Boiling Point Calculator
Find the boiling point of water at any altitude or atmospheric pressure, or calculate how much a dissolved solute raises the boiling point of a solvent. Switch between altitude and pressure entry, choose metric or imperial units, and see the full worked calculation including the barometric formula and the ebullioscopic constant.
Why does altitude change the boiling point of water?
Water boils when its vapour pressure equals the surrounding atmospheric pressure. At sea level that pressure is about 1013 hPa, so water boils at 100 °C (212 °F). As you climb higher, air pressure falls because there is less atmosphere above you pushing down. With lower external pressure, water molecules need less energy to escape into vapour, so boiling happens at a lower temperature. For every 300 m (roughly 1,000 ft) you ascend, the boiling point drops by about 1 °C (1.8 °F). At the summit of Mount Everest (8,849 m), pressure is only about 337 hPa and water boils at roughly 70 °C (158 °F).
How the altitude boiling-point formula works
This calculator uses two linked equations. First, the barometric formula estimates atmospheric pressure from altitude: P = 29.921 × (1 - 0.0000068753 × h_ft)^5.2559 (pressure in inHg). That pressure is then fed into an empirical boiling-point equation: BP_F = 49.161 × ln(P_inHg) + 44.932, which is a logarithmic fit to the Clausius-Clapeyron relation for water over the range of pressures found between sea level and the summit of Everest. The result is accurate to within about 0.5 °C across that range. You can also enter a pressure directly to skip the altitude step - useful when you have a weather station or barometric reading on hand.
Boiling point elevation: how solutes raise the boiling point of a solution
When you dissolve a substance in a solvent, the solute particles lower the vapour pressure of the liquid (Raoult's law). The solution therefore needs a higher temperature to reach the surrounding pressure, which means it boils at a higher temperature than the pure solvent. This effect is described by the formula DeltaT = i × Kb × m, where i is the Van't Hoff factor (how many ions or particles the solute produces), Kb is the ebullioscopic constant of the solvent (a fixed property, e.g. 0.512 °C·kg/mol for water), and m is the molality in moles of solute per kilogram of solvent. Because the effect depends on the number of dissolved particles, not their identity, it is called a colligative property. Dissolving 1 mol/kg of sugar (i = 1) raises water's boiling point by 0.512 °C, while 1 mol/kg of NaCl (i = 2) raises it by about 1.024 °C.
Practical effects on cooking and food safety
Boiling-point changes have real consequences in the kitchen and in food safety. At altitudes above about 2,000 m, water boils cool enough that recipes must be adjusted: pasta and rice need longer cooking times, bread rises faster (less time for gluten to develop), and candy or fudge recipes calibrated by temperature may set at the wrong consistency. Critically, for food safety, boiling water at very high altitude may not kill all pathogens reliably if the temperature never reaches the required threshold - the CDC recommends that at elevations above 2,000 m (6,500 ft), water should be boiled for at least one full minute, and above 5,000 m (16,400 ft) for three minutes, rather than relying solely on the rolling boil that signals doneness at sea level.
Boiling point of water at notable altitudes
| Location | Altitude (m) | Pressure (hPa) | Boiling point (°C) | Boiling point (°F) |
|---|---|---|---|---|
| Sea level | 0 | 1013.2 | 100 | 212 |
| Denver, CO (5,280 ft) | 1609 | 834.3 | 94.7 | 202.5 |
| Mexico City | 2240 | 771.5 | 92.6 | 198.6 |
| Machu Picchu | 2430 | 753.4 | 91.9 | 197.4 |
| Bogota, Colombia | 2625 | 735.2 | 91.2 | 196.2 |
| La Paz, Bolivia | 3640 | 645.9 | 87.7 | 189.9 |
| Lhasa, Tibet | 3656 | 644.5 | 87.7 | 189.8 |
| Everest Base Camp | 5364 | 514.4 | 81.5 | 178.7 |
| Mount Everest summit | 8849 | 314.4 | 68 | 154.5 |
Calculated using the standard barometric formula. Values at sea level are exact; others are rounded to one decimal place.
Frequently asked questions
At what temperature does water boil at 5,000 feet (1,524 m)?
At 5,000 feet (approximately 1,524 m), atmospheric pressure is about 843 hPa and water boils at roughly 95 °C (203 °F). The pressure and boiling-point fields in this calculator will both update as you change the altitude.
Does salt water boil at a higher temperature than fresh water?
Yes, but the effect is small. Dissolving common salt (NaCl) at 1 mol/kg concentration raises the boiling point of water by about 1.02 °C with a Van't Hoff factor of 2. Typical cooking salt concentrations are much lower, so the practical effect on cooking time is negligible - usually less than half a degree.
What is the Clausius-Clapeyron equation?
The Clausius-Clapeyron equation describes how the vapour pressure of a substance changes with temperature: d(ln P)/dT = DeltaH_vap / (R × T^2). For practical boiling-point calculations from altitude, the integrated form or empirical logarithmic fits are commonly used, which is what this calculator does. The calctool.org Clausius-Clapeyron calculator lets you work with two arbitrary pressures and the heat of vaporisation if you need that level of detail.
What is the Van't Hoff factor and how do I choose it?
The Van't Hoff factor i is the number of separate particles produced when one formula unit dissolves. Non-electrolytes like sugar or glucose do not dissociate (i = 1). Strong electrolytes like sodium chloride produce two ions (Na+ and Cl-, so i = 2), calcium chloride produces three (Ca2+ and 2 Cl-, so i = 3), and aluminium chloride four. Real solutions deviate slightly from ideal i values due to ion-pair formation, so use theoretical values for exam problems and measured values for precise work.
Why is the boiling point different for different solvents?
Each solvent has its own boiling point because of the intermolecular forces between its molecules. Water boils at 100 °C because its hydrogen bonds are relatively strong and need more energy to break. Ethanol (78.4 °C) has weaker forces; ether or pentane would be even lower. The ebullioscopic constant Kb also varies because it reflects how strongly the solvent's vapour pressure responds to added solute, which depends on the solvent's enthalpy of vaporisation and molar mass.
Can the boiling point be lower than sea level?
In the altitude-pressure sense, the boiling point is only lower than 100 °C at altitude because pressure is reduced. If you could somehow apply pressure above sea-level standard (a pressure cooker does exactly this), water boils above 100 °C - at 2 atm it boils at about 121 °C. Conversely, you cannot go below sea level very far; the Dead Sea at -430 m has a slightly higher pressure and boiling point of about 100.13 °C, a difference too small to matter in practice.