Vapor Pressure of Water Calculator
Find the vapor pressure of water from its temperature with five formulas.
Liquid water and ice, three temperature scales, and twelve pressure units.
Updated October 13, 2026
Introduction
Water is evaporating and condensing at the same time, always. The vapor pressure is the balance point of that exchange: the pressure of the vapor that sits in equilibrium with liquid water — or with ice — at a given temperature. Enter one temperature here and you get that pressure from five different published fits at once, so you can read the answer and see how much the choice of fit changes it.
The number is the one that decides whether water evaporates or condenses. Compare it with the vapor that is already in the air: a surface whose saturation pressure is higher than the vapor pressure around it gives up molecules, and a surface whose saturation pressure is lower takes them in. Drying, fogged windows, dew on grass, frost on a freezer wall and the humidity set point of a greenhouse are all the same comparison at different temperatures.
That makes this a working number for people who deal with water in air rather than with abstract thermodynamics: instrument and building-services work, where dew-point and humidity sensors are calibrated against exactly this curve; greenhouse and crop work; drying and evaporation estimates; freeze-drying; and coursework where a second method is needed to check a table. Two things this page deliberately does not do: it covers pure water only, and it reports the equilibrium pressure rather than an evaporation rate. For a different liquid, or to work between two measured points and the heat of vaporization, the general vapor pressure calculator covers the wider case.
−273.15 to 374 °C
Entered in Celsius, Fahrenheit or kelvin
7 formulas, 8 rows
Five fits over water, two over ice; the Antoine pair swaps at 100 °C
12 pressure units
Each result row reads in its own unit
Quick start
Type the temperature of the water
25 °C is already loaded as an example. Replace it with your own reading, and switch the box to °F or K if that is how you measured it — the formulas themselves read degrees Celsius, so the value is converted for you. What matters is the temperature of the water surface, not the temperature of the room it stands in.
Read the Pressure group
Between 0 °C and 100 °C five rows appear, each the same physical quantity computed by a different published fit. At room temperature they agree closely — at 25 °C the five land between 3.15674 and 3.16853 kPa, a spread of about 0.4% — so any of them is a fair answer there. Away from room temperature they separate, and the page keeps each row on screen only inside the range it was fitted for.
Put the row into the unit your instrument uses
Every result row carries its own pressure menu — pascals, bars, pounds per square inch, technical and standard atmospheres, torr, hectopascals, kilopascals, megapascals, gigapascals, inches of mercury and millimeters of mercury. Switching it rescales that one row; the calculation underneath is unchanged.
Check the working against a fit you know
At 25 °C the Buck fit evaluates as follows — this is the value the row shows, before any unit change:
That is the same pressure as 31.6853 hPa, 23.7659 mmHg, 0.459557 psi or 0.031271 atm — switch the row rather than converting by hand.
Read the number against the pressure around it
3.16853 kPa is roughly 3% of standard sea-level pressure (101.325 kPa). That ratio explains the behavior everyone knows: a puddle at 25 °C dries out slowly rather than boiling, because the water only has to reach 3.17 kPa of vapor pressure to be in balance with its own vapor. It reaches a whole atmosphere — the boiling point — at 100 °C, where the rows land between 100.716 and 104.077 kPa around the 101.325 kPa that defines it.
The five formulas and where each one holds
None of these rows is the definition of vapor pressure. Each is an empirical fit of measured saturation data, written with two to four constants, and each is accurate where it was fitted and unreliable outside that. The page shows you all of them together because their agreement is itself information.
The simplified fit
Two constants, an old and widely copied approximation. It answers in millimeters of mercury, which the page converts to pascals at the exact factor of 133.322387415 Pa per mmHg before the unit menu touches it. It is the crudest row here: at 0 °C it reads 0.652124 kPa against 0.60558 kPa from Antoine, about 8% high. It is also the row with a hard edge — its exponent divides by , which is zero at −273 °C, so it is only offered from 0 °C upwards.
The Antoine equation
The equation most handbooks and process tables quote, and the reason the page shows two rows with the same name: one coefficient set describes water up to its normal boiling point and a second one takes over above it. Only the set that matches your temperature is on screen.
| Fitted range | A | B | C |
|---|---|---|---|
| 0 – 100 °C | 8.07131 | 1730.63 | 233.426 |
| 100 – 374 °C | 8.14019 | 1810.94 | 244.485 |
Both sets answer in millimeters of mercury. At 100 °C they hand over to each other almost seamlessly — the low set reads 101.337 kPa and the high set 101.893 kPa, against the 101.325 kPa that defines the boiling point.
Magnus, Tetens and Buck
These three are written in kilopascals and multiplied by 1000 to reach the pascal base. Magnus and Tetens carry three constants each and track Buck closely through the middle of the range: between 0 °C and 50 °C the three stay within a few tenths of a per cent of one another. Buck carries a fourth constant and holds that agreement further up — at 100 °C it reads 101.308 kPa against the 101.325 kPa that defines the boiling point, while Magnus is already 2.7% above it.
Below 0 °C the curve changes
Ice and liquid water do not have the same saturation curve, and below freezing the difference is not small. At −10 °C the two over-ice rows read 0.259452 and 0.259947 kPa, while the liquid-water fits — the ones this page hides below 0 °C — would give about 0.287 kPa at the same temperature, roughly a tenth higher. That gap is the reason the rows swap at 0 °C, and the reason frost appears on a cold surface while the air around it still holds water vapor: over ice, the air is supersaturated sooner.
How far apart the five rows are
All values below are the kilopascal readings the page itself shows; the Antoine column is whichever coefficient set is on screen at that temperature.
| T (°C) | Simplified | Antoine | Magnus | Tetens | Buck |
|---|---|---|---|---|---|
| 0 | 0.652124 | 0.60558 | 0.61094 | 0.61078 | 0.61121 |
| 25 | 3.15674 | 3.15793 | 3.16174 | 3.16767 | 3.16853 |
| 50 | 11.9708 | 12.3056 | 12.3606 | 12.3364 | 12.3494 |
| 80 | 46.1904 | 47.2671 | 48.0397 | 47.5233 | 47.4103 |
| 100 | 100.716 | 101.337 | 104.077 | 102.212 | 101.308 |
| 300 | 12,266.72 | 8,692.1 | 10,343.31 | 9,411.05 | 7,159.92 |
| 374 | 34,167.19 | 21,730.38 | 26,638.17 | 23,693.49 | 15,226.21 |
Read the table by column, not by row. Around room temperature the five are within a few tenths of a per cent of each other, so the formula you pick barely matters; what matters is the temperature. One degree of error at 25 °C moves the answer by about 6% — from 3.16853 to 3.3624 kPa between 25 °C and 26 °C — which is more than ten times the spread between the rows themselves.
At the top of the range the rows fall apart: at 374 °C, the critical temperature of water, they span 15,226.21 to 34,167.19 kPa, and the page marks the ones that overshoot water's critical pressure in red rather than pretending they are still measurements.
These rows describe pure water
Anything dissolved in the water pushes the vapor pressure down: salt water, sugar syrup and glycol loops all sit below the rows on this page, in proportion to how much of the mixture is water. The Raoult's law calculator handles that case. The rows also report an equilibrium pressure, not how fast water is leaving a surface — evaporation rate also depends on airflow, the exposed area and how much vapor the surrounding air already holds.
Worked examples
Checking a humidity sensor at 25 °C
Situation: A humidity logger reports 31.9 hPa as the saturation pressure at its own 25 °C reading, and you want to know whether the instrument or the arithmetic is off.
Inputs
Result
That is 0.7% below the logger's 31.9 hPa. The curve climbs by about 6% per degree at this temperature, so a probe that is half a degree out explains a 3% pressure gap on its own — the temperature reading is the first thing to check, not the last.
In practice: Verify the temperature channel before the sensor: half a degree of probe error is worth 3% of pressure here, several times the gap you are investigating.
Frost point and freeze-drying at −10 °C
Situation: A product sits at −10 °C in a freeze-dryer and you need the chamber pressure that keeps its ice subliming.
Inputs
Result
The Buck over-ice row reads 0.259947 kPa, within 0.2% of Tetens. Warm the product and the ceiling climbs: the page gives 0.61078 kPa at 0 °C and 3.16853 kPa at 25 °C, more than twelve times the −10 °C value.
In practice: Keep the chamber below the saturation pressure of ice at the product temperature, and re-read it whenever the product warms: the ceiling rises steeply through the cycle.
Near the critical point: what the red rows mean
Situation: The temperature is pushed to the top of the range to see how the fits behave where the liquid–vapor boundary is about to disappear.
Inputs
Result
The Antoine high-temperature set is the closest of the five at 21,730.38 kPa, about 1.5% below water's critical pressure of 22.064 MPa. The simplified, Magnus and Tetens rows have all passed it; they keep their numbers and are drawn in red, so you can see how far each fit has run away instead of being told nothing.
In practice: Read the red rows as the page's own range warning: near-critical and supercritical work belongs in a steam table or in measured data, not in one fit from this page.
Getting a number you can trust
Measure the water, not the room
The curve is steep — 6% per degree at 25 °C — so a thermometer a meter away from the surface, or a thermostat set point, can easily be a degree out and several per cent wrong in pressure.
Read each row inside its own range
The page hides rows that do not belong to the current temperature: below 0 °C you are looking at the over-ice curves, above 100 °C at the second Antoine set. A row that disappears has not failed; it is out of scope.
Do not average the rows
They are five fits of the same underlying saturation data, not five independent measurements, so their mean is not a better estimate. Pick the fit whose range contains your temperature and quote the range with it.
Switch the row instead of converting by hand
3.16853 kPa is 31.6853 hPa, 23.7659 mmHg or 0.031271 atm. Doing that arithmetic outside the page invites a second rounding on top of the display rounding.
Always record the temperature beside the pressure
A bare "3.17 kPa" means nothing on its own; it is the saturation pressure at 25 °C, and the same pressure belongs to other temperatures on other curves, including the over-ice ones. Write the pair down together — gauge reading, temperature, and the unit the gauge was set to — and the number stays checkable months later.
Questions
Which of the five rows should I quote?
Choose by range, then by agreement with the others. Between 0 °C and 100 °C the Buck and Antoine rows are the two tightest of the five — at 100 °C they read 101.308 and 101.337 kPa against the 101.325 kPa that defines the boiling point — and Antoine is the equation most references expect to see, so either is defensible. Below 0 °C the two ice rows agree to within 0.2% of each other — 0.259452 against 0.259947 kPa at −10 °C — so either will do, with Buck carrying the extra constant. Above 100 °C, use the second Antoine set. The row to avoid at the edges is the simplified fit: at 0 °C it reads 0.652124 kPa where Antoine gives 0.60558 kPa, about 8% high.
Can I work backwards from a pressure to a temperature?
Not on this page — the result rows are read-only, because each one is a separate empirical fit and inverting them one by one would give you five different answers with no reason to prefer any of them. The reverse question, "at what temperature does water boil at this pressure", is a Clausius–Clapeyron problem, and the boiling point calculator solves it in both directions with tabulated substances.
Why do rows appear and disappear as I change the temperature?
Visibility follows the range each fit was made for. From 0 °C upwards five liquid-water rows are shown; the second Antoine row replaces the first above 100 °C; and below 0 °C the two over-ice rows take over, with the Tetens ice fit shown only above −265 °C, close to where its denominator would pass through zero. Nothing is broken when a row vanishes — it simply is not part of the answer at that temperature.
What does the red warning at the top of the range mean?
It means the fit produced a value above water's critical pressure of 22.064 MPa, where no liquid–vapor boundary exists and a saturation pressure therefore has no physical meaning. The number stays visible so you can judge how far the fit has drifted, but the row is flagged instead of being presented as a measurement. In the same situation a temperature outside −273.15 °C to 374 °C is refused outright, because there is no curve to evaluate at all.
How is this different from relative humidity or VPD?
This page gives the saturation pressure at a temperature — the amount of vapor the air could hold at equilibrium. Relative humidity compares the vapor actually present with that ceiling, and the vapor pressure deficit is the difference between the two, which is what drives transpiration in plants. If your question is about a greenhouse or a crop rather than about water itself, the vapor pressure deficit calculator is the next step.
How far to trust it
- These are empirical fits, not a thermodynamic model. Each row is only as good as the data it was fitted to and the range those data cover, which is why the page hides rows outside their range and flags any row that overshoots 22.064 MPa.
- Above about 100 °C the rows stop agreeing with each other. At 300 °C they already span 7,159.92 to 12,266.72 kPa, and at 374 °C 15,226.21 to 34,167.19 kPa. For steam, near-critical or supercritical conditions, use measured data or an authoritative steam table rather than any single row from this page.
- Pure water, and equilibrium only. Dissolved solids and other solutes lower the vapor pressure and are not modeled here, and the result says nothing about how quickly water will actually evaporate from a real surface.
- The temperature you enter is the water surface temperature. Air temperature, a set point or a shaded logger reading can differ by a degree or more, and near room temperature that is worth about 6% of the pressure per degree.
- Treat the output as an estimate, not a specification. Boiler and pressure-vessel work, sterilizer and autoclave cycles, pharmaceutical freeze-drying recipes and any calculation where being a few per cent out carries a real cost should be verified against measured data or an authoritative steam table, and the decision reviewed by a qualified engineer or chemist.
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