Last updated: July 3, 2026
Vapor Pressure of Water Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The vapor pressure of water calculator estimates saturation vapor pressure from temperature. It uses the Antoine equation for pure water from about 1 to 100 °C as the primary result in mmHg, converts to kPa and atm, and also reports the Tetens/Magnus kPa approximation used in humidity calculations.
Water vapor pressure is the equilibrium pressure of water vapor above pure liquid water at a given temperature. At 25 degrees Celsius it is about 23.7 millimeters of mercury, or 3.16 kilopascals.
Key Takeaways
- Water vapor pressure is the saturation pressure of water vapor in equilibrium with liquid water.
- This calculator uses Antoine constants A = 8.07131, B = 1730.63, C = 233.426 for about 1–100 °C.
- At 25 °C, pure water vapor pressure is about 23.7 mmHg or 3.16 kPa.
- At 100 °C, water vapor pressure is about 760 mmHg, equal to one atmosphere.
- Tetens/Magnus gives a convenient kPa estimate for humidity and meteorology applications.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
log10(P_mmHg) = A − B/(C + T); P_kPa = P_mmHg × 101.325/760; Tetens: P_kPa = 0.61078 × exp(17.27T/(T + 237.3))
Where:
- P=Saturation vapor pressure of pure water(mmHg or kPa)
- T=Water temperature(°C)
- A=Antoine constant for water, 1–100 °C(8.07131)
- B=Antoine constant for water, 1–100 °C(1730.63)
- C=Antoine constant for water, 1–100 °C(233.426 °C)
- P_Tetens=Tetens/Magnus approximate saturation pressure(kPa)
Worked Examples
Boiling point of water at 1 atm
At 100 °C the saturation vapor pressure of pure water is approximately one standard atmosphere.
- 1Use log10(P) = 8.07131 − 1730.63/(233.426 + 100).
- 2The exponent is about 2.88092.
- 3Raise 10 to that power to obtain about 760 mmHg, or 101.3 kPa.
Room-temperature water
Estimate the vapor pressure of water on a typical 25 °C laboratory bench.
- 1Substitute T = 25 °C in the Antoine equation.
- 2Compute log10(P) = 8.07131 − 1730.63/258.426 ≈ 1.3745.
- 3P = 10^1.3745 ≈ 23.7 mmHg, which is about 3.16 kPa.
Meteorology comparison at 20 °C
Use the Tetens/Magnus approximation often used for saturation vapor pressure in air calculations.
- 1Compute 17.27 × 20/(20 + 237.3) ≈ 1.3424.
- 2Evaluate exp(1.3424) ≈ 3.828.
- 3Multiply by 0.61078 to get approximately 2.338 kPa.
Introduction
The vapor pressure of water is the pressure exerted by water vapor when liquid water and vapor are in equilibrium at a specified temperature. This calculator uses the Antoine equation as the primary model for pure water from about 1 to 100 °C, then converts the result between mmHg, kPa, and atm. It also reports the Tetens/Magnus approximation used in humidity work, complementing tools such as the vapor pressure calculator and boiling point at altitude calculator. Authoritative reference data are available from NIST Chemistry WebBook and the IUPAC Gold Book.
What is the vapor pressure of water?
Water molecules continually leave and return to the liquid surface. In a closed space, evaporation and condensation eventually balance; the resulting equilibrium gas pressure is the saturation vapor pressure. It rises strongly with temperature because warmer molecules escape the liquid more easily.
Low vapor pressure means little water vapor is needed for saturation.
At 100 °C, water vapor pressure is close to 760 mmHg, so boiling occurs at 1 atm.
At 25 °C, saturation occurs near 23.7 mmHg (3.16 kPa).
For gas mixtures, combine saturation pressure with partial pressure concepts.
Antoine equation used by this calculator
The primary calculation is log10(P) = A − B/(C + T), where P is in mmHg and T is in degrees Celsius. For water over about 1–100 °C, this calculator uses A = 8.07131, B = 1730.63, and C = 233.426. The result is then converted with 1 atm = 760 mmHg = 101.325 kPa.
Do not mix Antoine constant sets. A, B, and C are unit-specific and range-specific.
Tetens/Magnus approximation
The Tetens expression P(kPa) = 0.61078 × exp(17.27T/(T + 237.3)) is common in meteorology and psychrometrics. It directly gives saturation vapor pressure in kPa for temperature in °C. Around room temperature it agrees well with tabulated water saturation pressures, although Antoine is the primary output here for chemistry work.
Tetens is convenient for humidity, dew point, and air–water calculations; Antoine is preferred when you need a chemistry-style vapor pressure in mmHg.
How to calculate water vapor pressure
Enter the water temperature in degrees Celsius. The calculator evaluates the Antoine logarithm, converts the pressure to kPa and atm, and separately evaluates the Tetens approximation. A validity note reminds you whether the entered temperature is inside the recommended Antoine range.
Measure or choose T in °C.
Evaluate A − B/(C + T).
Calculate P = 10^exponent in mmHg.
Convert to kPa using P × 101.325/760.
Compare with Tetens if the application is humidity or weather related.
Common saturation pressures for water
The table gives useful check values for pure water. Small differences can occur among references because equations are fitted over finite temperature intervals.
| Temperature | Approx. vapor pressure | Use case |
|---|---|---|
| 10 °C | ≈ 9.2 mmHg (1.23 kPa) | cool room or groundwater |
| 20 °C | ≈ 17.5 mmHg (2.33 kPa) | humidity calculations |
| 25 °C | ≈ 23.7 mmHg (3.16 kPa) | laboratory room temperature |
| 50 °C | ≈ 92.3 mmHg (12.3 kPa) | warm bath or process water |
| 100 °C | ≈ 760 mmHg (101.3 kPa) | normal boiling point |
Assumptions and limitations
The calculator describes pure liquid water in equilibrium with its vapor. Dissolved solutes lower water activity and reduce vapor pressure; for ideal dilute solutions this is treated by Raoult's law. Nonideal solutions, high pressures, and temperatures outside the fitted range require more specialized correlations or steam tables.
Valid Antoine range here: about 1–100 °C.
Pressure is saturation pressure, not relative humidity.
Solutes, curvature, and nonideal vapor behavior can shift the real value.
For osmotic or colligative effects, see the osmotic pressure calculator.
Reference data and standards
Vapor pressure equations are empirical fits to experimental data. For critical work, compare with primary references such as the NIST Chemistry WebBook, the NIST/TRC Web Thermo Tables, or peer-reviewed formulations for the thermodynamic properties of water.
Quick Reference Card
Water Vapor Pressure — Quick Reference
Quick reference • Vapor Pressure of Water Calculator
log10(P_mmHg) = 8.07131 − 1730.63/(233.426 + T°C)Valid range: Antoine constants used here are intended for pure water from about 1 °C to 100 °C
Common Values
⚠ Watch Out
- •Use °C with these Antoine constants; using kelvin will give the wrong answer.
- •Do not extrapolate far outside 1–100 °C without selecting a better constant set.
- •The result is for pure water, not salt solutions or mixtures.
- •Saturation vapor pressure is not the same as actual humidity unless the air is saturated.
Pro Tips
- →Convert mmHg to kPa by multiplying by 101.325/760.
- →Use Tetens for quick humidity calculations near normal weather temperatures.
- →At boiling, vapor pressure equals the surrounding pressure.
- →For solutions, estimate vapor-pressure lowering with Raoult's law before comparing to pure water.
FAQs
What is water vapor pressure at 25 °C?
Using the Antoine constants in this calculator, the saturation vapor pressure of pure water at 25 °C is about 23.7 mmHg, equivalent to about 3.16 kPa or 0.031 atm.
Why is water vapor pressure 760 mmHg at 100 °C?
At the normal boiling point, the saturation vapor pressure of liquid water equals the external pressure of one standard atmosphere. One atmosphere is defined as 760 mmHg, so water boils near 100 °C at sea level.
Is vapor pressure the same as humidity?
No. Saturation vapor pressure is the maximum equilibrium pressure of water vapor at a temperature. Relative humidity compares the actual water-vapor partial pressure in air with that saturation value.
What units does the Antoine equation use here?
With A = 8.07131, B = 1730.63, and C = 233.426, temperature must be in degrees Celsius and the pressure comes out in mmHg. The calculator converts that result to kPa and atm.
When should I use Tetens instead of Antoine?
Use Tetens/Magnus for quick humidity, dew-point, and meteorological estimates. Use Antoine for chemistry vapor-pressure estimates over its specified fitted range.
Does salt water have the same vapor pressure as pure water?
No. Dissolved salts lower the activity of water, so the vapor pressure above salt water is lower than that above pure water at the same temperature. Raoult's law is the ideal starting point for that correction.