A practical near-reference equation
Arden Buck's equations come from the Buck Research CR-1A hygrometer manual; the 1996 revision refined the coefficients he first published in 1981. The form is an enhanced Magnus-type exponential whose extra temperature term in the numerator buys a large accuracy gain for almost no extra cost. Across −80 to +50 °C it stays within roughly 0.1% of IAPWS-95, and it remains excellent through 100 °C.
That combination — one exponential, six tidy constants, near-reference accuracy — is why the Buck form shows up inside humidity sensors and meteorological software. On this site it is the recommended choice when you want IAPWS-grade numbers without evaluating the full reference correlation.
1996, not 1981
The widely copied 1981 coefficients are superseded. They are still floating around the web and in old code, but the 1996 set is more accurate and is the one you should use; this site exposes the 1981 form only as a clearly labeled legacy option for reproducing older output.
For sub-zero work, Buck also published a companion set of constants over ice — the main calculator switches to those automatically in its 'over ice' mode.
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.
- Humidity Conversion Equations (rev. 7/96) — Buck Research CR-1A manual — the Buck 1996 coefficients
- IAPWS R6-95(2018) / Wagner & Pruss 2002 — International Association for the Properties of Water and Steam — the reference standard
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