Clausius-Clapeyron vapor pressure calculator

Reviewed by the CalculatorHub team, edited by James Graham. Method from NIST Chemistry WebBook. Last verified 2026-06-23.

The Clausius-Clapeyron vapor pressure calculator computes clausius-clapeyron vapor pressure from the relation ln(P2/P1) = -(dHvap/R)(1/T2 - 1/T1). It takes 4 inputs (known pressure in Pa, enthalpy of vaporisation in J/mol, known temp in K, target temp in K) and returns the vapor pressure in Pa. Because this is a pure mathematical or physical formula rather than a jurisdiction-specific rule, the result never changes over time: the same inputs always produce the same answer, so you can rely on it whether you are checking homework, sizing a design, or sanity-checking another tool. Enter your values in the fields below and the result updates instantly; you can also share a permalink that pre-fills the exact calculation, which is useful for teaching, reports, or collaboration. For example, with known pressure = 101325 Pa, enthalpy of vaporisation = 40700 J/mol, known temp = 373.15 K, target temp = 373.15 K, the vapor pressure works out to 101325 Pa, and the worked example further down the page shows every step so you can follow the arithmetic and reproduce it by hand. The method is the standard form documented by NIST Chemistry WebBook, and the figure above each result carries the date it was last verified. This tool is general information and is not a substitute for professional engineering, medical, financial, or scientific advice; always check critical results against the primary source and your own judgement.