Dew Point Calculator
Enter air temperature and relative humidity to get dew point, wet bulb and a straight comfort read — dew point, not RH, is the real comfort metric.
Last updated: 2026-09-15
How the calculation works
- The Magnus-Tetens formula inverts the vapor-pressure relation: dew point is where the current moisture content becomes saturated.
- The wet-bulb approximation (Stull) gives the temperature of evaporative cooling — the physical limit of swamp coolers and the metric for heat stress.
- Comfort bands come from empirical dew-point comfort research, not the humidity percentage.
Formula
γ = ln(RH/100) + a·T/(b+T) Td = b·γ / (a − γ) (Magnus-Tetens, a=17.62, b=243.12)
| Symbol | Meaning | Unit |
|---|---|---|
Td | Dew point temperature | °C |
T | Air temperature | °C |
RH | Relative humidity | % |
Worked example
Interpreting the result
Dew point is the amount of moisture in the air, expressed as a temperature — it doesn't change with air temperature the way RH does. Air conditioning works largely by cooling air below its dew point and wringing water out. Surfaces (cold pipes, single-pane windows) at or below dew point will condense water — that's the number for mold prevention.
Assumptions
- Standard atmospheric pressure near sea level; the Magnus approximation shifts slightly at altitude.
- Ranges: −40 to 50 °C input, ±0.35 °C accuracy.
Limitations
- Does not replace full psychrometric charts when precise enthalpy or humidity ratio is needed.
- Wet bulb is an approximation (Stull), accurate within ~1 °C for normal conditions.
Frequently asked questions
What dew point is uncomfortable?
Above 60 °F (15.5 °C) feels muggy to most people; above 70 °F (21 °C) is oppressive. Below 55 °F (13 °C) feels crisp and dry.
Why does dew point matter more than humidity %?
RH changes with temperature even when moisture content is constant — 50% RH at 30 °C is far more humid air than 50% RH at 15 °C. Dew point measures the actual water content.