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WaterVaporPressure

Antoine Equation Calculator

Compute the vapor pressure of water from the Antoine equation using NIST’s range-specific constants. The calculator picks the correct A/B/C set for your temperature, shows the worked equation, outputs the result in any unit, and reports how far it deviates from the IAPWS-95 reference.

Vapor pressure of water · Antoine (NIST)
3.1667 kPa
31.667 hPa23.753 mmHg0.031667 bar0.031253 atm3166.7 Pa
log₁₀(P/bar) = 5.4022 − 1838.7 / (298.15 − 31.737) = -1.4994
P = 10^-1.4994 bar = 0.031667 bar
Active NIST set: 273–303 K (Bridgeman & Aldrich 1964)-0.097% vs IAPWS-95

NIST Antoine constants for water

log₁₀(P/bar) = A − B/(T + C), with T in kelvin and P in bar. The set covering your temperature is highlighted.

Range (K)ABCSource
273–303active5.402211838.675-31.737Bridgeman & Aldrich 1964
304–3335.203891733.926-39.485Bridgeman & Aldrich 1964
334–3635.07681659.793-45.854Bridgeman & Aldrich 1964
344–3735.083541663.125-45.622Bridgeman & Aldrich 1964
379–5733.55959643.748-198.043Liu & Lindsay 1970

Constants from the NIST Chemistry WebBook (SRD 69). The 379–573 K set (Liu & Lindsay) must never be used below 100 °C — it is wildly wrong there; this tool auto-selects the correct set for you.

Using the Antoine equation correctly

The Antoine equation, log₁₀(P) = A − B/(T + C), is deceptively easy to get wrong. Two traps dominate. First, units: the NIST constants for water return pressure in bar with temperature in kelvin; plug in °C, or read the answer as atm, and you are off by orders of magnitude. Second, range: the constants are fitted piecewise, and the high-temperature set is wildly inaccurate near room temperature. This tool handles both for you — it converts from bar to your chosen unit and auto-selects the right NIST set, flagging when your temperature is outside the validated band.

For reference-grade work, the IAPWS-95 formulation is more accurate than any Antoine fit, which is why this site shows the deviation from it on every result. Antoine remains the right tool when you need a transparent, hand-checkable equation with published constants — the chemistry-lab and textbook standard.

Frequently asked questions

What is the Antoine equation?

The Antoine equation is a three-parameter correlation between the vapor pressure of a pure substance and temperature: log₁₀(P) = A − B/(T + C). The constants A, B and C are fitted to experimental data for each substance and each temperature range. It is the workhorse of chemical engineering for estimating vapor pressure, because it is simple, invertible (you can solve for the boiling temperature at a given pressure), and accurate within its fitted band.

What units does the NIST Antoine equation for water use?

NIST's Chemistry WebBook lists Antoine constants for water with temperature in kelvin and pressure in bar — not °C, atm, mmHg or kPa. This is the single most common mistake in online Antoine implementations. This calculator takes that bar output and converts it to whatever unit you choose, so the result is correct in Pa, kPa, mmHg, atm and the rest.

Why are there several sets of Antoine constants for water?

A single A/B/C triple cannot fit the vapor-pressure curve of water accurately across its whole liquid range, so NIST publishes range-specific sets: four overlapping Bridgeman & Aldrich (1964) sets covering roughly 273–373 K, and a Liu & Lindsay (1970) set for 379–573 K. Using the high-temperature set near room temperature gives errors of tens of percent. This tool auto-selects the correct set for your temperature and shows which one it used.

How accurate is the Antoine equation compared with IAPWS-95?

Within the Bridgeman & Aldrich sets (about 0–100 °C), the Antoine equation is accurate to roughly 0.1% against the IAPWS-95 reference — excellent for lab and teaching work. The high-temperature set is rougher, reaching several percent toward 300 °C. The calculator displays the live deviation from IAPWS-95 so you always know how much to trust the number at your temperature.

References

Every formula on this page is implemented from, and validated against, the following primary standards and papers.

Reviewed by Jimmy Raymond, Engineer
B.S. Environmental Engineering · B.S. Computer Science · Last reviewed June 4, 2026

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