Relative Humidity Calculator
Work out relative humidity from the air temperature and dew point, from dry-bulb and wet-bulb psychrometer readings, or see what happens to humidity when air is heated or cooled. You also get vapor pressure, vapor pressure deficit, absolute humidity, mixing ratio and an indoor comfort rating.
Last updated:
from a sling or ventilated psychrometer
station pressure; 1013.25 hPa = 29.92 inHg at sea level
How to use the Relative Humidity Calculator
- Choose your inputs: dew point, wet-bulb reading, or a heating/cooling scenario.
- Pick °F or °C and enter the air temperature.
- Enter the second value: dew point, wet-bulb temperature, or starting humidity and new temperature.
- For wet-bulb readings, set the air pressure (sea level is 1013.25 hPa).
- Press Calculate to see the humidity, comfort level and the working.
Relative humidity formulas
- Saturation vapor pressure (Magnus):
es(T) = 6.1094 × exp(17.625 T ÷ (T + 243.04))hPa, T in °C. - From dew point:
RH = 100 × es(Td) ÷ es(T). - From a psychrometer:
e = es(Tw) − 0.000653 (1 + 0.000944 Tw) × P × (T − Tw), thenRH = 100 × e ÷ es(T). An ice-covered bulb uses 0.000575 and saturation over ice. - Heating or cooling:
RH2 = RH1 × es(T1) ÷ es(T2), capped at 100% once the air reaches its dew point.
Psychrometer table (sea level, °F)
| Dry bulb ↓ / wet-bulb depression → | 2 °F | 4 °F | 6 °F | 8 °F | 10 °F | 14 °F | 18 °F |
|---|---|---|---|---|---|---|---|
| 50 °F | 87% | 74% | 62% | 50% | 38% | 16% | — |
| 60 °F | 89% | 78% | 68% | 58% | 48% | 30% | 14% |
| 70 °F | 90% | 81% | 72% | 64% | 56% | 40% | 26% |
| 80 °F | 91% | 83% | 76% | 68% | 61% | 47% | 35% |
| 90 °F | 92% | 85% | 78% | 71% | 65% | 53% | 42% |
| 100 °F | 93% | 86% | 80% | 74% | 68% | 57% | 47% |
Wet-bulb depression is the dry-bulb reading minus the wet-bulb reading. Values come from the psychrometer equation above for a ventilated (sling or fan) psychrometer at 1013.25 hPa.
Indoor humidity guide
| Indoor RH | What to expect |
|---|---|
| Below 30% | Dry skin and eyes, static shocks, wood floors and instruments may shrink or crack |
| 30–50% | Generally recommended range for comfort and health |
| 50–60% | Acceptable; dust mites start to thrive above 50% |
| Above 60% | Mold, mildew and window condensation become likely; consider a dehumidifier |
Frequently asked questions
How do you calculate relative humidity from temperature and dew point?
RH = 100 × e_s(Td) ÷ e_s(T), where e_s is the saturation vapor pressure from the Magnus formula e_s(T) = 6.1094 × e^(17.625 T ÷ (T + 243.04)) hPa with T in °C. For example, 25 °C air with a 15.6 °C dew point has a relative humidity of about 56%.
How do you get relative humidity from wet-bulb and dry-bulb readings?
Use the psychrometer equation e = e_s(Tw) − A × P × (T − Tw), where Tw is the wet-bulb temperature, P the air pressure in hPa and A ≈ 0.000653 per °C for a ventilated psychrometer. Then RH = 100 × e ÷ e_s(T). A 25 °C dry bulb and 20 °C wet bulb at sea level give about 63%.
Why does relative humidity drop when air is heated?
Heating does not remove water vapor, but warm air can hold much more before saturating. Outdoor air at 0 °C and 80% RH, warmed to 21 °C indoors, drops to about 20% RH, which is why heated homes feel dry in winter.
What is a good indoor relative humidity?
Around 30–50% is generally recommended for comfort and health, and keeping it below 60% helps limit mold and dust mites. Below about 30% air feels dry and static builds up.
What is vapor pressure deficit (VPD)?
VPD is the difference between saturation vapor pressure and actual vapor pressure, e_s − e, usually given in kPa. It measures the drying power of the air. Growers often aim for roughly 0.4–1.6 kPa depending on plant stage.
What is the difference between relative and absolute humidity?
Relative humidity is how close the air is to saturation at its current temperature, in percent. Absolute humidity is the actual mass of water vapor per volume of air, in g/m³. Absolute humidity stays the same when air warms; relative humidity falls.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.