The reference every other formula is measured against
IAPWS-95 is the formulation adopted by the International Association for the Properties of Water and Steam as the standard for the thermodynamic properties of ordinary water. The saturation-pressure correlation published by Wagner and Pruss (2002) reproduces the experimental vapor-pressure curve from the triple point (273.16 K) to the critical point (647.096 K) to better than ±0.025% — closer to reality than any simple closed form can manage.
This page uses the compact auxiliary equation (Wagner & Pruss Eq. 2.5), which needs only six coefficients and the exp/pow functions — not the full 56-term Helmholtz energy equation of state. The leading (Tc/T) multiplier in front of the polynomial is essential: dropping it is a common transcription bug that shifts the whole curve.
When to use it
Use IAPWS-95 whenever the number matters: engineering design, instrument calibration, building reference tables, or simply checking another formula. It is the baseline for the deviation column in this site's main calculator, and the ground truth its unit tests assert against.
The only cost over a one-line Magnus form is a handful of extra constants — negligible on any modern processor. If you have a free choice, there is rarely a reason to use anything less accurate for liquid water above freezing.
Compare with other formulas
See this and every other formula side by side, with the live deviation from IAPWS-95 at your temperature, on the main calculator. The Antoine equation has its own page.
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
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