Water Vapor Pressure Calculator | Formula

Calculate water vapor pressure from temperature, or find temperature from vapor pressure, using the Antoine equation in °C, °F, K, kPa, atm, mmHg, or psi.

Water Vapor Pressure Formula

The calculator uses the Antoine equation for the saturation vapor pressure of water. Temperature is converted to degrees Celsius before the formula is applied. Vapor pressure is calculated in mmHg first, then converted to your selected unit.

\log_{10}(P_{mmHg}) = A - \frac{B}{C + T_C}

P_{mmHg} = 10^{A - \frac{B}{C + T_C}}

T_C = \frac{B}{A - \log_{10}(P_{mmHg})} - C

P_mmHg = water vapor pressure in millimeters of mercury
T_C = temperature in degrees Celsius
A = 8.07131
B = 1730.63
C = 233.426

If you enter temperature, the calculator uses the Antoine equation to find the vapor pressure. If you enter vapor pressure, it uses the rearranged Antoine equation to find the temperature. The pressure value must be greater than 0 when solving for temperature.

The calculator also applies these unit conversions when needed:

T_C = (T_F - 32)\frac{5}{9}

T_C = T_K - 273.15

1\ atm = 101.325\ kPa

1\ mmHg = 0.133322\ kPa

1\ psi = 6.89476\ kPa

Water Vapor Pressure Reference Values

These values are useful checks for water vapor pressure near common temperatures. Values are approximate saturation vapor pressures.

Temperature	Vapor Pressure	Common meaning
0°C	about 0.61 kPa	Near freezing point
20°C	about 2.33 kPa	Room temperature
25°C	about 3.16 kPa	Standard lab reference temperature
50°C	about 12.3 kPa	Warm water
100°C	about 101.3 kPa	Boiling point at 1 atm

Pressure Unit Conversions

Unit	Equivalent in kPa	Equivalent in mmHg
1 kPa	1 kPa	7.5006 mmHg
1 atm	101.325 kPa	760 mmHg
1 mmHg	0.133322 kPa	1 mmHg
1 psi	6.89476 kPa	51.7149 mmHg

Example Calculations

Example 1: Find vapor pressure from temperature

Find the water vapor pressure at 25°C.

\log_{10}(P) = 8.07131 - \frac{1730.63}{233.426 + 25}

\log_{10}(P) \approx 1.3745

P \approx 10^{1.3745} = 23.69\ mmHg

Convert to kPa:

23.69 \times 0.133322 \approx 3.16\ kPa

Example 2: Find temperature from vapor pressure

Find the temperature when the water vapor pressure is 101.325 kPa.

First convert kPa to mmHg:

P_{mmHg} = \frac{101.325}{0.133322} \approx 760.00\ mmHg

Then use the inverse Antoine equation:

T_C = \frac{1730.63}{8.07131 - \log_{10}(760.00)} - 233.426

T_C \approx 100.00^\circ C

FAQ

What does water vapor pressure mean?

Water vapor pressure is the pressure exerted by water vapor when liquid water and its vapor are in equilibrium at a given temperature. As temperature rises, more water molecules enter the vapor phase, so the vapor pressure increases.

Why does the calculator use the Antoine equation?

The Antoine equation is a common empirical formula for estimating the vapor pressure of pure substances. For water, the constants used here are suited for common liquid-water temperature calculations, especially near the 1°C to 100°C range.

Can vapor pressure be higher than atmospheric pressure?

Yes, at temperatures above the normal boiling point, the saturation vapor pressure of water is higher than 1 atm. At 100°C, water vapor pressure is about 1 atm. Below 100°C, it is lower than 1 atm under normal conditions.