Saturation Vapor Pressure Calculator

How it is calculated

The input is air temperature T in degrees Celsius. The tool applies the Tetens equation: es = 6.1078 × 10(7.5 × T ÷ (T + 237.3)) in hectopascals (hPa), and divides by 10 for kilopascals (kPa). The same water-surface constants are used for every temperature, including below 0 °C, where the value is the pressure over supercooled liquid water rather than over ice. The equation is an approximation that is best between roughly 0 and 50 °C; at 100 °C it returns 1,021.94 hPa, slightly above the 1,013 hPa of standard atmosphere, so do not use it near boiling. Any temperature above −237.3 °C is accepted. The tool gives saturation pressure only, not relative humidity or dew point.

Common mistakes

• Entering the temperature in Fahrenheit or Kelvin. The field is Celsius only: 77 °F must be entered as 25, and 298 K as 25.
• Confusing saturation vapor pressure with actual vapor pressure. The result is the maximum at that temperature; the real value is lower unless the air is saturated.
• Mixing up hPa and kPa when feeding other formulas. 31.67 hPa equals 3.17 kPa, a factor of ten.
• Using the result for water at its boiling point or for ice below freezing. The formula is tuned to liquid water in the usual weather range, so errors grow outside it.