# Boiling Point Calculator (Clausius–Clapeyron)

URL: https://speedycalculator.com/calculators/chemistry/boiling-point/
Last updated: 2026-10-30

> The boiling point calculator uses the Clausius–Clapeyron equation to find the temperature at which a liquid boils at any pressure, from a known boiling point and the enthalpy of vaporization – with built-in data for water, ethanol, acetone, ammonia, propane and other common substances.

## Inputs

- Substance
- Enthalpy of vaporization ΔHvap (kJ/mol, J/mol, kcal/mol, cal/mol, eV (per particle))
- Known boiling point T₁ (°C, K, °F)
- Pressure at the known boiling point P₁ (kPa, atm, bar, hPa (mbar), mmHg (Torr), psi, inHg, Pa)
- New pressure P₂ (kPa, atm, bar, hPa (mbar), mmHg (Torr), psi, inHg, Pa)

## Results

- Boiling point at P₂
- Change from the reference boiling point
- Reference boiling point T₁
- Reference pressure P₁
- Enthalpy of vaporization used
- Pressure ratio P₂/P₁
- Direction

## Formula

```
ln(P₂/P₁) = −(ΔHvap/R) · (1/T₂ − 1/T₁)  ⇒  T₂ = 1 / (1/T₁ − R·ln(P₂/P₁)/ΔHvap) · R = 8.314 J/(mol·K), temperatures in kelvin
```

## Explanation

A liquid boils when its vapour pressure reaches the pressure of its surroundings. Lower the outside pressure – on a mountain, in a vacuum distillation flask or inside a freeze-dryer – and bubbles form at a lower temperature; raise it, as in a pressure cooker or a steam boiler, and the liquid must get hotter before it boils. This is why water boils at about 100 °C at sea level but near 90 °C at 3,000 m.

The Clausius–Clapeyron equation links the two pressures and temperatures through one property of the liquid: its molar enthalpy of vaporization ΔHvap, the energy needed to turn one mole of liquid into vapour. Substances with strong intermolecular forces (hydrogen-bonded water, alcohols) have a large ΔHvap and their boiling point shifts slowly with pressure; weakly bound liquids such as methane or hydrogen shift quickly.

Pick a substance to use its normal boiling point at 1 atm and its tabulated ΔHvap as the reference point, or choose Custom to enter any known boiling point, its pressure and your own enthalpy of vaporization. The result is most accurate when the new pressure is not too far from the reference, because ΔHvap itself changes slowly with temperature.

### How to use the Boiling Point Calculator

- **Substance**
- **Enthalpy of vaporization ΔHvap**
- **Known boiling point T₁**
- **Pressure at the known boiling point P₁**
- **New pressure P₂**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Convert the reference boiling point to kelvin, take the natural logarithm of the pressure ratio P₂/P₁, multiply by R/ΔHvap and subtract the product from 1/T₁. The reciprocal of that number is the new boiling point in kelvin, which the calculator converts to the unit you choose.

### Worked example

At about 3,000 m the air pressure is near 70 kPa. For water, T₁ = 99.97 °C = 373.12 K at P₁ = 101.325 kPa, and ΔHvap = 40,650 J/mol. ln(70/101.325) = −0.3699, so 1/T₂ = 1/373.12 − 8.314 × (−0.3699)/40,650 = 0.0026801 + 0.0000757 = 0.0027558 K⁻¹. T₂ = 362.88 K = **89.7 °C**, about 10 °C below the sea-level boiling point.

### Reference data used for the presets

SubstanceNormal boiling point (1 atm)ΔHvap at the boiling pointWater99.97 °C40.65 kJ/molEthanol78.29 °C38.56 kJ/molMethanol64.7 °C35.21 kJ/molAcetone56.05 °C29.10 kJ/molBenzene80.09 °C30.72 kJ/molToluene110.63 °C33.18 kJ/moln-Hexane68.73 °C28.85 kJ/molDiethyl ether34.5 °C26.52 kJ/molChloroform61.17 °C29.24 kJ/molAmmonia−33.33 °C23.33 kJ/molMethane−161.48 °C8.19 kJ/molPropane−42.1 °C19.04 kJ/moln-Butane−0.5 °C22.44 kJ/molHydrogen−252.87 °C0.90 kJ/molValues are the normal boiling points and enthalpies of vaporization at the boiling point as listed in the CRC Handbook of Chemistry and Physics and the NIST Chemistry WebBook.

### When to use it

Use this page to predict boiling points for vacuum distillation, rotary evaporation, pressure cooking or storage of liquefied gases such as propane and butane. For water at different altitudes, the [boiling point at altitude calculator](/chemistry/boiling-point-altitude/) works out the air pressure for you; to go the other way – vapour pressure at a given temperature – use the [vapor pressure calculator](/chemistry/vapor-pressure/).

### Inputs explained

FieldWhat to enterSubstanceChoose one: Custom – enter your own values, Water (bp 99.97 °C, ΔHvap 40.65 kJ/mol), Ethanol (bp 78.29 °C, ΔHvap 38.56 kJ/mol), Methanol (bp 64.7 °C, ΔHvap 35.21 kJ/mol) …Enthalpy of vaporization ΔHvapUnits: kJ/mol, J/mol, kcal/mol, cal/mol, eV (per particle)Known boiling point T₁Units: °C, K, °FPressure at the known boiling point P₁Units: kPa, atm, bar, hPa (mbar), mmHg (Torr), psi …New pressure P₂Units: kPa, atm, bar, hPa (mbar), mmHg (Torr), psi …

### What each result means

- **Change from the reference boiling point** = [Boiling point at P₂] - ([tref] - 273.15)
- **Reference boiling point T₁** = [tref] - 273.15
- **Reference pressure P₁** = [pref]
- **Enthalpy of vaporization used** = [Hv]
- **Pressure ratio P₂/P₁** = [New pressure P₂]÷[pref]

## Worked example

Inputs — Substance: Water (bp 99.97 °C, ΔHvap 40.65 kJ/mol); New pressure P₂: 70 kPa.

Results — **Boiling point at P₂ = 89.728 °C**; Change from the reference boiling point = -10.242 °C (= K); Reference boiling point T₁ = 99.97 °C; Reference pressure P₁ = 101.325 kPa; Enthalpy of vaporization used = 40.65 kJ/mol; Pressure ratio P₂/P₁ = 0.6908; Direction = Lower pressure – the liquid boils at a lower temperature.

## Key terms

- **Boiling point**: The temperature at which the vapour pressure of a liquid equals the surrounding pressure.
- **Normal boiling point**: The boiling point at exactly 1 atm (101.325 kPa).
- **Enthalpy of vaporization (ΔHvap)**: The heat absorbed when one mole of liquid turns into vapour at constant temperature and pressure.
- **Clausius–Clapeyron equation**: A relation between vapour pressure and temperature derived from the Clapeyron equation, assuming an ideal-gas vapour and a constant ΔHvap.

## FAQ

**What is the boiling point of water at 70 kPa?**
About 89.7 °C by Clausius–Clapeyron with ΔHvap = 40.65 kJ/mol; steam tables give 89.9 °C, so the estimate is within 0.2 °C.

**Why does the boiling point drop when pressure drops?**
A liquid boils when its vapour pressure equals the outside pressure. At lower outside pressure, a lower vapour pressure – and therefore a lower temperature – is enough.

**What temperature does water reach in a pressure cooker?**
A typical cooker adds about 100 kPa (15 psi) of gauge pressure, so the absolute pressure is about 200 kPa and water boils at roughly 120 °C.

**How accurate is the Clausius–Clapeyron equation?**
It assumes ΔHvap is constant and the vapour is an ideal gas. Within ±30 °C of the reference point the error is usually below 1 °C; far from it (or near the critical point) the error grows.

**Which value of ΔHvap should I use?**
Use the enthalpy of vaporization at or near the known boiling point. Water's is 40.65 kJ/mol at 100 °C but 44.0 kJ/mol at 25 °C.

**Can I use any pressure unit?**
Yes – only the ratio P₂/P₁ enters the equation, and each field has its own unit menu, so mixing kPa and mmHg is fine.

**Why do the presets use 1 atm?**
Tabulated "normal boiling points" are defined at exactly 1 atm (101.325 kPa); the newer "standard boiling point" uses 1 bar, which is about 0.4 °C lower for water.


## Sources

- [NIST Chemistry WebBook](https://webbook.nist.gov/chemistry/)
- [OpenStax – Chemistry 2e](https://openstax.org/details/books/chemistry-2e)
- [LibreTexts Chemistry](https://chem.libretexts.org/)