Water Vapor Pressure Calculator - Antoine Equation Saturation Pressure
Use the vapour pressure of water calculator with the Antoine equation: enter a temperature and read the saturation pressure in mmHg, kPa, atm, bar, Pa, or psi.
Water Vapor Pressure Calculator
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What Is Water Vapor Pressure Calculator?
The vapour pressure of water calculator finds the saturation pressure of liquid water at a temperature you choose, using the Antoine equation. Instead of looking up a steam table, you type a temperature in Celsius, Fahrenheit, or Kelvin and the tool returns the pressure in mmHg, kPa, atm, bar, Pa, or psi.
- • Chemistry and Physics Classes: Students verify how fast the saturation pressure of water climbs with temperature and check their hand calculations against a trusted empirical formula.
- • Lab and Process Planning: Technicians estimate evaporation and vacuum-drying behavior by reading the pressure water exerts at a given bath or oven temperature.
- • Weather and Humidity Work: Meteorology learners connect this saturation pressure to dew point and relative humidity, where it acts as the reference ceiling for water vapor in air.
This number matters because the saturation pressure of water sets the line between liquid and vapor for every process that involves boiling, condensation, drying, or humidity. The same value tells you both how hard water pushes to become vapor and at what temperature it will boil under a given pressure. Use this calculator when you need a quick, traceable saturation pressure without opening a printed table.
Because relative humidity compares the actual vapor pressure of water in air to this saturation value, the relative humidity calculator shows how the two pressures combine to describe how humid the air feels.
How Water Vapor Pressure Calculator Works
The Antoine equation is an empirical fit that captures how the saturation pressure of water rises with temperature.
- T: Temperature of the liquid water in degrees Celsius
- P: Saturation pressure of water, returned in mmHg by the Antoine equation
- A, B, C: Antoine constants for water, which change between the low and high temperature bands
The constants are not universal; they change with the substance and with the temperature band. For water we use one set from 1 °C to 100 °C and a second set from 99 °C up to 374 °C. The calculator picks the band automatically and converts the mmHg result into whichever unit you selected, so you never have to do the unit math by hand.
According to the NIST Chemistry WebBook, the vapour pressure of water reaches 1 atm (760 mmHg) at its normal boiling point of 100 °C, and the Antoine set for water reproduces measured pressures closely across the 1–100 °C range.
Worked Example: Water at 25 °C
Temperature T = 25 °C, use the 1–100 °C Antoine set (A = 8.07131, B = 1730.63, C = 233.426).
1. Compute C + T = 233.426 + 25 = 258.426. 2. Divide B by that sum: 1730.63 / 258.426 = 6.7010. 3. Subtract from A: 8.07131 − 6.7010 = 1.3703 (this is log10(P)). 4. Raise 10 to the power: P = 10^1.3703 = 23.46 mmHg. 5. Convert to kPa: 23.46 × 0.133322 = 3.13 kPa.
Saturation pressure of water at 25 °C ≈ 23.46 mmHg, or 3.13 kPa.
Published tables list 23.8 mmHg (3.17 kPa), so the equation is within 1% in this band.
According to NIST Chemistry WebBook — Water, the normal boiling point of water occurs at 100 °C where the pressure reaches 1 atm (760 mmHg).
Water reaches its boiling point when the saturation pressure you calculate here equals the surrounding atmospheric pressure, which the boiling point calculator turns into a boiling temperature.
Key Concepts Explained
Four ideas explain why the saturation pressure of water behaves the way it does.
Saturation Pressure
A substance's saturation pressure is the pressure its vapor exerts when liquid and vapor are in balance in a closed space. At that point evaporation and condensation happen at equal rates, and no more vapor can be held at that temperature. The IUPAC Gold Book defines vapour pressure as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase at a given temperature in a closed system.
Temperature Dependence
The vapour pressure of water rises steeply with temperature: about 4.6 mmHg at 0 °C, 23.8 mmHg at 25 °C, and 760 mmHg at 100 °C. Doubling the temperature in Celsius more than doubles the pressure, which is why a small temperature rise sharply speeds evaporation and drying.
Boiling and Pressure
Water boils when its saturation pressure equals the external pressure. At sea level that happens at 100 °C; on a mountain where the air pressure is lower, it happens sooner. The partial pressure calculator shows how water vapor contributes its own pressure inside a gas mixture once the air is saturated.
Antoine Constants and Range
The Antoine equation needs substance-specific constants and a valid temperature band. The 1–100 °C set is accurate within a few percent across that range but drifts near 100 °C, while the 99–374 °C set covers steam conditions. The calculator switches bands automatically and flags values outside the validated range.
Keeping these ideas separate helps you read the result correctly: the temperature sets the physics, the Antoine constants encode the fit, and the unit only rescales the final number.
In a gas mixture the water vapor contributes its own partial pressure equal to this saturation value when the air is fully saturated, a relationship the partial pressure calculator works through for mixed gases.
How to Use This Calculator
Follow these steps to get a saturation pressure from any temperature.
- 1 Enter the Temperature: Type the temperature of the water in the Temperature field. Use any value from below freezing up to 374 °C.
- 2 Pick the Temperature Unit: Choose Celsius, Fahrenheit, or Kelvin to match your entry. The calculator converts internally to Celsius before applying the Antoine equation.
- 3 Pick the Pressure Unit: Choose mmHg, kPa, Pa, atm, bar, or psi for the result. The conversion is applied to the Antoine mmHg value automatically.
- 4 Read the Result: The Saturation Pressure of Water updates as you type. Compare it with a known value or feed it into a larger problem.
- 5 Use It Downstream: Take the pressure at, say, 50 °C and use it in a gas or humidity calculation, or check where condensation begins as air cools to that temperature.
Pressure of water at 50 °C: enter 50 in the Temperature field, leave the unit on Celsius, and choose kPa. The Antoine 1–100 °C set gives log10(P) = 8.07131 − 1730.63/(233.426 + 50) = 8.07131 − 6.1097 = 1.9616, so P = 10^1.9616 = 92.5 mmHg, which is 12.34 kPa. Water at 50 °C exerts about 12.34 kPa (92.5 mmHg); cool saturated air to 50 °C and condensation begins there, as the dew point calculator shows.
Cooling moist air until its pressure falls to this saturation value is exactly what produces condensation, the process the dew point calculator quantifies.
Benefits of Using This Calculator
Using this calculator avoids manual table lookups and unit juggling.
- • No Steam-Table Lookup: The saturation pressure of water is computed from any temperature in seconds, ideal for homework and quick lab estimates.
- • All Common Units: Every common temperature and pressure unit is supported, so you can match textbook, lab, and engineering conventions without manual conversion.
- • Automatic Constant Selection: The correct Antoine constant set is chosen for the temperature band, removing the most common source of error in hand calculations.
- • Transparent Formula: The formula and constants in use are shown, so you can check the output against your own working.
- • Range Warnings: Results outside the validated Antoine range are flagged, so you know when a number is an extrapolation rather than a measured-fit value.
- • Connects to Other Tools: Once you have the pressure, the ideal gas calculator turns it into moles or volume using the ideal gas law.
These benefits matter most when you chain several steps, such as moving from a temperature to a pressure and then into a gas or humidity calculation.
Once you know the pressure, the ideal gas calculator lets you convert that water vapor pressure into moles or volume using the ideal gas law.
Factors That Affect Your Results
A few factors change how you should read the result.
Temperature Band
The Antoine constants change between the 1–100 °C set and the 99–374 °C set. Near 100 °C the low-band set reads about 751.6 mmHg versus the accepted 760 mmHg for the vapour pressure of water, a small known bias at the edge of its range.
Purity of the Water
These constants describe pure water. Dissolved salts or other substances lower the pressure (Raoult's effect), so this calculator answers the pure-water question, not a solution's pressure.
Unit Choice
The Antoine equation returns mmHg; every other unit is a conversion. The mmHg-to-Pa factor is 133.322, and kPa is simply mmHg × 0.133322. Pick the unit your source data uses to avoid an extra conversion step.
Extrapolation Below 1 °C
Below 1 °C the 1–100 °C set is extended past its validated range. The number is still useful for trends, but treat it as an estimate rather than a measured-fit value.
- • The Antoine equation is an empirical fit, not a physical law; it is accurate to a few percent within its stated band and drifts outside it.
- • It describes pure liquid water only. Mixtures, supercooled water, and ice require different treatments and are out of scope here.
According to Antoine Equation (Wikipedia), the constants A = 8.07131, B = 1730.63, C = 233.426 describe water in mmHg and °C over the 1–100 °C band.
According to IUPAC Gold Book — Vapour Pressure, vapour pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase at a given temperature in a closed system.
Frequently Asked Questions
Q: What is the saturation pressure of water at 25 °C?
A: At 25 °C the saturation pressure of water is about 23.8 mmHg, which equals roughly 3.17 kPa or 0.0313 atm. The Antoine equation returns 23.46 mmHg (3.13 kPa) in the 1–100 °C band, within 1% of published tables.
Q: How does the Antoine equation calculate this saturation pressure?
A: The Antoine equation is log10(P) = A − B/(C + T), with T in °C and P in mmHg. For water between 1 °C and 100 °C the constants are A = 8.07131, B = 1730.63, and C = 233.426. You add C to the temperature, divide B by that, subtract from A to get log10(P), then raise 10 to that power.
Q: What is water's saturation pressure?
A: Water's saturation pressure is the pressure its vapor exerts when liquid water and vapor are in equilibrium in a closed container at a given temperature. It rises from about 4.6 mmHg at 0 °C to 760 mmHg at 100 °C. The IUPAC Gold Book defines vapour pressure as the pressure of a vapor in thermodynamic equilibrium with its condensed phase.
Q: Why does the saturation pressure of water increase with temperature?
A: Warmer water has more molecules with enough kinetic energy to escape the liquid surface, so more molecules enter the vapor phase and the pressure above the liquid climbs. The relationship is steep and nonlinear, which is why the Antoine equation uses a reciprocal temperature term rather than a straight line.
Q: At what temperature does water reach 1 atm of pressure?
A: Water reaches a pressure of 1 atm (760 mmHg) at its normal boiling point of 100 °C at sea level. The Antoine 1–100 °C set returns 760.09 mmHg at 100 °C, matching the accepted value per the NIST Chemistry WebBook.
Q: How does water vapor relate to relative humidity?
A: Relative humidity is the ratio of the actual water vapor pressure in the air to the saturation pressure at that air temperature. When the two are equal the air is saturated at 100% humidity and condensation begins. The relative humidity calculator uses this exact ratio, so the saturation value from this tool is its denominator.