Humidity Calculator
Find relative humidity, absolute humidity and vapour pressure from temperature and dew point.
Last updated · View algorithm, constants and default inputs (JSON)
Relative humidity (%)
Dots show recalculated samples. Other inputs stay fixed.
About this calculator
Uses the Magnus approximation. Enter air temperature and dew point in the same unit; dew point cannot be higher than air temperature in this model. Mixing ratio assumes standard sea-level pressure (1013.25 hPa). Comfort bands are descriptive, not health guidance.
Worked example
calculated with the default values below
Air temperature 24 °C · Dew point 16 °C · Temperature unit Celsius Relative humidity 60.9 % · Absolute humidity 13.25 g/m³ · Vapour pressure 18.17 hPa · Saturation vapour pressure 29.83 hPa · Mixing ratio 11.36 g/kg · Comfort level Slightly humid
General background references
Disclaimer
These calculators give general estimates. Sizing, pricing, availability and standards vary by brand, retailer and country, so treat the output as a starting point and confirm it against the seller's or authority's own figures.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
es = 6.112 × e^(17.67T ÷ (T + 243.5)) ; e = 6.112 × e^(17.67D ÷ (D + 243.5)) ; RH = 100 × e ÷ es ; AH = 216.7 × e ÷ (T + 273.15)
- T
- air temperature (°C)
- D
- dew point (°C)
- es
- saturation vapor pressure (hPa)
- e
- actual vapor pressure (hPa)
Valid only when dew point ≤ air temperature; RH is not clamped. Mixing ratio assumes 1013.25 hPa. °F inputs convert to °C.
Frequently asked questions
Why can relative humidity fall even when you add no water to the air?
For a fixed dew point, the model holds actual vapour pressure steady while saturation pressure rises with air temperature, so relative humidity falls. Mixing ratio also assumes fixed standard pressure of 1013.25 hPa. Absolute humidity is vapour mass per volume and depends on temperature, so it is not constant as air warms.