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Dew Point Calculator

Find the exact temperature at which air becomes saturated. Essential for HVAC, gardening, meteorology, and comfort assessment.

%

Dew Point

52.3

°F

Comfort Level

Pleasant

Heat Index (≥80°F & ≥40% RH)

—

Feels Like (humidity-adjusted)

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Dew Point vs. Relative Humidity: What's the Difference?

Relative Humidity tells you how saturated the air is at its current temperature — but the same 60% RH can feel completely different on a hot day versus a cold one. The dew point is an absolute measure: it tells you exactly how much water vapour is in the air, regardless of temperature.

Meteorologists and HVAC engineers prefer the dew point because it never lies. A dew point of 70°F (21°C) is oppressive whether the thermometer reads 75°F or 95°F.

The Magnus-Tetens Formula

This calculator uses the industry-standard Magnus-Tetens approximation (a = 17.625, b = 243.04°C), accurate to within ±0.4°C for temperatures between −45°C and 60°C. The formula computes the intermediate value α from your inputs, then derives the dew point directly.

Frost Point

When the calculated dew point falls below 0°C (32°F), it becomes the frost point — the temperature at which water vapour deposits directly as ice crystals rather than liquid dew. Gardeners use this to predict overnight frost events.

Comfort Scale

Below 50°F (10°C): dry and crisp. 50–60°F: comfortable. 60–65°F: starting to feel sticky. 65–70°F: noticeably humid and uncomfortable. Above 70°F: oppressive — the body struggles to cool itself through perspiration.

Heat Index

When the air temperature is 80°F+ and RH is 40%+, the NWS Heat Index formula calculates how hot it actually feels. High dew points dramatically amplify the apparent temperature because sweat evaporates more slowly in moist air.

When Does Dew Form?

Dew forms on surfaces when their temperature drops to or below the dew point of the surrounding air. Clear nights allow surfaces to radiate heat into space, cooling rapidly. If a surface — grass, a car roof, a window — cools to the dew point, water condenses on it. The higher the dew point, the more dew will form and the less cooling is required.

Frequently Asked Questions

This calculator uses the Magnus-Tetens formula: α = ln(RH/100) + (a·T)/(b+T), then Td = (b·α)/(a−α), where a = 17.625 and b = 243.04°C. All inputs in Fahrenheit are converted to Celsius first, then the result is converted back.
No. The dew point can equal the air temperature (100% relative humidity = fog or rain), but it can never exceed it. If your result shows dew point = air temp, condensation is already occurring.
Relative humidity changes throughout the day as temperature rises and falls, even if the actual moisture content of the air stays constant. The dew point remains stable and directly reflects how much water vapour is present, making it a more reliable comfort indicator.
Yes, at higher altitudes air pressure is lower, which affects vapour pressure and the relationship between temperature and moisture. For ground-level weather and HVAC applications the standard formula used here is highly accurate.
HVAC engineers set supply air temperatures above the dew point to prevent condensation on ductwork and coils, which can cause mould and corrosion. In dehumidification design, the dew point determines exactly how much the air must be cooled to shed moisture.
A dew point above 75°F (24°C) is considered extreme and potentially hazardous. Dew points above 80°F (27°C) — recorded in the Persian Gulf region — represent the upper limit of human survivability for prolonged outdoor exposure, because the body cannot cool itself at all.
If the overnight low is forecast to fall to or below the current dew point and skies are clear, frost is likely. High dew points also favour fungal diseases such as powdery mildew and botrytis. Tracking the dew point helps gardeners time irrigation and fungicide applications correctly.

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