Enter the absolute water pressure into the calculator below to determine the boiling point.
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Boiling Point Formula
The calculator uses two formulas depending on which tab you choose.
Water boiling point from pressure (Antoine equation):
T = B / (A - log10(P)) - C
- T = boiling temperature in °C
- P = absolute pressure in mmHg
- A, B, C = Antoine constants for water (A = 8.07131, B = 1730.63, C = 233.426 for P ≤ 760 mmHg; A = 8.14019, B = 1810.94, C = 244.485 above that)
Pressure from altitude (barometric formula):
P = 101.325 * (1 - 2.25577e-5 * h)^5.25588
- P = pressure in kPa
- h = elevation above sea level in meters
The result feeds into the Antoine equation to estimate the boiling point of water.
Other liquids (Clausius-Clapeyron equation):
1/T2 = 1/T1 + (R / ΔHvap) * ln(P1 / P2)
- T1 = known boiling point in K at pressure P1
- T2 = boiling point in K at the target pressure P2
- P1, P2 = reference and target pressures in the same units
- ΔHvap = heat of vaporization in J/mol
- R = 8.314462618 J/(mol·K)
The Pressure tab applies the Antoine equation directly. The Altitude tab estimates pressure from elevation, then runs the Antoine equation. The Other Liquid tab uses Clausius-Clapeyron with built-in constants for common solvents or values you supply for a custom liquid.
Reference Values
Boiling point of water at common elevations, calculated from the standard atmosphere:
| Elevation | Pressure (kPa) | Water boils at |
|---|---|---|
| Sea level (0 ft) | 101.3 | 100 °C / 212 °F |
| 1,000 ft | 97.7 | 98.9 °C / 210 °F |
| 3,000 ft | 90.8 | 96.7 °C / 206 °F |
| 5,280 ft (Denver) | 83.4 | 94.2 °C / 201.6 °F |
| 7,000 ft | 78.2 | 92.4 °C / 198.3 °F |
| 10,000 ft | 69.7 | 89.7 °C / 193.5 °F |
| 14,000 ft | 59.5 | 85.5 °C / 185.9 °F |
Normal boiling points and heats of vaporization for common liquids at 1 atm:
| Liquid | Boiling point | ΔHvap (kJ/mol) |
|---|---|---|
| Water | 100.0 °C | 40.65 |
| Ethanol | 78.37 °C | 38.56 |
| Methanol | 64.7 °C | 35.21 |
| Acetone | 56.05 °C | 31.30 |
| Isopropyl alcohol | 82.6 °C | 45.40 |
| Ammonia | -33.34 °C | 23.35 |
Example Problems and FAQ
Example 1: Water in a pressure cooker. A pressure cooker holds steam at 15 psi gauge above atmospheric, so the absolute pressure is about 29.7 psi (205 kPa). Enter 205 kPa on the Pressure tab. The Antoine equation gives a boiling point near 121 °C (250 °F). That higher temperature is why food cooks faster.
Example 2: Ethanol under vacuum. You want to distill ethanol at 0.2 atm. Select Ethanol on the Other Liquid tab, enter 0.2 atm. Using T1 = 351.52 K, P1 = 1 atm, ΔHvap = 38,560 J/mol, the calculator returns about 41 °C. Lower pressure means lower boiling temperature, which is useful for heat-sensitive compounds.
Why does water boil at a lower temperature at high altitude? Boiling happens when the liquid's vapor pressure equals the surrounding pressure. Atmospheric pressure drops with elevation, so vapor pressure matches it at a lower temperature.
How accurate is the altitude estimate? The barometric formula assumes the standard atmosphere. Real pressure varies with weather and temperature, so expect the result to be within roughly 1 to 2 °F of the actual local boiling point on most days.
When does the Clausius-Clapeyron estimate break down? The equation assumes ΔHvap is constant and the vapor behaves ideally. Accuracy is good within a factor of about 10 in pressure around the reference point. Far from that range, or near the critical point, errors grow.
What pressure should I enter, gauge or absolute? Always absolute pressure. If you have a gauge reading, add the local atmospheric pressure (about 14.7 psi or 101.3 kPa at sea level) before entering it.
