Choose saturation vapor pressure or saturation temperature for pure liquid water from 1–100 °C using the labeled temperature and absolute-pressure units.

Pure liquid-water saturation estimate, 1–100 °C only. This is absolute saturation pressure, not gauge pressure or the actual water-vapor partial pressure in unsaturated air.

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.

log₁₀(PmmHg) = A - (B) / (C + TC)
PmmHg = 10A - (B) / (C + T<sub>C< / sub>)
TC = (B) / (A - log₁₀(PmmHg)) - C
  • PmmHg = water vapor pressure in millimeters of mercury
  • TC = 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. Pressure must be positive and within this fit’s 1–100 °C range, approximately 0.6513–101.3365 kPa absolute, when solving for temperature.

The calculator also applies these unit conversions when needed:

TC = (TF - 32)(5) / (9)
TC = TK - 273.15
1 atm = 101.325 kPa
1 mmHg = 0.133322 kPa
1 psi = 6.89476 kPa

Water Vapor Pressure Reference Values

These are approximate reference values. The 0 °C row provides context and is outside this calculator’s enforced 1–100 °C fit range.

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.

log₁₀(P) = 8.07131 - (1730.63) / (233.426 + 25)
log₁₀(P) ≈ 1.3745
P ≈ 101.3745 = 23.69 mmHg

Convert to kPa:

23.69 × 0.133322 ≈ 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:

PmmHg = (101.325) / (0.133322) ≈ 760.00 mmHg

Then use the inverse Antoine equation:

TC = (1730.63) / (8.07131 - log₁₀(760.00)) - 233.426
TC ≈ 100.00^° 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. This calculator retains the A=8.07131, B=1730.63, C=233.426 fit with °C and mmHg and enforces 1–100 °C in both directions. It does not extrapolate to ice or high-temperature steam.

Can vapor pressure be higher than atmospheric pressure?

Yes, saturation vapor pressure exceeds 1 atm above the normal boiling point. This calculator is limited to 1–100 °C and returns approximately 1 atm at its upper limit. All pressure inputs and results are absolute, not gauge pressure.