Vapour Pressure Calculator
How a liquid's vapour pressure changes with temperature — the Clausius-Clapeyron equation, and the boiling point at any pressure.
ln(P₂/P₁) = −ΔHvap/R × (1/T₂ − 1/T₁).
How the vapour pressure calculator works
ln(P₂/P₁) = −ΔHvap/R × (1/T₂ − 1/T₁). Vapour pressure rises exponentially with temperature, which is why a small rize in temperature produces a large rize in evaporation.
Run in reverse it gives the boiling point at altitude. Water boils at 100 °C at sea level and about 93 °C at 2,000 m, which is why high-altitude cooking takes longer.
Formula: ln(P₂/P₁) = −ΔHvap/R (1/T₂ − 1/T₁)
Worked examples
| Inputs | Result | Note |
|---|---|---|
| Water at 80 °C | 48.20216 | 47.4 kPa |
| Boiling point at 80 kPa | 93.39719 | about 93 °C — roughly 2,000 m altitude |
| Finding ΔHvap | 41.61934 | about 40.7 kJ/mol |
FAQFrequently asked questions
What is the Clausius-Clapeyron equation?
It relates a liquid's vapour pressure to temperature through its enthalpy of vaporization.
Why does water boil below 100 °C at altitude?
Because boiling happens when vapour pressure equals ambient pressure. Lower ambient pressure means a lower temperature suffices — about 93 °C at 2,000 m.
Why is high-altitude cooking slower?
Because the water is cooler even though it is boiling. Cooking rate depends on temperature, not on whether bubbles are forming.
How accurate is it?
Good over modest temperature ranges, where ΔHvap is roughly constant. Over wide ranges the enthalpy itself changes and the Antoine equation fits better.
Why does a pressure cooker work?
It raises the pressure to about 200 kPa, which lifts the boiling point to roughly 120 °C — and reaction rates roughly quadruple over that rize.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.