Humidity Calculator
Calculate relative humidity, dew point, or wet bulb temperature from known values. See also Dew Point Calculator and Heat Index Calculator.
Understanding Humidity Types
Humidity refers to the amount of water vapor present in the air. There are several ways to express humidity:
- Relative Humidity (RH) — The ratio of current moisture to the maximum moisture the air can hold at that temperature, expressed as a percentage. This is the most commonly reported value in weather forecasts.
- Absolute Humidity — The actual mass of water vapor per unit volume of air (g/m3). This does not change with temperature.
- Specific Humidity — The mass of water vapor per unit mass of total air (including vapor), expressed in g/kg. Used primarily in meteorology.
- Dew Point — The temperature at which air becomes saturated. A direct measure of moisture content.
- Wet Bulb Temperature — The lowest temperature achievable by evaporative cooling. Always between dew point and air temperature.
Formulas Used
Relative Humidity from Dew Point:
RH = 100 x exp((a x Td)/(b + Td) - (a x T)/(b + T))
Dew Point from RH (Magnus):
alpha = (a x T)/(b + T) + ln(RH/100)
Td = (b x alpha)/(a - alpha)
Wet Bulb (Stull 2011):
Tw = T x atan(0.151977 x (RH + 8.313659)^0.5)
+ atan(T + RH) - atan(RH - 1.676331)
+ 0.00391838 x RH^1.5 x atan(0.023101 x RH)
- 4.686035
Where: a = 17.27, b = 237.7, T in Celsius, RH in %
Ideal Indoor Humidity
The ideal indoor relative humidity is between 30% and 60%. Below 30%, air feels dry and can cause dry skin, irritated sinuses, and static electricity. Above 60%, moisture promotes mold growth, dust mites, and can damage building materials. In winter, indoor humidity often drops below 20% due to heating, making a humidifier beneficial. In summer, air conditioning naturally dehumidifies. The EPA recommends keeping indoor humidity below 60% and ideally between 30-50%.
Humidity Comfort Reference
| RH Range | Comfort | Health Effects | Recommendation |
|---|---|---|---|
| 0-25% | Too Dry | Dry skin, nosebleeds, static | Use humidifier |
| 25-40% | Slightly Dry | Minor dryness possible | Acceptable in winter |
| 40-60% | Ideal | Optimal comfort and health | Maintain this range |
| 60-70% | Slightly Humid | Mild discomfort possible | Monitor for mold |
| 70-80% | Humid | Mold risk, dust mites thrive | Use dehumidifier |
| 80-100% | Very Humid | High mold risk, structural damage | Dehumidify immediately |
Frequently Asked Questions
What is the difference between dew point and wet bulb temperature?
Dew point is the temperature at which air becomes saturated (100% RH). Wet bulb is the lowest temperature achievable through evaporative cooling — it is always between the dew point and air temperature. Wet bulb is important for HVAC design and heat stress assessment.
Why does 100% humidity not mean it is raining?
100% relative humidity means the air is saturated — it holds the maximum water vapor possible at that temperature. This can result in fog, dew, or clouds, but rain requires additional conditions like condensation nuclei and updrafts to form droplets large enough to fall.
How do I measure humidity at home?
Use a hygrometer (humidity meter), available at hardware stores for under $20. Digital hygrometers are most convenient. Place it away from direct sunlight, vents, and exterior walls for accurate readings. Many smart thermostats also measure humidity.
What is a dangerous wet bulb temperature?
A wet bulb temperature above 95 degrees F (35 degrees C) is considered the upper limit of human survivability — the body cannot cool itself through sweating at this point. Even fit, healthy individuals in shade with unlimited water cannot survive prolonged exposure above this threshold.
Does humidity affect how cold it feels in winter?
Yes. High humidity in cold weather makes it feel colder because moist air conducts heat away from your body faster than dry air. This is why a damp 35 degrees F day can feel colder than a dry 25 degrees F day.
How does altitude affect humidity?
As altitude increases, air pressure decreases and air can hold less moisture. Relative humidity often increases with altitude (since temperature drops), which is why clouds form at higher elevations. Absolute humidity generally decreases with altitude.