Dew Point Calculator
Calculate dew point from temperature and relative humidity, convert between dew point and relative humidity, and find absolute humidity — using the precise Magnus formula. Supports °F and °C.
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Dew Point Calculator
August-Roche-Magnus approximation
Dew Point
18.0°C
Temp − DP Spread
7.0°
Pleasant
Slightly humid but still comfortable.
Abs. Humidity
14.97 g/m³
COMFORT REFERENCE
Dew Point Comfort Scale
How different dew point temperatures feel to the human body.
<10°C / <50°F
🏜️ Very Dry
Arid, desert-like. Dry skin and lips likely.
10-15°C / 50-59°F
😌 Comfortable
Ideal conditions. Most people feel comfortable.
15-18°C / 59-65°F
🙂 Pleasant
Slightly humid but still comfortable.
18-21°C / 65-70°F
😅 Humid
Sticky. Some people start feeling uncomfortable.
21-24°C / 70-75°F
🥵 Oppressive
Very humid. Sweating and discomfort common.
24°C+ / 75°F+
🚨 Dangerous
Extremely uncomfortable. Heat stress risk with prolonged exposure.
QUICK REFERENCE
Dew Point, Temperature & Relative Humidity
Approximate relative humidity at different temperature / dew point combinations.
| Air Temp | Dew Point | Rel. Humidity | Comfort Level | Conditions |
|---|---|---|---|---|
| 35°C / 95°F | 24°C / 75°F | ~55% | Dangerous | Tropical heat wave, heat stroke risk |
| 30°C / 86°F | 21°C / 70°F | ~60% | Oppressive | Typical summer in southeastern US |
| 28°C / 82°F | 18°C / 64°F | ~55% | Humid | Warm humid summer day |
| 25°C / 77°F | 15°C / 59°F | ~52% | Pleasant | Comfortable summer conditions |
| 22°C / 72°F | 10°C / 50°F | ~46% | Comfortable | Ideal spring/fall conditions |
| 15°C / 59°F | 5°C / 41°F | ~54% | Very Dry | Cool and dry, comfortable jacket weather |
| 10°C / 50°F | -1°C / 30°F | ~43% | Very Dry | Cool and mildly humid |
| 0°C / 32°F | 0°C / 32°F | ~100% | Very Dry | Air fully saturated, fog likely |
Dew Point Is More Reliable Than Relative Humidity
RH changes with air temperature and falls fast — even if the air's actual moisture content stays exactly the same. On a warm afternoon, RH may drop to 40% while dew point at 15°C (59°F) stays constant. Meteorologists use dew point because it measures actual moisture instead of a temperature-relative ratio.
💧 How Dew and Frost Form
Dew forms when a surface cools below the dew point temperature — water vapor in contact with it condenses into liquid droplets. This is why grass and car windshields are wet on clear mornings even when it hasn't rained. Frost forms the same way, except the dew point is below freezing (0°C / 32°F), so vapor deposits directly as ice crystals rather than passing through a liquid stage first.
⛈️ Dew Point in Weather Forecasting
Meteorologists use dew point as a key severe-thunderstorm forecasting variable. High surface dew points mean more moisture and latent heat available to fuel storm development — the higher the surface dew point, the more energetic thunderstorms can become, and dew points above roughly 21°C (70°F) are commonly associated with an elevated tornado risk in severe weather setups.
How Dew Point Is Calculated — The Magnus Formula
The dew point is calculated using the August-Roche-Magnus approximation, one of the most widely used formulas in meteorology and HVAC engineering. It provides accuracy within ±0.4°C for temperatures between -45°C and 60°C, making it suitable for virtually all real-world applications.
Dew Point from Temperature and Humidity
Magnus constants: a = 17.625, b = 243.04°C. α(T, RH) = ln(RH/100) + (a×T)/(b+T). Dew Point (°C) = (b×α)/(a−α)Example: T = 25°C, RH = 65% → α = ln(0.65) + (17.625×25)/(243.04+25) ≈ 1.2325. Dew Point ≈ (243.04×1.2325)/(17.625−1.2325) ≈ 18.3°C.
Relative Humidity from Temperature and Dew Point
RH = 100 × exp((a×Td)/(b+Td)) / exp((a×T)/(b+T))Example: T = 25°C, Td = 15°C → RH ≈ 100 × exp(1.028)/exp(1.310) ≈ 76%.
Absolute Humidity (Tetens Saturation Vapor Pressure)
Saturation vapor pressure: es = 6.1078 × exp((17.27×T)/(T+237.3)) [hPa]. Actual vapor pressure: e = es × (RH/100). Absolute Humidity [g/m³] = 216.7 × e / (273.15+T)Example: T = 25°C, RH = 65% → es ≈ 31.67 hPa, e ≈ 20.59 hPa, AH ≈ 216.7×20.59/298.15 ≈ 14.96 g/m³.
Dew Point vs Relative Humidity — What's the Difference?
Relative humidity and dew point both measure atmospheric moisture, but they behave very differently — and understanding the difference helps explain why weather apps and meteorologists prefer dew point for comfort assessments.
Why Relative Humidity Changes Through the Day
Relative humidity is the ratio of actual water vapor to the maximum water vapor air can hold at that temperature. Warmer air can hold more moisture, so as temperatures rise through the morning, the air's capacity to hold water rises faster than moisture actually enters or leaves it — relative humidity drops, though no water has actually left the air. A typical summer day might start at 90% RH at dawn and fall to 50% RH by mid-afternoon, with the dew point remaining constant throughout.
Why Dew Point Is a Better Comfort Indicator
Since dew point only changes when actual moisture content changes, it gives a stable, consistent measure of how humid the air truly is — regardless of time of day or temperature. A 70°F dew point feels equally oppressive at 75°F air temperature or at 95°F air temperature; in both cases, your body's ability to cool itself by sweating is severely impaired because the surrounding air is already heavily loaded with moisture.
Dew Point and Indoor Comfort (HVAC)
In buildings and HVAC engineering, indoor dew point is used to control condensation on surfaces and prevent mold growth. The recommended indoor dew point range is roughly 40-55°F (4-13°C), corresponding to about 40-60% relative humidity at typical indoor temperatures of 68-72°F (20-22°C).
— FAQ