Choosing a formula
For engineering and reference work, use IAPWS-95 — the international standard, accurate to ±0.025%. For a fast meteorological estimate near ambient conditions, Buck (1996) and the Magnus (Alduchov–Eskridge) form are within a few tenths of a percent. The Antoine equation, with NIST’s range-specific constants, is the chemistry-lab standard. Older forms like Tetens and Goff–Gratch remain useful for historical or atmospheric work. The comparison table above makes the trade-offs visible at your exact temperature.
Frequently asked questions
Which water vapor pressure formula is most accurate?
IAPWS-95 (the Wagner–Pruss formulation) is the international reference standard, accurate to better than ±0.025% from the triple point to the critical point — use it as the ground truth. Of the simple closed-form equations, Buck (1996) is the best, within about 0.1% through 100 °C. The Magnus (Alduchov–Eskridge) form is excellent for meteorology between −40 and +60 °C but drifts above that. This calculator shows every formula's deviation from IAPWS-95 so you can choose with eyes open.
What is saturation vapor pressure?
It's the pressure exerted by water vapor when the air is fully saturated (100% relative humidity) at a given temperature — the point where evaporation and condensation balance. It depends only on temperature, rising steeply as temperature increases. Relative humidity is the ratio of the actual vapor pressure to this saturation value.
Does water really boil at 100 °C?
Almost — and the small discrepancy is a useful credibility check. The old definition fixed 100 °C as boiling at 1 atm (101.325 kPa). On the modern ITS-90 temperature scale, IAPWS-95 gives a saturation pressure of 101.42 kPa at exactly 100 °C, which means water actually boils at 1 atm at about 99.97 °C. Both are correct on their respective scales.
What about temperatures below freezing — dew point or frost point?
Below 0 °C, switch the calculator to 'over ice' to get the frost point (saturation over ice), computed with the reference-grade Murphy–Koop equation. Saturation over supercooled liquid water is different and slightly higher; meteorological relative humidity is conventionally reported over liquid water even below freezing (WMO convention).
What units can I use?
Input temperature in °C, °F, or K, and read the result in Pa, hPa/mbar, kPa, MPa, bar, mmHg (Torr), atm, psi, or inHg. Every value is computed live in your browser — nothing is uploaded.
References
Every formula on this page is implemented from, and validated against, the following primary standards and papers.
- IAPWS R6-95(2018) / Wagner & Pruss 2002 — International Association for the Properties of Water and Steam — the reference standard
- Humidity Conversion Equations (rev. 7/96) — Buck Research CR-1A manual — the Buck 1996 coefficients
- Improved Magnus Form Approximation of Saturation Vapor Pressure — Alduchov & Eskridge 1996, J. Appl. Meteorol. 35:601
- Goff–Gratch equation (Smithsonian Met. Tables 1984) — Vömel (CIRES) tabulation — corrected form
- NIST Chemistry WebBook (SRD 69) — Water — NIST — Antoine constants & validation reference
- Review of the vapour pressures of ice and supercooled water — Murphy & Koop 2005, QJRMS 131:1539
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