The vapour pressure calculator above answers one question about one substance: at a given temperature, what pressure does a pure liquid's vapour exert when it is in equilibrium with the liquid in a closed space? It offers two routes. The Antoine equation uses three fitted coefficients per substance and reproduces measured data closely across its stated temperature range. The Clausius-Clapeyron relation needs only an enthalpy of vaporisation and one known pressure-temperature pair, so it works for any liquid you can find those two numbers for.
Arb Digital publishes free calculators that make the limits of a model as visible as its output. Here that means telling you which equation produced the number, showing how fast the answer is changing with temperature, and putting the result next to one atmosphere so you can see immediately whether the liquid is close to boiling. All three matter more than the headline figure on its own.
What This Vapour Pressure Calculator Does
Vapour pressure is the pressure of a substance's own vapour above its liquid at equilibrium. It depends only on the substance and the temperature — not on the volume of the container, not on how much liquid is present, and not on what other gases are in the space. That last point surprises people: a puddle in a sealed jar of air reaches the same water vapour pressure as a puddle in a sealed evacuated jar.
The tool reports that pressure in your chosen unit, then adds three things a plain lookup does not. It gives the percentage rise per degree at your working temperature, which is the number that tells you how tightly a temperature must be controlled. It back-calculates the enthalpy of vaporisation implied by the slope, which is a useful sanity check on the fit. And it finds the temperature at which the vapour pressure would double.
Two boundaries. Our boiling point calculator solves the same physical relation for the opposite unknown: given a surrounding pressure, at what temperature does the vapour pressure match it? That temperature is the boiling point. And our Raoult's law calculator takes the pure-liquid values this page produces and works out what happens when two liquids are mixed. This page deals with one pure substance only.
How to Use It
- Pick the method. Antoine if your liquid is in the list, Clausius-Clapeyron if it is not or if you want to work from a reference point you trust.
- Choose the liquid. The selection loads Antoine coefficients and also fills the Clausius-Clapeyron boxes with that liquid's enthalpy of vaporisation and normal boiling point, so you can compare the two methods side by side.
- Set the temperature in degrees Celsius, and choose the pressure unit you want the answer in.
- In Clausius-Clapeyron mode, check the reference pair. The reference pressure must be entered in the same unit you selected for the output.
- Read the rise-per-degree figure before trusting the headline number. If the pressure is moving by six percent per degree, a two-degree thermometer error is a twelve percent pressure error.
The Formula and How It Is Calculated
The Antoine equation is log10(P) = A − B / (T + C), with T in kelvin and P in bar in the parameter set used here. The three coefficients are fitted to measured vapour pressure data for one substance over a stated temperature window, and they are not physically meaningful on their own — they are simply the numbers that make the curve fit.
The Clausius-Clapeyron relation in its integrated two-point form is ln(P2/P1) = −(ΔHvap/R) × (1/T2 − 1/T1), with both temperatures in kelvin and R = 8.314 J mol−1 K−1. It comes from assuming the vapour is ideal, that the liquid's volume is negligible next to the vapour's, and that the enthalpy of vaporisation is constant over the temperature interval.
Work the default. Water at 25 °C, which is 298.15 K, with Antoine coefficients A = 4.6543, B = 1435.264 and C = −64.848 taken from the NIST Chemistry WebBook phase-change data for water. B / (T + C) = 1435.264 / 233.302 = 6.1519, so log10P = 4.6543 − 6.1519 = −1.4976 and P = 0.03181 bar, which is 3.18 kPa or 23.9 mmHg. The accepted value is 3.169 kPa, so the fit is within half a percent. Coefficient sets for the other liquids here come from the same NIST Chemistry WebBook, and the ethanol entry is a good example of a page carrying several sets for different temperature ranges.
Why Clausius-Clapeyron Drifts, and by How Much
Run the same water calculation through Clausius-Clapeyron from the normal boiling point and the answer comes out around 3.75 kPa rather than 3.17 — about 18 percent high. That is not a mistake in the arithmetic. It is the constant-enthalpy assumption failing across a 75-degree interval.
Enthalpy of vaporisation is itself temperature dependent, and it falls as temperature rises, reaching zero at the critical point. For water it is about 44.0 kJ/mol at 25 °C and 40.65 kJ/mol at 100 °C. Using the boiling-point value across the whole gap understates how strongly the molecules are held at room temperature, so it overestimates the escaping tendency there. Substitute 44.0 and the prediction drops to about 2.9 kPa — closer, and now erring the other way, because the true value varies continuously rather than switching between two numbers.
The practical rule is simple. Clausius-Clapeyron is good to a few percent within roughly twenty degrees of the reference point and degrades steadily beyond that. Use the enthalpy value quoted nearest to your working temperature, not the one nearest the boiling point, and prefer Antoine whenever coefficients exist for your liquid and your range. This calculator lets you run both on the same substance precisely so the size of the gap is visible rather than theoretical.
The Rise per Degree Is the Number That Matters
Because the relationship is exponential, the sensitivity of vapour pressure to temperature is large and it changes with temperature. Water near 25 °C gains roughly six percent of its vapour pressure per degree. Near 100 °C the fractional rise is smaller, about three and a half percent per degree, but the absolute rise is about 3.4 kPa per degree rather than 0.19 kPa.
This is why the grid reports the figure at all. It converts a temperature tolerance into a pressure tolerance without any further work. If a vacuum process needs the pressure held within five percent and the liquid is gaining six percent per degree, the bath must hold to better than a degree. It also explains why a pressure cooker works: raising the internal pressure to about two atmospheres pushes the boiling point of water to roughly 121 °C, and that same steep curve means a modest pressure increase buys a large temperature increase.
The doubling-temperature tile is the same idea expressed differently. For water at 25 °C the vapour pressure doubles by about 37 °C — a twelve-degree rise. Near the boiling point the doubling interval stretches to about twenty-one degrees. The interval widens as temperature rises, which is the signature of an exponential with a curved exponent rather than a plain one.
Volatility, Boiling and What Vapour Pressure Actually Predicts
A liquid boils when its vapour pressure reaches the pressure of its surroundings. That single sentence links this page to every altitude-cooking and vacuum-distillation question. At 2,000 metres the surrounding pressure is near 80 kPa, and water's vapour pressure reaches 80 kPa at about 93 °C, so that is where it boils. Under a rotary evaporator at 5 kPa it boils near 33 °C, which is why heat-sensitive compounds can be concentrated without cooking them.
Vapour pressure is also the honest definition of volatility. A liquid with a high vapour pressure at ambient temperature evaporates fast and produces a lot of vapour in an enclosed space — acetone at 25 °C sits near 30 kPa, which is roughly thirty percent of an atmosphere, against water's three percent. Comparing two liquids at the same temperature ranks them properly, whereas comparing boiling points can mislead when the substances behave differently.
One thing vapour pressure does not tell you is evaporation rate in open air. That depends on airflow, surface area, humidity and diffusion as well as on the equilibrium pressure. Vapour pressure sets the ceiling — the concentration a still, closed space would eventually reach — not the speed of getting there. The temperature converter and pressure converter handle the unit changes that usually surround these comparisons, and the partial pressure calculator covers what happens once that vapour is mixed with other gases.
Where the Antoine Fit Stops Being Valid
Every Antoine coefficient set carries a temperature range, and outside it the fit is not merely less accurate — it can be badly wrong, because a three-parameter fit extrapolates poorly. Many substances have two or three published sets covering different windows, and choosing the wrong one is a common source of error. The sets used here cover ordinary laboratory temperatures for each liquid, and the tool flags a temperature that falls outside the range it is using rather than quietly returning a number.
Two hard limits apply to any vapour pressure calculation. Below the melting point there is no liquid, so what you would be calculating is the sublimation pressure of a solid, which follows a different curve. Above the critical temperature there is no liquid either — no pressure will condense the substance — so vapour pressure ceases to be defined at all. Water's critical temperature is 374 °C, so ordinary work never approaches it, but for ammonia it is 132 °C and for carbon dioxide only 31 °C, which is why a carbon dioxide cylinder at room temperature is a genuinely different problem. The enthalpy figure the tool derives from the local slope is related to the thermodynamics handled by the Gibbs free energy calculator, where the same quantity governs whether a phase change proceeds.
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Browse All Free Tools Suggest a ToolCommon Mistakes to Avoid
- Using Celsius in the equations — both Antoine and Clausius-Clapeyron need absolute temperature. The tool converts for you, but hand calculations go badly wrong without it.
- Extrapolating an Antoine set past its range — a three-parameter fit degrades quickly outside the window it was built for, and many liquids have several published sets.
- Taking the boiling-point enthalpy of vaporisation to room temperature — for water that alone puts the Clausius-Clapeyron answer about 18 percent high at 25 degrees.
- Mixing units in the reference pair — the reference pressure must be in the same unit as the output, or the ratio is meaningless.
- Treating vapour pressure as an evaporation rate — it sets the equilibrium ceiling, not how quickly an open dish empties.
Related Free Tools From Arb Digital
Solve the same relation for temperature with the boiling point calculator, then take the pure values here into a mixture using the Raoult's law calculator or the partial pressure calculator. Change units with the temperature converter and the pressure converter, and follow the thermodynamics into the Gibbs free energy calculator. The full free online tools hub lists everything else.
Frequently Asked Questions
It is the pressure exerted by a substance's own vapour when that vapour is in equilibrium with its liquid in a closed space. It depends only on the substance and the temperature, not on the size of the container or how much liquid is present.
About 3.17 kilopascals, which is 23.8 millimetres of mercury or roughly 3.1 percent of one atmosphere. The Antoine coefficients used here return 3.18 kilopascals, within half a percent of the accepted value.
Antoine is an empirical three-coefficient fit to measured data for one substance over a stated temperature range, and it is accurate inside that range. Clausius-Clapeyron is derived from thermodynamics and needs only an enthalpy of vaporisation and one reference point, but it assumes that enthalpy is constant.
Because the enthalpy of vaporisation falls as temperature rises. Using water's boiling-point value of 40.65 kilojoules per mole to predict the pressure at 25 degrees overestimates it by around 18 percent, while using the 25-degree value of about 44 kilojoules per mole undershoots slightly.
A liquid boils when its vapour pressure equals the pressure pressing on its surface. That is why water boils below 100 degrees at altitude and why a rotary evaporator at 5 kilopascals boils water near 33 degrees.
No. As long as some liquid remains, the equilibrium vapour pressure is the same whether the container holds a drop or a litre. Only the substance and the temperature set it.
No, because above the critical temperature no amount of pressure will produce a liquid, so there is no liquid-vapour equilibrium to have a pressure. Water's critical temperature is 374 degrees Celsius, but carbon dioxide's is only 31.
Not directly. It sets the vapour concentration a still, closed space would eventually reach. The actual rate in open air also depends on airflow, surface area, humidity and diffusion.
This calculator is provided for education and general reference. It describes how vapour pressure is computed and is not laboratory, ventilation or safety guidance; follow the procedures and risk assessments issued by your own institution.
