Heat capacity ratio of steam
| Value | 1.33 — |
| Status | Reference value; the source states no uncertainty |
| Source | IAPWS-95 (IAPWS R6-95(2018)), computed with CoolProp: 1.337 for water vapour at 100 °C and 1 atm |
| Categories | ThermodynamicEngineering & Trade |
| Unit | Value |
|---|---|
| ratio | 1.33 — |
| percent | 133 % |
| part per trillion | 1,330,000,000,000 ppt |
| part per billion | 1,330,000,000 ppb |
| part per million | 1,330,000 ppm |
| per cent mille | 133,000 pcm |
| basis point | 13,300 bp |
| per mille | 1,330 per mille |
Learning zone
Water vapour is a bent triatomic molecule with three rotational degrees of freedom instead of two, so cv is larger and γ falls to about 1.33 near 100 °C and atmospheric pressure. It keeps sliding downward with pressure and temperature as vibrational modes contribute, reaching roughly 1.28 in superheated steam at typical turbine inlet conditions.
The number matters in steam nozzle and safety-valve sizing, where the critical pressure ratio for choked flow is [2/(γ+1)]^(γ/(γ−1)): 0.545 for steam against 0.528 for air. Steam engineering also uses polytropic exponents that look like γ but are not: n ≈ 1.135 for wet steam and 1.3 for superheated steam are Zeuner's empirical expansion exponents, not ratios of specific heats.
Cite this page
formula.expert. (2026, November 1). Heat capacity ratio of steam. https://formula.expert/constants/heat-capacity-ratio-steam