The boiling point calculator above answers a question that has no single answer: at what temperature does this liquid boil? A liquid boils when its vapour pressure equals the pressure pushing down on it, so the boiling point is a property of the liquid and the surroundings together. Change the pressure and the boiling point moves. This tool computes that relationship properly, either from Antoine coefficients published by NIST for water and ethanol, or from the Clausius-Clapeyron relation when you have one known boiling point and an enthalpy of vaporisation.
Arb Digital builds free calculators that name the model they are using, because in vapour-pressure work the model choice is most of the accuracy. The result panel states which correlation produced the number and, for the Antoine sets, which coefficient block was selected. A page that gives you a boiling point without telling you how it got there cannot be checked, and an unchecked number in this area is usually wrong by a degree or two.
What This Boiling Point Calculator Does
Pick water or ethanol and the tool uses the Antoine equation with coefficients taken from the NIST Chemistry WebBook. Because Antoine fits are only valid over limited temperature windows, several coefficient sets are stored per substance and the tool selects the block appropriate to the answer, then re-solves inside it. That two-pass approach is what keeps water at 100.00 °C at one atmosphere rather than the half-degree error a single wide-range fit produces.
Choose custom Antoine coefficients and you can enter A, B and C for any substance you have data for, using the NIST convention of pressure in bar and temperature in kelvin. Choose Clausius-Clapeyron and you supply one reference boiling point, the pressure it was measured at, and an enthalpy of vaporisation; the tool then projects to the pressure you asked about.
Conditions can be given either as a pressure in six common units or as an altitude, which is converted to pressure using the International Standard Atmosphere. The result appears in Celsius, kelvin and Fahrenheit, with the shift from the normal one-atmosphere value shown alongside so you can see the size of the effect at a glance.
How to Use It
- Choose the substance or method. Water and ethanol come with NIST coefficients loaded; anything else needs either custom Antoine data or the Clausius-Clapeyron route.
- Decide how to specify conditions. Pressure if you have a gauge reading or a target; altitude if you are asking about cooking or fieldwork somewhere high.
- Check the pressure unit. Standard atmospheric pressure is 101.325 kPa, 1.01325 bar, 1 atm, 760 mmHg or 14.696 psi — all the same condition.
- Read the method line under the answer. It names the correlation and, for Antoine, the temperature window whose coefficients were used.
- Use the bars to see the trend across altitudes rather than treating one number as the whole story.
The Formula and How It Is Calculated
The Antoine equation is log₁₀ P = A − B / (T + C), with P in bar and T in kelvin in the NIST convention used here. Rearranged for the boiling point at a given pressure it becomes T = B / (A − log₁₀ P) − C, written with C as a negative number in the tables. The coefficients for water are published on the NIST Chemistry WebBook page for water, which lists several blocks including 5.40221, 1838.675, −31.737 for 273–303 K and 5.08354, 1663.125, −45.622 for 344–373 K. Ethanol's coefficients come from the corresponding WebBook page for ethanol, where the 292.77–366.63 K block is 5.24677, 1598.673, −46.424.
Check it by hand at standard pressure. One atmosphere is 1.01325 bar, and log₁₀ of that is 0.005717. Using the 344–373 K water block, A minus that is 5.077823, and 1663.125 divided by 5.077823 gives 327.53. Subtracting C, which is −45.622, gives 373.15 K, or exactly 100.00 °C. Running ethanol's block the same way returns 351.45 K, or 78.30 °C, against a literature normal boiling point of 78.24 °C.
Clausius-Clapeyron takes a different route: ln(P₂/P₁) = −(ΔHvap/R)(1/T₂ − 1/T₁), rearranged to solve for T₂. It needs only one reference point rather than a fitted curve, which is its whole appeal, but it assumes the enthalpy of vaporisation does not change with temperature. That assumption is good over tens of degrees and poor over hundreds, since ΔHvap falls steadily toward zero at the critical point.
Why Pressure Controls Boiling, Not Temperature
Boiling is not "reaching 100 degrees". A liquid boils when bubbles of its own vapour can form and survive inside the bulk of the liquid, which requires the vapour pressure inside a bubble to match the pressure of the surroundings squeezing it shut. Vapour pressure rises steeply and non-linearly with temperature, so there is exactly one temperature at which the two balance for any given ambient pressure. That temperature is the boiling point.
This immediately explains the altitude effect. At 1,500 metres the standard atmosphere gives about 84.6 kPa, so water needs less vapour pressure to boil and reaches the balance point around 95 °C. Food cooked in that water cooks more slowly, because cooking rate depends on temperature, not on whether the water is visibly bubbling. It also explains why the fix is a pressure cooker: raising the pressure inside the vessel raises the boiling point, commonly to around 120 °C at typical operating settings.
It runs the other way too. Pull a strong enough vacuum and water boils at room temperature, which is the physical basis of freeze drying and of vacuum distillation. Heat-sensitive compounds that would decompose at their atmospheric boiling point can be distilled intact at reduced pressure because the whole process happens cooler. If you need to move between pressure units while working through any of this, the pressure converter handles the arithmetic, and the pressure calculator covers force-over-area conversions.
Antoine, Clausius-Clapeyron, and Which to Trust
Antoine coefficients are fitted to real measured vapour-pressure data over a stated temperature range. Inside that range they are typically accurate to a fraction of a percent in pressure, which translates to a fraction of a degree in boiling point. Outside it they can go badly wrong, and quietly, because the equation still returns a plausible-looking number. This is the reason the tool stores several blocks per substance and picks the right one instead of using a single fit everywhere.
Clausius-Clapeyron makes three simplifications: that the vapour behaves ideally, that the liquid's volume is negligible next to the vapour's, and that the enthalpy of vaporisation is constant. The first two are good at ordinary pressures. The third is the one that degrades. For water, ΔHvap is about 40.65 kJ/mol at 100 °C but nearer 44 kJ/mol at 25 °C, so a projection made from the 100 °C value down to near room temperature carries a systematic error of a few degrees.
The practical rule is to use Antoine when coefficients exist for your substance and your range, and Clausius-Clapeyron when they do not, choosing a reference point as close as possible to the conditions you care about. Both are correlations rather than laws, and neither knows about your specific sample's purity. Our specific heat calculator covers the sensible-heat side of a heating problem, while the latent heat of vaporisation is the separate quantity feeding the calculation here.
What This Calculator Deliberately Does Not Model
Dissolved solutes raise the boiling point through a colligative effect that depends on the number of dissolved particles, not on their identity. Salted water boils slightly above 100 °C at sea level — though far less than kitchen folklore suggests, since a heavily salted pot rises by under a degree. That calculation belongs to the colligative properties calculator, which handles boiling point elevation and freezing point depression from molality and the van 't Hoff factor. This page changes the pressure; that page changes the liquid.
Mixtures of volatile liquids are also outside scope. An ethanol and water mixture does not have a single boiling point that interpolates between the two pure values; it boils over a range, and at one particular composition it forms an azeotrope that distils unchanged at a temperature below either component's boiling point. Treating a mixture as a pure substance with an averaged Antoine fit will mislead you.
Finally, superheating is real. Very clean water in a smooth vessel can exceed its boiling point without boiling, because there is no nucleation site for a bubble to start on, and then boil violently when one appears. The temperature this calculator returns is the equilibrium boiling point — the temperature at which boiling can occur — not a guarantee about what a particular vessel will do. The ideal gas law calculator and the molar mass calculator are useful companions when you are working through the vapour side of a problem.
Arb Digital publishes hundreds of free calculators across chemistry, physics, maths and finance — no sign-up, no limits. If something you need is missing, tell us and we will look at building it.
Browse All Free Tools Suggest a ToolCommon Mistakes to Avoid
- Mixing Antoine conventions — coefficients published for mmHg and Celsius give nonsense in a bar-and-kelvin equation. Always check which units a coefficient set was fitted in.
- Using one Antoine block outside its range — the fit still returns a number, and that number can be several degrees out. The valid window is part of the data, not a footnote.
- Assuming ΔHvap is constant over a wide range — for water it changes by about 8 percent between 25 and 100 °C, which is enough to bias a long Clausius-Clapeyron extrapolation.
- Treating altitude pressure as exact — the standard atmosphere is a reference model. Actual weather moves sea-level pressure by several percent either way.
- Expecting a mixture to have one boiling point — mixtures of volatile liquids boil over a range and may form azeotropes, so a pure-component correlation does not apply.
Related Free Tools From Arb Digital
Switch between pressure units with the pressure converter or work out a pressure from force and area with the pressure calculator. Move between temperature scales using the temperature converter, handle solute effects on boiling and freezing with the colligative properties calculator, and cover the sensible-heat part of a heating problem with the specific heat calculator. The ideal gas law calculator completes the vapour side, and the full free online tools hub lists everything else.
Frequently Asked Questions
It is the temperature at which a liquid's vapour pressure equals the pressure of its surroundings, so bubbles of vapour can form and survive inside the liquid. Because it depends on the surrounding pressure, a liquid does not have one fixed boiling point.
For water and ethanol it uses the Antoine equation with coefficients from the NIST Chemistry WebBook, selecting the coefficient block that matches the answer's temperature range. You can also enter custom Antoine coefficients or switch to the Clausius-Clapeyron relation.
Atmospheric pressure falls with height, so less vapour pressure is needed to reach the balance point. At about 1,500 metres the pressure is near 84.6 kPa and water boils around 95 degrees Celsius rather than 100.
Roughly 3.5 degrees Celsius per 1,000 metres near sea level, or about 1 degree per 285 metres. The relationship is a curve rather than a straight line, so that rule of thumb drifts once you are several kilometres up.
It is an empirical three-parameter fit of vapour pressure against temperature, written as log base ten of P equals A minus B divided by the sum of T and C. It is accurate inside the temperature range its coefficients were fitted to and unreliable outside it.
When no Antoine coefficients are available for your substance. It needs only one known boiling point, its pressure and an enthalpy of vaporisation, but it assumes that enthalpy is constant, so keep the reference point close to the conditions you care about.
Yes, but only slightly, and by a colligative mechanism that depends on the number of dissolved particles rather than pressure. That calculation belongs to the colligative properties calculator; this page varies pressure on a pure liquid instead.
This calculator is provided for education and general reference. It evaluates published vapour-pressure correlations and is not laboratory, process or safety guidance; follow the procedures and risk assessments issued by your own institution.
