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Relative Humidity Calculator

Calculate relative and absolute humidity from air temperature and dew point.

Frequently Asked Questions

What is relative humidity?

The percentage of moisture in the air relative to the maximum the air can hold at that temperature. 100% RH means saturation - fog or dew is forming.

Why does RH change with temperature?

Warmer air can hold more water vapor. The same absolute moisture gives lower RH on a hot afternoon and higher RH on a cool morning, even with no rain.

What is the Magnus formula used for?

It approximates the saturation vapor pressure of water as a function of temperature, allowing accurate dew point and RH calculations from basic measurements.

What indoor RH is healthy?

30-50% is the comfort sweet spot. Below 30% causes dry skin and static; above 60% promotes mold, dust mites, and condensation on cold surfaces.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

This calculator uses the Magnus formula to estimate saturation vapor pressure at two temperatures: the actual air temperature and the dew point. Relative humidity is the ratio of actual vapor pressure to saturation vapor pressure. The dew point is the temperature at which air becomes saturated (100% RH) if cooled without adding or removing moisture. If you know the dew point, you can calculate relative humidity directly.

Why Humidity Matters

Comfort

Most people feel comfortable at 40-60% RH. Below 30%, skin becomes dry, lips chap, and static electricity increases. Above 70%, air feels muggy, sweat doesn't evaporate efficiently, and the risk of mold and dust mites increases.

Health

Respiratory health: very low RH (<20%) dries airways; very high RH (>80%) promotes mold and dust mites, triggering asthma and allergies.
Skin: low RH causes dryness; high RH traps heat and makes sweating less effective.

Building and Materials

Mold growth: begins at RH >60% in cool areas; higher temperatures (like bathrooms) risk mold at even lower RH.
Wood and materials: expand in high humidity, shrink in low humidity, causing warping and structural damage.
Electronics: condensation in high humidity; static electricity in low humidity.

Relationship Between Temperature and Relative Humidity

Relative humidity is temperature-dependent. Warm air can hold more moisture; cool air cannot. Example: air at 20°C with 11 g/m³ of moisture has 50% RH. Cool that same air to 10°C without removing moisture, and the RH rises to 90% because 10°C air can hold less moisture. This is why dew forms at night - temperature drops, RH rises to 100%, and water condenses.

How to Measure Humidity

Hygrometer

A device that measures relative humidity. Digital hygrometers ($10-50) are accurate and common. Mechanical (analog) hygrometers are cheaper but less accurate. Always check calibration: in a sealed bag with a wet paper towel for 12+ hours, humidity should read ~95%.

Dew Point Estimate

If you have an accurate thermometer and know the current RH (from a hygrometer), you can estimate dew point: roughly, every 5% drop in RH below 100% drops the dew point about 1°C below the air temperature near typical room conditions.

Desiccants and Humidity Control

Desiccants (Dehumidify)

Silica gel: absorbs moisture; rechargeable by heating. Common in storage boxes and electronics packaging.
Calcium chloride: very hygroscopic; used in dehumidifiers.
Charcoal: absorbs moisture and odors; good for closets and cars.

Humidifiers (Increase Humidity)

Evaporative: blow air through wet wick; slower, safer, maintains RH ~50%.
Ultrasonic: vibrate water to create mist; faster, can over-humidify, may leave mineral deposits.
Steam: boil water; useful but uses energy.

Practical Tips

Target 40-60% RH for homes.
Use exhaust fans in bathrooms to reduce humidity.
Increase ventilation if mold appears.
In winter, heating lowers RH; use a humidifier if RH drops below 30%.
In summer, AC lowers RH; if mold appears, check for leaks or poor ventilation.