Dew Point Calculator
Temperature plus humidity in, dew point out: with how muggy it feels, the frost and wet-bulb points, and whether your windows will fog.
By Manzoor Ahmad, pharmacist turned web dveloper · Updated How do you calculate dew point?
Td = 243.04 × γ ÷ (17.625 − γ), where γ = ln(RH ÷ 100) + 17.625 × T ÷ (243.04 + T) and T is in °C. At 25 °C (77 °F) and 60% relative humidity, the dew point is 16.7 °C (62 °F): air that feels somewhat humid. A quick check for humid air: Td ≈ T − (100 − RH) ÷ 5.
Dew point comfort chart
Dew point tracks how sticky the air feels far better than relative humidity, because it measures the actual moisture in the air.
| Dew point | °F | How it feels |
|---|---|---|
| Below 10 °C | below 50 | Dry; static and dry skin indoors |
| 10–13 °C | 50–55 | Comfortable |
| 13–16 °C | 55–60 | Pleasant, slightly humid |
| 16–18 °C | 60–65 | Somewhat humid |
| 18–21 °C | 65–70 | Humid and muggy |
| 21–24 °C | 70–75 | Very humid, uncomfortable |
| 24 °C and above | 75+ | Oppressive; heat stress risk |
Dew point table by temperature and humidity (°C)
| Air temp | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|
| 10 °C (50 °F) | -6.8 | -3.0 | 0.0 | 2.6 | 4.8 | 6.7 | 8.4 |
| 15 °C (59 °F) | -2.4 | 1.5 | 4.7 | 7.3 | 9.6 | 11.6 | 13.4 |
| 20 °C (68 °F) | 1.9 | 6.0 | 9.3 | 12.0 | 14.4 | 16.4 | 18.3 |
| 25 °C (77 °F) | 6.2 | 10.5 | 13.9 | 16.7 | 19.1 | 21.3 | 23.2 |
| 30 °C (86 °F) | 10.5 | 14.9 | 18.4 | 21.4 | 23.9 | 26.2 | 28.2 |
| 35 °C (95 °F) | 14.8 | 19.4 | 23.0 | 26.1 | 28.7 | 31.0 | 33.1 |
Worked example: will the bathroom mirror fog?
After a shower the bathroom is 24 °C at 85% RH, giving a dew point of 21.3 °C. A mirror at 19 °C is below that, so it fogs. Warm the mirror above 21.3 °C or dry the air below about 69% RH (dew point under 18 °C) and it stays clear. To convert the other way, use the relative humidity calculator.
Dew point FAQs
The dew point is the temperature to which air must be cooled, at constant pressure and moisture content, for it to become saturated (100% relative humidity). Cool it further and water vapor condenses as dew, fog or cloud.
Use the Magnus formula: γ = ln(RH/100) + 17.625T ÷ (243.04 + T), then Td = 243.04γ ÷ (17.625 − γ), with T in °C. At 25 °C and 60% RH the dew point is 16.7 °C (62 °F).
For humidity above about 50%, Td ≈ T − (100 − RH) ÷ 5 in °C. At 25 °C and 60% RH that gives 17 °C, close to the exact 16.7 °C.
Most people find a dew point of 10–16 °C (50–60 °F) comfortable. Above 18 °C (65 °F) the air feels muggy, and above 24 °C (75 °F) it feels oppressive because sweat evaporates slowly.
Dew point measures how much water vapor is actually in the air. Relative humidity is that amount compared with the maximum the air could hold at its current temperature, so RH changes during the day even when the dew point stays the same.
Because it does not depend on air temperature. 100% RH at 5 °C is far drier air than 50% RH at 30 °C; the dew points (5 °C vs 18.4 °C) show that directly.
When the glass surface is at or below the room air's dew point. Indoor air at 21 °C and 50% RH has a dew point of about 10 °C, so any window or pipe colder than that will fog or drip.
The frost point is the temperature at which water vapor becomes saturated over ice rather than liquid water. Below 0 °C it is slightly higher than the dew point, and frost forms on surfaces that cool to it.
No. At most it equals the air temperature, which happens at 100% relative humidity. If a reading shows otherwise, a sensor is wrong or the air is supersaturated, which is short-lived.
In a well-mixed afternoon atmosphere, the cloud base sits roughly 125 m above the ground for every 1 °C of spread between temperature and dew point (about 228 ft per °F). A 10 °C spread suggests clouds near 1,250 m.
The lowest temperature air can reach by evaporating water into it. It lies between the dew point and the air temperature and is important for heat stress; this calculator uses Stull's (2011) formula, accurate to within about 0.3 °C for normal conditions.
The Magnus formula with Alduchov–Eskridge coefficients is accurate to within about 0.1–0.4 °C between −40 °C and 50 °C, better than most humidity sensors, which are typically ±2–3% RH.
Sources
- Alduchov & Eskridge (1996), Improved Magnus Form Approximation of Saturation Vapor Pressure, Journal of Applied Meteorology
- NOAA National Weather Service: Dew Point vs Humidity
- Stull (2011), Wet-Bulb Temperature from Relative Humidity and Air Temperature, Journal of Applied Meteorology and Climatology
- Lawrence (2005), The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air, BAMS
- NOAA National Weather Service Glossary: Dew Point
Results use the Magnus formula at sea-level pressure and are for education, comfort and building checks. For aviation, agriculture or safety-critical work, use calibrated instruments and official forecasts.