Vapor Pressure of Water Calculator

Related Calculators

Saturation vapor pressure: The pressure where liquid water and water vapor are in equilibrium at a given temperature.

Absolute pressure: Pressure measured relative to a perfect vacuum. This is the pressure needed for the Boiling temperature calculation.

Gauge pressure: Pressure measured relative to the surrounding air. Gauge pressure is not the same as Absolute pressure (kPa).

mmHg: Millimeters of mercury, a pressure unit often used in chemistry and older vapor-pressure tables.

kPa: Kilopascals, a common SI pressure unit used in engineering and weather work.

Antoine equation: A fitted equation that estimates water vapor pressure from temperature over a limited temperature range.

IAPWS reference fit: A standard water-property method based on accepted steam-table style saturation relations over a wider liquid-water range.

Critical point: The temperature and pressure where the usual liquid and vapor boundary ends, so ordinary saturation vapor pressure is no longer defined the same way.

Mistake: Entering gauge pressure into Absolute pressure (kPa) .
Fix: Use absolute pressure for Absolute pressure (kPa) . If you only know gauge pressure, add local atmospheric pressure before calculating Boiling temperature .

Mistake: Picking Temperature unit as F or K, then typing a value that was really in C.
Fix: Match the number in Water temperature (C) to the chosen Temperature unit before you calculate Vapor pressure of water .

Mistake: Using Antoine equation outside its allowed range.
Fix: Read Range note . If your input is outside the Antoine range, switch Calculation method to the IAPWS reference fit.

Mistake: Confusing Vapor pressure in mmHg with Vapor pressure in kPa when comparing to a table or homework problem.
Fix: Compare the output that uses the same unit as your source. The calculator shows both Vapor pressure in kPa and Vapor pressure in mmHg to help you check.

Mistake: Entering zero or a negative value in Absolute pressure (kPa) for boiling mode.
Fix: Use a pressure greater than 0 so the calculator can find a physical Boiling temperature .

Mistake: Expecting this tool to give ice-surface vapor pressure below freezing.
Fix: For Vapor pressure of water , this calculator is for liquid water saturation. If your Water temperature (C) is below 0 C, use an ice-surface formula instead.

• This calculator gives saturation vapor pressure for pure water at equilibrium. It does not model salt water, dissolved substances, or mixed gases.
• Liquid-water results are limited by the selected method range. Antoine is restricted to its stated range, and the calculator stops instead of silently extrapolating.
• Inputs below 0 C are blocked for saturation output because this tool does not include ice-surface vapor-pressure formulas.
• Inputs at or above the critical temperature of water are blocked because ordinary liquid-vapor saturation is not defined there in the same way.
• In boiling mode, pressure must be absolute and greater than zero. Using gauge pressure can give the wrong boiling temperature.
• Small differences between sources are normal because different references may use different fitted equations, constants, or rounding.

How the calculator works

In vapor-pressure mode, the tool converts your Water temperature to degrees C or K as needed, applies the selected method, and then converts the answer into the chosen pressure unit plus the standard check outputs in kPa and mmHg.

In boiling-point mode, the tool converts your Absolute pressure to a working pressure unit, then solves for the temperature where water's saturation vapor pressure equals that absolute pressure.

Formulas used

P = P_c * exp((T_c / T) * (a1*τ + a2*τ^(3/2) + a3*τ^3 + a4*τ^(7/2) + a5*τ^4 + a6*τ^(15/2)))

τ (distance from critical temperature) = 1 - T / T_c

log10(P_mmHg) = A - B / (C + T_C)

T_C = B / (A - log10(P_mmHg)) - C

P_kPa = P_Pa / 1000 = P_hPa / 10 = P_bar * 100 = P_atm * 101.325 = P_psi * 6.894757293168 = P_mmHg * 0.133322387415

Here, T is absolute temperature in K, T_C is temperature in C, and P is pressure. The IAPWS method uses water critical constants T_c = 647.096 K and P_c = 22.064 MPa. The Antoine method returns pressure in mmHg first, then the calculator converts it to the other pressure units.

Worked mini-example

If you choose vapor pressure mode, enter 25 C, use the IAPWS reference fit, and keep pressure in kPa, the result is about 3.17 kPa. The same pressure is about 23.77 mmHg, which matches common reference values for room temperature.

If you choose boiling mode, enter 1 atm as Absolute pressure, the solved Boiling temperature in C is about 100 C.

Range handling and interpretation

The calculator checks the selected method before solving. Antoine is only used inside its supported temperature window, so it will show an error instead of extrapolating outside that range. The IAPWS option is the better general reference for liquid water over a wider range.

Range note tells you whether your input is inside the allowed range. Method used helps when your class, lab, or source table uses a different equation and you want to compare results.

Assumptions behind the result

This tool assumes pure water, phase equilibrium, and liquid-water saturation behavior. Real systems can differ if water is mixed with other substances, if the pressure entered is gauge instead of absolute, or if your source uses a different fitted equation or rounded constants.