About

Water boils at 100°C only at sea level where atmospheric pressure equals 1013.25hPa. At 3000m elevation, pressure drops to approximately 701hPa, reducing water's boiling point to 90°C. This matters: eggs take longer to hard-boil in Denver, pressure cookers become essential in La Paz, and industrial distillation processes require recalibration. The relationship between pressure and boiling point follows the Clausius-Clapeyron equation, which models phase transition thermodynamics using the liquid's enthalpy of vaporization ΔH_vap.

This calculator implements the barometric formula to derive atmospheric pressure from altitude, then applies Clausius-Clapeyron to compute the adjusted boiling temperature. Results assume standard atmospheric conditions and pure substances. Real-world factors like dissolved solutes, local weather systems, and humidity introduce deviations of 0.5 - 2°C.

Formulas

Atmospheric pressure decreases exponentially with altitude according to the barometric formula:

P = P₀ × exp(−MghRT)

Where P₀ = 1013.25hPa (sea level pressure), M = 0.02896kg/mol (molar mass of air), g = 9.80665m/s², h is altitude in meters, R = 8.314J/(mol·K), and T = 288.15K (standard temperature).

The Clausius-Clapeyron equation relates vapor pressure to temperature:

ln(P₂P₁) = ΔH_vapR × (1T₁ − 1T₂)

Solving for T₂ (boiling point at altitude):

T₂ = 11T₁ − R × ln(P₂/P₁)ΔH_vap

Where T₁ is normal boiling point in Kelvin, P₁ = 1013.25hPa, P₂ is pressure at altitude, and ΔH_vap is enthalpy of vaporization in J/mol.

Reference Data

Liquid	Normal B.P. (°C)	ΔH_vap (kJ/mol)	B.P. at 2000m (°C)	B.P. at 4000m (°C)
Water	100.0	40.66	93.3	86.0
Ethanol	78.4	38.56	71.9	64.9
Methanol	64.7	35.21	58.4	51.7
Acetone	56.1	31.30	50.0	43.5
Diethyl Ether	34.6	26.52	28.8	22.7
Benzene	80.1	30.72	72.9	65.3
Toluene	110.6	33.18	103.0	94.9
Hexane	68.7	28.85	61.8	54.5
Chloroform	61.2	29.24	54.4	47.2
Acetic Acid	118.1	23.70	109.7	100.9
Isopropanol	82.5	39.85	76.1	69.2
Butanol	117.7	43.29	110.9	103.6
Glycerol	290.0	61.00	281.8	272.9
Ammonia	-33.3	23.35	-39.3	-45.7
Carbon Disulfide	46.2	26.74	40.3	34.0
Formic Acid	100.8	22.69	92.6	84.0

Frequently Asked Questions

Water boils at a lower temperature due to reduced atmospheric pressure. At 2500m, water boils at approximately 91.3°C instead of 100°C. Since cooking speed depends on temperature, not merely boiling status, food requires 25-50% longer cooking times. A pressure cooker restores internal pressure to sea-level equivalent, enabling normal cooking temperatures.

The simplified barometric formula assumes isothermal atmosphere at 288.15K (15°C) and provides accuracy within ±2% for altitudes below 11,000m. Real atmospheric pressure varies with weather systems, humidity, and temperature gradients. For critical applications, use local barometric readings from weather stations rather than altitude-derived estimates.

The equation assumes ideal behavior and constant enthalpy of vaporization across the temperature range. This holds reasonably well for most organic solvents and water within ±20°C of the normal boiling point. For liquids with strong hydrogen bonding or near their critical points, deviations of 1-3°C may occur. Associating liquids like acetic acid require modified parameters.

Atmospheric pressure increases below sea level. At the Dead Sea (approximately -430m), pressure reaches about 1065 hPa, raising water's boiling point to approximately 101.4°C. The calculator handles negative altitudes using the same barometric formula, which remains valid for modest depths. Deep mines may experience pressures exceeding 1100 hPa.

Boiling point elevation from solutes adds to the altitude effect. Saturated salt water (approximately 26% NaCl) raises boiling point by about 3.8°C. At sea level this yields 103.8°C; at 3000m altitude, the combined boiling point would be approximately 94.1°C (base 90.3°C + 3.8°C elevation). The calculator assumes pure substances; add solute corrections separately.

Yes. The calculator provides a pressure override option. Enter the local barometric reading in hPa (millibar) directly. This accounts for weather-induced pressure variations and provides more accurate results than altitude-based estimation. Ensure you use absolute pressure, not altitude-corrected station pressure reported by weather services.