Vapour Pressure of Water Calculator
Find the equilibrium vapor pressure of liquid water or ice at any temperature from −100 °C to the critical point (374 °C). Results are given in Pa, kPa, bar, atm, and mmHg, with a cross-check against the Buck (1981) equation.
Temperature unit
°C
Liquid water — NIST Antoine / Stull (1947), 1–100 °C
- 1
Denominator C + T
233.426 + 25 = 258.426 - 2
Antoine log₁₀(P/mmHg)
8.07131 − 1,730.63 ÷ 258.426 = 1.374499 - 3
Vapor pressure (mmHg)
10^1.374499 = 23.6864 - 4
Vapor pressure (kPa)
23.6864 × 0.133322368 = 3.1579
How does this calculator work?
The vapor pressure of liquid water follows log₁₀(P/mmHg) = 8.07131 − 1730.63/(233.426 + T°C) from 1 to 100 °C (NIST/Stull 1947), with separate Antoine coefficients to 150 °C and the Sonntag (1990) ice formula below 0 °C. Enter a temperature (°C or K) to get pressure in Pa, kPa, bar, atm, and mmHg, plus the complete pressure-temperature curve.
Formula
How this is calculated
Water has three regimes for vapor pressure: over ice (below 0 °C), liquid (0–100 °C at 1 atm, up to 374 °C under pressure), and supercritical above 374 °C (where liquid and vapour become indistinguishable). This calculator covers the first two with equation sets matched to the thermodynamic phase.
For liquid water between 1 and 100 °C, the NIST Antoine equation (Stull 1947 constants: A = 8.07131, B = 1730.63, C = 233.426) gives vapor pressure in mmHg from log₁₀(P) = A − B/(C + T°C). A second Antoine set (A = 8.14019, B = 1810.94, C = 244.485) extends coverage to 150 °C. For sub-zero temperatures, the Sonntag (1990) equation is used for ice-surface vapor pressure, which is lower than liquid water vapor pressure at the same temperature — this is why ice sublimes in dry air. A Buck (1981) cross-check is also shown for 0–60 °C.
The calculator shows the full vapor pressure curve from −50 to 150 °C. The normal boiling point — where vapor pressure equals 1 atm (101.325 kPa) — appears at approximately 100 °C. At altitude, where atmospheric pressure is lower, water boils at a lower temperature.
Frequently asked questions
Water molecules in a crystal lattice are held more tightly than in the liquid, so fewer can escape into the vapour phase. Below 0 °C, if liquid water could be supercooled, its vapor pressure would be higher than that of ice — this is why supercooled cloud droplets can evaporate while ice crystals grow (the Bergeron process in cloud physics).
At the critical point (374.14 °C, 220.64 bar) liquid and vapour water become indistinguishable and the distinction between phases disappears. Above this temperature only supercritical fluid exists regardless of pressure, so there is no meaningful vapor pressure. This calculator's valid range stops at 374 °C for this reason.
The NIST Antoine equation agrees with published steam tables to within ±0.5 % over its valid ranges. For precision engineering use certified IAPWS-95 steam tables. This calculator is suitable for chemistry, physics, and educational purposes.
Also known as
TG we-Calculate Editorial Team. (2026). Vapour Pressure of Water Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/vapour-pressure-of-water-calculator
TG we-Calculate Editorial Team. "Vapour Pressure of Water Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/vapour-pressure-of-water-calculator.
TG we-Calculate Editorial Team, "Vapour Pressure of Water Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/vapour-pressure-of-water-calculator
@misc{wecalculate_vapour_pressure_of_water_calculator, title = {Vapour Pressure of Water Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/vapour-pressure-of-water-calculator}}, year = {2026}, note = {TG we-Calculate} }
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