Dew Point vs. Relative Humidity: The Formula, Comfort Levels, and Condensation
October 29, 2026
Relative humidity (RH) tells you how close the air is to saturation at its current temperature, so it changes as the temperature changes even when the amount of moisture is the same. The dew point is the temperature at which the air becomes saturated and dew or condensation begins. It is a direct measure of moisture content and a better guide to how humid it feels.
A quick estimate (accurate within about 1°C when RH is above 50%): dew point ≈ T − (100 − RH) ÷ 5, with T in °C. At 30°C and 60% RH: 30 − 40 ÷ 5 = 22°C. The more exact Magnus formula gives 21.4°C.
Skip the math and get your answer instantly:
Open the Temperature Converter →The Magnus Formula
Dew point = (b × γ) ÷ (a − γ), where γ = ln(RH ÷ 100) + (a × T) ÷ (b + T), with a = 17.62 and b = 243.12°C. For T = 30°C and RH = 60%: γ = ln(0.6) + (17.62 × 30) ÷ 273.12 = −0.511 + 1.935 = 1.425, and dew point = (243.12 × 1.425) ÷ (17.62 − 1.425) = 21.4°C. At 30°C and 40%: 14.9°C. At 20°C and 50%: 9.3°C. At 25°C and 80%: 21.3°C. At 35°C and 70%: 28.7°C, which feels extremely oppressive.
| Temperature | Relative humidity | Dew point (°C) | Dew point (°F) |
|---|---|---|---|
| 20°C (68°F) | 50% | 9.3 | 48.7 |
| 30°C (86°F) | 40% | 14.9 | 58.8 |
| 30°C (86°F) | 60% | 21.4 | 70.5 |
| 35°C (95°F) | 70% | 28.7 | 83.7 |
How Dew Points Feel
Below about 10°C (50°F): dry and comfortable. 10°C to 16°C (50°F to 60°F): pleasant. 16°C to 18°C (60°F to 65°F): slightly humid. 18°C to 21°C (65°F to 70°F): muggy. 21°C to 24°C (70°F to 75°F): oppressive. Above 24°C (75°F): extremely uncomfortable and dangerous with heat. Sweat evaporates slowly when the dew point is high, which makes the body's cooling less efficient. Air conditioners remove moisture as they cool.
Condensation and Mold at Home
Condensation forms on any surface that is colder than the dew point of the room air. In winter, indoor air at 21°C (70°F) and 50% RH has a dew point of 10.2°C (50°F); a window pane at 8°C will fog. Reduce indoor humidity to 30% to 40% in cold weather, use bathroom and kitchen fans, and improve insulation or use double glazing. Keep indoor RH between 30% and 50% to limit dust mites and mold. A cold water pipe at 10°C in a humid basement sweats if the dew point is above 10°C.
Practical Notes
Weather reports give the dew point, which is often the best comfort indicator. Compare the dew point with the overnight low: if the low is predicted to fall to the dew point, expect fog or dew. Hygrometers give RH; use the formula or a chart to find the dew point. The estimate rule is less accurate at low RH.
Frequently Asked Questions
What is the dew point?
The temperature at which air becomes saturated and condensation begins.
How do I estimate it quickly?
Dew point ≈ T − (100 − RH) ÷ 5, in °C.
What dew point feels oppressive?
Above about 21°C (70°F).
Why do windows fog in winter?
Their surface is colder than the dew point of the room air.
What indoor humidity is recommended?
Around 30% to 50%.