Vapor Pressure of Water Calculator
Estimate saturation pressure of water between 0 and 100 C and relate it to relative humidity or partial pressure.
Accurate for liquid water near atmospheric pressure.
Optional. Leave blank if you only need saturation pressure.
Saturation pressure
3.16 kPa
23.69 mmHg | 0.0312 atm | 0.458 psi
How to Use This Calculator
Enter water temperature
Provide the liquid temperature between 0 and 100 C.
Optional: relative humidity
Add ambient relative humidity to compute the actual water vapor partial pressure.
Review saturation pressure
Results list pressure in kPa, mmHg, atm, and psi.
Use in design or weather analysis
Compare saturation and actual vapor pressures to study evaporation or HVAC processes.
Formula
log10(Psat) = 8.07131 - 1730.63 / (233.426 + T)
Psat is in mmHg. Convert to kPa by multiplying by 0.133322368. Partial pressure = Psat * RH / 100 when relative humidity is provided.
Example
At 25 C: log10(Psat) = 1.336, so Psat = 21.7 mmHg (2.89 kPa). At 60 percent RH, partial pressure equals 1.73 kPa.
Full Description
Water vapor pressure rises rapidly with temperature and influences evaporation, boiling, weather, and indoor climate control.
The Antoine correlation offers a quick estimate using readily available constants, providing accurate results for many engineering calculations between freezing and boiling points.
Frequently Asked Questions
Can I use temperatures below 0 C?
The constants provided are most accurate above 0 C. For subzero values, use frost point correlations.
Does altitude matter?
Atmospheric pressure affects boiling but not the saturation pressure itself, which depends primarily on temperature.
How can I invert the calculation?
Use a numerical solver or reference tables to find temperature from vapor pressure if needed.
Is this the same as dew point?
Dew point is the temperature where partial pressure equals saturation. This calculator provides the saturation value for the current temperature.
Why use mmHg constants?
Historical data uses mmHg; the calculator converts to kPa, atm, and psi for convenience.