Dew Point Calculator

Dew point is the best single measure of how humid the air actually feels — better than relative humidity alone. This dew point calculator finds it instantly from temperature and humidity, helping with comfort assessment, HVAC planning, and weather analysis.

What Is Dew Point?

The dew point is the temperature at which air must be cooled (at constant pressure) for water vapor to condense into liquid water (dew). The closer the dew point is to the actual air temperature, the more humid it feels. When they're equal, relative humidity = 100% (fog or rain).

How to Use This Calculator

1. Enter the air temperature in °F or °C.
2. Enter the relative humidity as a percentage.
3. Click Calculate to see the dew point and comfort level.

Dew Point Formula (Magnus Approximation)

γ(T, RH) = ln(RH/100) + (17.625 × T) ÷ (243.04 + T)

Td = 243.04 × γ ÷ (17.625 − γ)

• T = Temperature in °C
• RH = Relative humidity (%)
• Td = Dew point in °C

Example Calculation

Temperature: 85°F (29.4°C), Humidity: 70%

• γ = ln(70/100) + (17.625 × 29.4) ÷ (243.04 + 29.4)
• = −0.357 + 1.899 = 1.542
• Td = 243.04 × 1.542 ÷ (17.625 − 1.542) = 23.4°C (74°F)
• Comfort level: Humid — uncomfortable for most people

Dew Point Comfort Scale

• Below 55°F (13°C) — Dry and comfortable
• 55–60°F (13–16°C) — Comfortable for most
• 60–65°F (16–18°C) — Getting humid; noticeable
• 65–70°F (18–21°C) — Humid and oppressive for many
• Above 70°F (21°C) — Extremely humid; dangerous for heat illness

Why Use This Calculator?

HVAC engineers use dew point to prevent condensation in buildings. Brewers and winemakers monitor it for fermentation conditions. Meteorologists use it to predict fog and precipitation. And everyday people use it to understand whether it will feel muggy outside.

Common Mistakes to Avoid

• Confusing relative humidity with dew point — 80% humidity at 50°F feels less oppressive than 60% humidity at 90°F. Dew point gives you the actual comfort measure.
• Assuming low relative humidity always feels comfortable — in very hot conditions, even 30% humidity can feel oppressive if the dew point is high.

Frequently Asked Questions

What is a comfortable dew point?

Most people find dew points below 60°F (16°C) comfortable. Above 65°F (18°C) starts to feel noticeably sticky and oppressive.

Why does dew form on grass in the morning?

At night, surfaces cool faster than the air. When a surface temperature drops below the dew point of the surrounding air, moisture condenses on it — forming dew.

Is dew point the same as humidity?

No. Relative humidity is the percentage of maximum possible moisture at the current temperature. Dew point is an absolute measure of moisture content that doesn't change with temperature.

Can the dew point be higher than the air temperature?

No. When the dew point equals the air temperature, relative humidity is 100%. The dew point can never exceed the air temperature under normal atmospheric conditions.

Conclusion

Dew point is the humidity measure that actually tells you how the air will feel. Whether you're a homeowner checking HVAC conditions, a runner planning a morning jog, or just checking the weather, the dew point gives you the real picture.

Related: Heat Index Calculator | Wind Chill Calculator