Relative Humidity Calculator
Calculate relative humidity from dry bulb and wet bulb temperatures.
Supports Fahrenheit and Celsius with psychrometric formulas.
How Humidity Is Calculated
Humidity has several related but distinct measurements. Relative humidity is the most commonly reported — it describes how much water vapor is in the air as a percentage of the maximum possible at that temperature.
Relative Humidity formula:
RH (%) = (Actual Vapor Pressure ÷ Saturation Vapor Pressure) × 100
A simpler approximation using temperature and dew point:
RH ≈ 100 − 5 × (Temperature − Dew Point) [both in °C]
Worked example:
- Temperature: 25°C
- Dew point: 15°C
RH ≈ 100 − 5 × (25 − 15) = 100 − 50 = 50% relative humidity
Key humidity measurements:
- Relative Humidity (RH): Water vapor as % of maximum possible at current temperature
- Absolute Humidity: Actual mass of water per cubic meter of air (g/m³)
- Specific Humidity: Mass of water vapor per mass of moist air (g/kg)
- Dew Point: Temperature at which air becomes saturated (RH = 100%)
Absolute humidity formula:
AH (g/m³) = (6.112 × e^(17.67 × T/(T+243.5)) × RH × 2.1674) ÷ (273.15 + T)
Where T = temperature in °C.
Comfort ranges:
| RH Level | Perception |
|---|---|
| Below 30% | Very dry, irritates mucous membranes, static electricity |
| 30–50% | Comfortable, ideal for most people |
| 50–60% | Slightly humid but acceptable |
| Above 60% | Humid, promotes mold growth, dust mites |
| Above 80% | Oppressive, heat stress risk |
Dew point as a comfort indicator: Dew point above 60°F (16°C) feels humid; above 70°F (21°C) feels oppressive. Unlike RH, dew point doesn’t change with temperature — making it the more reliable comfort indicator.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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