The original Buck coefficients
Buck's 1981 paper introduced a set of practical vapor-pressure equations that were quickly adopted in hygrometers, datasheets, and atmospheric software. This is the simpler, pure Magnus-type form — a single exponential with two constants (17.502 and 240.97 °C) and the 611.21 Pa pre-factor. Near room temperature it is accurate to a few tenths of a percent against IAPWS-95.
In 1996 Buck revised the coefficients and added an extra temperature term in the numerator, which noticeably improves accuracy toward the edges of the range. That 1996 form is the one you should use today; the 1981 set is kept here only for compatibility.
Why it is still here — and when to use it
The 1981 coefficients are widely copied across the web and embedded in older code and instrument firmware. If you need to reproduce or validate the output of a system that used them, this page and the main calculator's labeled legacy option compute exactly that form.
For any new work, prefer the modern Buck (1996) equation, or IAPWS-95 for reference-grade accuracy. The comparison table on the main calculator shows how far the 1981 form drifts from IAPWS-95 at your temperature.
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.
- New Equations for Computing Vapor Pressure and Enhancement Factor — Buck 1981, J. Appl. Meteorol. 20:1527 — the superseded 1981 coefficients
- 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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