👤 By whycalculator Team 📅 Last Updated March 28, 2026
Dew Point Calculator
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This dew point calculator helps you quickly understand when moisture in the air will begin to condense based on air temperature and relative humidity. It gives a practical view of how close the air is to reaching saturation, which is useful for weather checks, HVAC work, and indoor comfort.
Along with dew point, this tool can also estimate absolute humidity and vapour pressure, giving a clearer picture of the actual moisture content in the air. Whether you are monitoring room comfort, checking storage conditions, or analyzing outdoor climate, this calculator makes moisture calculations fast and easy.
How to Calculate Dew Point
Dew point is the temperature at which air becomes fully saturated with moisture and water vapor starts turning into liquid droplets. To calculate dew point, you need only two values: air temperature and relative humidity (RH).
Dew Point Formula
The most commonly used formula is the Magnus formula:
α = (a × T / (b + T)) + ln(RH / 100)
Dew Point = (b × α) / (a − α)
Where:
- T = air temperature in °C
- RH = relative humidity (%)
- a = 17.27
- b = 237.7
Step-by-Step Example
Suppose the air temperature is 30°C and the relative humidity is 70%.
Step 1: Calculate α
α = (a × T / (b + T)) + ln(RH / 100)
α = (17.27 × 30 / (237.7 + 30)) + ln(70 / 100)
α = (518.1 / 267.7) + ln(0.7)
α = 1.935 − 0.3567 = 1.5783
Step 2: Calculate Dew Point
Dew Point = (b × α) / (a − α)
Dew Point = (237.7 × 1.5783) / (17.27 − 1.5783)
Dew Point = 375.86 / 15.6917
Dew Point = 23.95°C
Quick Approximate Formula
For a fast estimation, you can use this simple formula:
Dew Point ≈ T − ((100 − RH) / 5)
Using the same values:
Dew Point = 30 − ((100 − 70) / 5)
Dew Point = 30 − 6 = 24°C
Real-Life Example
If a room has an air temperature of 25°C and humidity of 60%, the dew point is approximately:
17°C
This means if any surface, such as a cold water pipe, mirror, or AC vent, drops below 17°C, water droplets may start forming on it.
Dew Point Sample Values Table
The table below shows sample dew point values for different combinations of air temperature and relative humidity. These examples help you quickly understand how dew point changes as temperature and moisture levels increase.
| Air Temperature (°C) | Relative Humidity (%) | Dew Point (°C) | Condition |
|---|---|---|---|
| 10 | 40 | -2.9 | Dry Air |
| 12 | 50 | 1.5 | Comfortable |
| 15 | 60 | 7.3 | Comfortable |
| 18 | 65 | 11.2 | Slightly Humid |
| 20 | 70 | 14.4 | Moderate |
| 22 | 75 | 17.3 | Humid |
| 24 | 80 | 20.3 | Sticky |
| 26 | 55 | 16.2 | Moderate |
| 28 | 60 | 19.6 | Humid |
| 30 | 70 | 24.0 | Muggy |
| 32 | 75 | 27.1 | Very Humid |
| 35 | 80 | 31.0 | Oppressive |
| 40 | 85 | 37.0 | Extreme Moisture |
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