Vapour Pressure of Water Calculator
Estimate the saturation vapour pressure of pure water from temperature and view the result in common pressure units.
Inputs
Live results
Nearby temperature reference
| Temperature | kPa | mmHg | hPa | psi |
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How to use this vapour pressure of water calculator
What this calculator does. It estimates the saturation vapour pressure of pure liquid water at a chosen temperature. Saturation vapour pressure is the pressure exerted by water vapour when liquid water and its vapour are in equilibrium. The tool is useful for quick laboratory checks, humidity and drying calculations, vacuum-system planning, and understanding why water boils at different pressures. It does not calculate the total pressure of a gas mixture, relative humidity by itself, or the behaviour of saline solutions and other mixtures.
When to use it. Use it to check the maximum equilibrium partial pressure of water vapour in a closed space, compare evaporation tendency at two temperatures, estimate the water-vapour contribution in a gas stream, or connect a boiling condition to ambient pressure. For authoritative thermophysical data and fitted coefficients, consult the NIST Chemistry WebBook entry for water.
How to calculate. The calculator opens with a complete demonstration: 25 °C and kilopascals. Its result and workbook are ready immediately. To use your own values:
- Replace Water temperature with the temperature of the liquid water.
- Choose the matching Temperature unit. Changing this control converts the current value, so the physical temperature is preserved.
- Select a Primary pressure unit for the large result. The secondary cards and nearby-temperature table provide additional units.
- Read the live result, then choose Download Excel to export the current inputs, outputs, formula notes, and nearby-temperature rows as a validated XLSX workbook.
- Choose Reset to clear the demonstration data. Reset intentionally leaves the calculator empty and disables export until a complete valid temperature is entered again.
Input guide. Water temperature is required and accepts ordinary decimal-point notation, such as 25, 37.5, or 212. Thousands separators, decimal commas, scientific notation, and pasted unit symbols are rejected to prevent ambiguous interpretation. The supported correlation range is 1 – 374 °C, equivalent to 33.8 – 705.2 °F or 274.15 – 647.15 K. Raising temperature increases vapour pressure nonlinearly. A common mistake is entering a Fahrenheit value while Celsius remains selected. Temperature unit is required and offers °C, °F, and K; it changes both interpretation and display but not the underlying physical state when switched. Primary pressure unit is required and controls only the large displayed result; it does not change the calculation.
Output guide. Saturation vapour pressure is the estimated equilibrium pressure in the selected unit. Pressure in pascals, Pressure in mmHg, and Pressure in atmospheres are exact unit conversions of the same model value. Absolute temperature reports the corresponding kelvin value. The summary pills show the active temperature and whether it lies inside the supported liquid-water range. The nearby-temperature table lists five points at – 10, – 5, 0, +5, and +10 °C relative to the entered temperature, clipped to the model range. Each row contains temperature, kPa, mmHg, hPa, and psi. A value approaching standard atmospheric pressure indicates that the boiling point is approaching that temperature at roughly sea-level pressure.
Worked example. At the startup value of 25 °C, the Antoine form used here is log10(PmmHg) = 8.07131 – 1730.63 ÷ (233.426 + 25). This gives approximately 23.686 mmHg. Multiplying by 133.322368 converts the result to about 3,157.92 Pa, or 3.158 kPa, matching the first-open result and the exported workbook.
Learn more. Vapour pressure rises rapidly with temperature because more molecules have enough energy to escape the liquid phase. The U.S. National Weather Service vapour-pressure reference explains the connection between temperature, saturation, and atmospheric moisture.
Formula, assumptions, and interpretation
The calculator uses a piecewise Antoine correlation for pure water. For 1 – 99 °C it uses A = 8.07131, B = 1730.63, and C = 233.426. For 100 – 374 °C it uses A = 8.14019, B = 1810.94, and C = 244.485. In both bands, temperature is in degrees Celsius and pressure is returned in mmHg before conversion. Antoine equations are empirical correlations, so they are most reliable within the fitted temperature band and should not be extrapolated beyond it.
At 100 °C, water's saturation vapour pressure is close to one standard atmosphere, which explains normal boiling near sea level. Boiling occurs when saturation vapour pressure equals the surrounding absolute pressure; therefore, lower ambient pressure lowers the boiling temperature. The Engineering ToolBox water saturation table provides a practical tabular comparison, while the NIH PubChem record for water supplies broader substance information.
Common mistakes and limits
Do not treat saturation vapour pressure as the total pressure of moist air: it is the water-vapour partial pressure at saturation. At lower relative humidity, the actual water-vapour partial pressure is lower. Do not use this pure-water result for concentrated salt, sugar, glycol, or solvent mixtures, because dissolved substances change water activity and generally reduce vapour pressure. Finally, use an appropriate reference standard or calibrated instrument for safety-critical process design, pressure-vessel work, medical devices, or legal metrology.