Humidity Comfort Index Calculator
Check if your indoor or outdoor humidity is in the comfort zone.
Get health and comfort recommendations based on relative humidity.
Relative humidity (RH) measures how much water vapor is in the air as a percentage of the maximum the air could hold at that temperature. Dew point is the temperature at which air must cool (at constant pressure) to become saturated and water droplets begin to form.
Relative humidity formula: RH (%) = (Actual Vapor Pressure ÷ Saturation Vapor Pressure) × 100
Magnus formula approximation for dew point: Td = (243.04 × γ) ÷ (17.625 − γ)
Where: γ = (17.625 × T) ÷ (243.04 + T) + ln(RH ÷ 100)
- T: air temperature in °C
- RH: relative humidity in %
- ln: natural logarithm
- Td: dew point temperature in °C
Heat Index (apparent temperature when hot and humid): HI = −42.379 + 2.04901523×T + 10.14333127×RH − … (Steadman’s full polynomial; simplified: HI ≈ T + 0.33×e − 0.70×ws − 4.00, where e = vapor pressure)
Comfort scale by dew point (°C):
- Below 10°C (50°F): Very dry, comfortable for most
- 10–15°C (50–60°F): Comfortable: ideal range
- 15–18°C (60–65°F): Slightly humid, still comfortable
- 18–21°C (65–70°F): Humid, uncomfortable for some
- 21–24°C (70–75°F): Very humid, oppressive
- Above 24°C (75°F): Dangerous: limits evaporative cooling
Health and comfort impacts:
- Too dry (RH < 30%): Dry skin, irritated nasal passages, increased static electricity, wood shrinkage
- Ideal (RH 40–60%): Optimal for human comfort and respiratory health
- Too humid (RH > 70%): Mold growth, dust mites, structural moisture damage, heat stress risk
Worked example: Air temperature: 28°C. Relative humidity: 75%.
γ = (17.625 × 28) ÷ (243.04 + 28) + ln(75 ÷ 100) = (493.5 ÷ 271.04) + ln(0.75) = 1.820 + (−0.2877) = 1.5323
Dew point = (243.04 × 1.5323) ÷ (17.625 − 1.5323) = 372.40 ÷ 16.0927 = 23.1°C
A dew point of 23°C is in the “very humid / oppressive” range — expect significant discomfort outdoors and elevated heat stroke risk during exercise.