# Vapour Pressure of Water Calculator

URL: https://speedycalculator.com/calculators/chemistry/vapour-pressure-of-water/
Last updated: 2026-10-30

> The vapour pressure of water calculator gives the saturation vapour pressure of water (or ice) at any temperature with five formulas side by side – Buck, Antoine, Magnus, Tetens and a simple exponential fit – plus the actual vapour pressure, absolute humidity and dew point at a chosen relative humidity.

## Inputs

- Temperature (°C, K, °F)
- Formula for the main result
- Surface below 0 °C
- Relative humidity (for humidity and dew point) (%)

## Results

- Saturation vapour pressure
- Buck
- Antoine
- Magnus
- Tetens
- Simple exponential fit
- Actual vapour pressure at this relative humidity
- Absolute humidity (water vapour per m³ of air)
- Saturation vapour density
- Dew point (Magnus)
- Share of atmospheric pressure (101.325 kPa)
- Validity

## Formula

```
Buck: P = 0.61121 · exp[(18.678 − T/234.5) · T/(257.14 + T)] kPa · Antoine: log₁₀ P = A − B/(C + T) (mmHg) · Magnus: P = 0.61094 · exp[17.625T/(T + 243.04)] kPa · Tetens: P = 0.61078 · exp[17.27T/(T + 237.3)] kPa · T in °C
```

## Explanation

The saturation vapour pressure of water is the pressure of water vapour in equilibrium with liquid water (or ice) at a given temperature – the most water vapour the air can hold before it condenses. It climbs steeply with temperature: 0.61 kPa at 0 °C, 2.34 kPa at 20 °C, 7.38 kPa at 40 °C and 101.3 kPa at 100 °C, the point at which water boils at sea level.

Meteorologists need it for relative humidity, dew point and cloud formation; HVAC engineers for drying and condensation; chemists for correcting the volume of gases collected over water; biologists for evaporation and transpiration. Because no simple formula is exact everywhere, several empirical equations are in use. The Buck (1981) equation is among the most accurate from −40 to +50 °C, Magnus and Tetens are compact and popular in meteorology, and the Antoine equation with two coefficient sets covers 1–374 °C.

Below 0 °C there are two answers: the vapour pressure over supercooled liquid water and the lower one over ice. Choose which surface you need. With a relative humidity, the calculator also gives the actual vapour pressure, the mass of water vapour per cubic metre of air and the dew point.

### How to use the Vapour Pressure of Water Calculator

- **Temperature**
- **Formula for the main result**
- **Surface below 0 °C**
- **Relative humidity (for humidity and dew point)**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the temperature and pick the formula used for the main result; all five values are shown for comparison. Antoine switches automatically to its high-temperature coefficients above 100 °C; Buck and Tetens switch to their ice versions below 0 °C if you choose the ice surface. The actual vapour pressure is RH × saturation pressure, absolute humidity is e·M/(R·T), and the dew point inverts the Magnus formula.

### Worked example

At 25 °C, Buck: (18.678 − 25/234.5) × 25/(257.14 + 25) = 18.571 × 0.08861 = 1.6456, so P = 0.61121 × e^1.6456 = **3.168 kPa**. Antoine: log₁₀ P = 8.07131 − 1730.63/(233.426 + 25) = 1.37437, P = 23.68 mmHg = 3.158 kPa. At 50% relative humidity the actual vapour pressure is 1.584 kPa, the air holds 11.5 g of water vapour per m³ and the dew point is about 13.9 °C.

### Vapour pressure of water table

TemperatureBuck (kPa)Antoine (kPa)Tetens (kPa)0 °C0.6110.6060.6115 °C0.8720.8660.87210 °C1.2281.2211.22815 °C1.7051.6981.70520 °C2.3382.3302.33825 °C3.1693.1583.16830 °C4.2454.2324.24337 °C6.2806.2606.27540 °C7.3827.3587.37550 °C12.34912.30612.33660 °C19.94519.87019.93270 °C31.20131.08731.21680 °C47.41047.26747.52390 °C70.15270.03070.516100 °C101.308101.337102.212Calculated with the formulas on this page (over liquid water).

### Antoine coefficients for water

RangeABC1–100 °C8.071311730.63233.42699–374 °C8.140191810.94244.485P in mmHg and T in °C. Multiply mmHg by 0.133322 to get kPa.

### Tips

- Use kelvin for every temperature in thermodynamic formulas – the unit menus convert for you.
- Watch the units: ΔH is usually in kJ/mol and ΔS in J/(mol·K).
- Tabulated values are for standard conditions (1 bar, usually 25 °C).
- Everything is calculated in your browser.

### Common mistakes to avoid

- Mixing kJ and J between ΔH and ΔS.
- Assuming ΔH and ΔS do not change with temperature over large ranges.
- Using Celsius in ln(T₂/T₁).

### Inputs explained

FieldWhat to enterTemperatureUnits: °C, K, °FFormula for the main resultChoose one: Buck (1981) – recommended, Antoine equation, Magnus (Alduchov & Eskridge), Tetens …Surface below 0 °CChoose one: Liquid water (supercooled), IceRelative humidity (for humidity and dew point)In %

### What each result means

- **Saturation vapour pressure** = [P]
- **Buck** = [pb]
- **Antoine** = [pa]
- **Magnus** = [pm]
- **Tetens** = [pt]
- **Simple exponential fit** = [ps]
- **Actual vapour pressure at this relative humidity** = [P]×[Relative humidity (for humidity and dew point)]÷100
- **Absolute humidity (water vapour per m³ of air)** = [P]×[Relative humidity (for humidity and dew point)]÷100×1000×18.015÷(8.314462618×([Temperature] + 273.15))
- **Saturation vapour density** = [P]×1000×18.015÷(8.314462618×([Temperature] + 273.15))
- **Dew point (Magnus)** = 243.04×ln([P]×[Relative humidity (for humidity and dew point)]÷100÷0.61094)÷(17.625 - ln([P]×[Relative humidity (for humidity and dew point)]÷100÷0.61094))
- **Share of atmospheric pressure (101.325 kPa)** = ([P]÷101.325)×100

## Worked example

Inputs — Temperature: 25 °C; Formula for the main result: Buck (1981) – recommended; Surface below 0 °C: Liquid water (supercooled); Relative humidity (for humidity and dew point): 50 %.

Results — **Saturation vapour pressure = 3.1685 kPa**; Buck = 3.1685 kPa; Antoine = 3.1579 kPa; Magnus = 3.1617 kPa; Tetens = 3.1677 kPa; Simple exponential fit = 3.1842 kPa; Actual vapour pressure at this relative humidity = 1.5843 kPa; Absolute humidity (water vapour per m³ of air) = 11.513 g/m³; Saturation vapour density = 23.026 g/m³; Dew point (Magnus) = 13.89 °C; Share of atmospheric pressure (101.325 kPa) = 3.127 %; Validity = Within the normal range of the chosen formula.

## Key terms

- **Saturation vapour pressure**: The equilibrium pressure of water vapour over a flat surface of pure water or ice at a given temperature.
- **Relative humidity**: The actual vapour pressure as a percentage of the saturation vapour pressure.
- **Dew point**: The temperature to which air must be cooled at constant pressure for its water vapour to start condensing.
- **Absolute humidity**: The mass of water vapour per unit volume of air, usually in g/m³.

## FAQ

**What is the vapour pressure of water at 25 °C?**
3.17 kPa (23.8 mmHg, 31.7 hPa). Buck gives 3.168 kPa and Antoine 3.158 kPa.

**What is the vapour pressure of water at 100 °C?**
101.325 kPa (1 atm) by definition of the normal boiling point; Buck gives 101.3 kPa.

**What is the vapour pressure of water at body temperature (37 °C)?**
About 6.28 kPa (47.1 mmHg) – the value used in respiratory physiology for alveolar air.

**Which formula is most accurate?**
Buck (1981) agrees with the reference IAPWS values to within about 0.05% between −40 and 50 °C. Antoine is excellent from 1 to 100 °C; Magnus and Tetens are within about 0.1–0.4% near room temperature.

**Why is vapour pressure lower over ice than over supercooled water?**
Molecules are more tightly held in ice, so fewer escape. At −10 °C: 0.260 kPa over ice versus 0.286 kPa over liquid water. This difference drives snow and hail growth in clouds.

**How do I correct a gas collected over water?**
Subtract the water vapour pressure at the water temperature from the total pressure. At 22 °C that is 2.64 kPa (19.8 mmHg).

**How is the dew point found?**
The actual vapour pressure e = RH × saturation pressure is fed back into the Magnus formula and solved for temperature. At 25 °C and 50% RH the dew point is about 13.9 °C.


## Sources

- [Buck, A. L. (1981). New equations for computing vapor pressure and enhancement factor. J. Appl. Meteor. 20, 1527–1532](https://doi.org/10.1175/1520-0450(1981)020%3C1527:NEFCVP%3E2.0.CO;2)
- [NIST Chemistry WebBook – Water (phase-change data)](https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185)
- [Alduchov, O. A. & Eskridge, R. E. (1996). Improved Magnus form approximation of saturation vapor pressure. J. Appl. Meteor. 35, 601–609](https://doi.org/10.1175/1520-0450(1996)035%3C0601:IMFAOS%3E2.0.CO;2)