Vapor Pressure Formulas
There is no single “water vapor pressure formula” — there is a family of them, each a trade-off between accuracy, simplicity and the temperature range it was fitted for. Here is every formula this site implements, with its equation, valid range, and a mini-calculator. To see them compared side by side at one temperature, use the main calculator.
- Antoine Equation
Range-specific NIST constants for chemistry; output in bar internally, with the active A/B/C set shown.
- IAPWS-95
IAPWS-95 is the international reference standard for the saturation vapor pressure of water — accurate to ±0.025% from the triple point to the critical point.
- Buck Equation
The Buck 1996 equation is the best simple closed form for saturation vapor pressure — within about 0.1% of IAPWS-95 through 100 °C.
- Buck (1981, legacy)
The original 1981 Buck coefficients — superseded by Buck 1996, kept only to reproduce older software and datasheet output.
- Magnus Formula
The Magnus formula is the simplest practical saturation-pressure equation; the Alduchov–Eskridge coefficients make it accurate to about 0.4% and let it invert to the dew point.
- Goff–Gratch
The Goff–Gratch equation was the long-standing WMO reference for saturation vapor pressure before IAPWS-95 — still seen in atmospheric science and older standards.
- Tetens Equation
The Tetens equation is a simple, historically important Magnus-type form, accurate to about 1% near ambient temperatures.
- Murphy–Koop (ice)
Murphy–Koop (2005) is the modern reference for the saturation vapor pressure over ice — the basis for the frost point.
Not sure which to use? For reference-grade accuracy choose IAPWS-95; for a fast, near-reference estimate choose Buck (1996); for meteorology and dew point choose the Magnus (Alduchov–Eskridge) form. The main calculator shows each one’s deviation from IAPWS-95 at your exact temperature.