it should not.
What happens at 100% humidity?
Dew point equals air temperature in this model.
Is dew point the same as relative humidity?
No.
Why does RH rise overnight?
Cooling lowers saturation vapor pressure even when moisture changes little.
Can dew form above freezing?
Yes; frost is associated with sufficiently cold surfaces.
Does this predict indoor condensation?
It gives a threshold; real surfaces and airflow still matter.
What to keep in mind
This is the NWS Magnus-type approximation for atmospheric water vapor. Sensor error, pressure, supercooled conditions and temperatures far outside ordinary weather observations can affect accuracy.
Results are rounded for display in US format, with money in dollars and cents; CSV exports keep full calculation precision. This is a calculation summary, not an official certificate.