Relative Humidity Calculator
Calculate RH from air temperature and dew point. This is a fast way to estimate indoor comfort, mold risk, and HVAC conditions.
Tip: Dew point cannot be higher than air temperature in normal conditions.
What is RH (Relative Humidity)?
Relative humidity (RH) is the percentage of water vapor currently in the air compared with the maximum amount the air could hold at the same temperature. At 50% RH, the air holds about half of its moisture capacity. At 100% RH, the air is saturated and condensation becomes likely.
This matters because warm air can hold more moisture than cold air. So RH changes when temperature changes, even if the actual amount of water vapor stays the same.
How this RH calculator works
This calculator uses your air temperature and dew point to estimate relative humidity with the Magnus approximation, a standard meteorological formula for practical use.
Formula used
Where:
- T = air temperature in °C
- Td = dew point temperature in °C
The tool also returns:
- Vapor Pressure Deficit (VPD): useful for plant growth, drying, and climate control
- Comfort interpretation: dry, comfortable, humid, or very humid
Why relative humidity matters
1) Health and comfort
Very low RH can dry your eyes, skin, throat, and sinuses. Very high RH can feel sticky and uncomfortable, and may aggravate allergies due to mold and dust mites.
2) Home protection
High humidity can promote mold growth, condensation on windows, paint damage, and warping in wood furniture and floors. Low humidity can crack wood and increase static electricity.
3) HVAC efficiency
Humidity affects how hot or cold a room feels. Managing RH well can make spaces feel better at the same thermostat setting, potentially reducing energy use.
4) Specialty use cases
- Greenhouse and indoor grow room control
- Wine cellars and cigar humidors
- Museums, archives, and instrument storage
- Server rooms and sensitive electronics spaces
What is a good indoor RH range?
For most homes, 40% to 60% RH is a practical target. Many people feel best around 45% to 50%.
- Below 30%: usually too dry for comfort
- 30% to 60%: generally acceptable and comfortable
- Above 60%: risk of dampness, dust mites, and mold rises
How to improve humidity balance
If RH is too low
- Use a clean humidifier
- Seal air leaks in winter
- Add houseplants (modest effect)
- Reduce over-drying from HVAC settings
If RH is too high
- Run a dehumidifier
- Increase ventilation in kitchens and bathrooms
- Fix leaks and moisture intrusion quickly
- Use exhaust fans and maintain AC systems
Common RH calculation mistakes
- Mixing units: Always confirm whether values are in °C or °F.
- Using bad sensor placement: Avoid measuring near vents, windows, or direct sunlight.
- Ignoring dew point: Dew point is often more stable and meaningful for moisture content than RH alone.
- Rounding too aggressively: Small temperature differences can noticeably shift RH values.
FAQ
Is RH the same as dew point?
No. RH is a percentage that changes with temperature; dew point is the temperature at which condensation starts and is a direct measure of moisture in air.
Why can RH go up at night?
As air temperature falls, the same moisture amount occupies a larger fraction of capacity, so RH rises.
Can RH be above 100%?
In normal indoor calculations, no. In the real atmosphere, short-lived supersaturation can occur in special conditions, but practical RH reporting is usually capped around 100%.
Bottom line
A simple RH calculator gives you immediate insight into comfort and moisture risk. Use it regularly if you manage indoor air quality, run HVAC equipment, or care for moisture-sensitive spaces. Keeping humidity in the right range protects health, improves comfort, and helps preserve your home and belongings.