The previous reference standard
Before IAPWS-95, the Goff–Gratch equation was the reference adopted by the World Meteorological Organization and tabulated in the Smithsonian Meteorological Tables. It is a multi-term logarithmic expression valid from about −50 to +102 °C over water, and it is still encountered throughout atmospheric science and in instruments calibrated decades ago.
There is a transcription trap worth knowing: the 1988 WMO reprint introduced a sign error in one term. The form implemented here follows the corrected Smithsonian (1984) tabulation, not the WMO-1988 reprint.
When to use it
Use Goff–Gratch when you need continuity with historical data, older literature, or a standard that specifically calls for it. For new work it offers no accuracy advantage over IAPWS-95 and is considerably more cumbersome to evaluate.
The comparison table on the main calculator shows its small but real deviation from IAPWS-95 at any temperature you choose.
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.
- Goff–Gratch equation (Smithsonian Met. Tables 1984) — Vömel (CIRES) tabulation — corrected form
- IAPWS R6-95(2018) / Wagner & Pruss 2002 — International Association for the Properties of Water and Steam — the reference standard
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