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Vapor Pressure Calculator

ChemistryGas Laws and Thermodynamics
Vapor pressure (mmHg) —
Vapor pressure (kPa)
—
Vapor pressure (atm)
—
Boils at sea-level pressure?
—

Vapor pressure follows the Antoine equation, log10 P = A − B ÷ (C + T), with P in mmHg and T in °C. For water (A 8.07131, B 1730.63, C 233.426) at 25 °C, log10 P = 1.3745, so P ≈ 23.7 mmHg, or about 3.16 kPa. At 100 °C the same constants give 760 mmHg, one atmosphere.

About this tool

Vapor pressure is the pressure a liquid's vapor exerts when it is in equilibrium with the liquid at a given temperature. Chemistry students meet it in Raoult's law and phase diagrams, lab staff need it to judge how fast a solvent evaporates, and HVAC and weather work rely on water's value for humidity. Pick a preset liquid or type your own Antoine constants, set the temperature, and the page returns the pressure in mmHg, kPa and atmospheres, the working of the equation, and whether the liquid would boil at sea-level pressure. The main limit: Antoine constants are a curve fit over a set temperature span, so values far outside it, especially near the critical point, can be off by several percent; the tool warns when a preset is used outside its fitted range.

How to use it

  1. Choose the substance

    Pick water, ethanol, benzene or acetone, or Custom Antoine constants to enter A, B and C from a data table.

  2. Enter the temperature

    Type the liquid temperature in degrees Celsius.

  3. Read the pressure

    The result shows mmHg, with kPa and atm below and the substituted equation in the working.

Examples

Water at 100 °C

Substance
Water
Temperature
100

Result Vapor pressure (mmHg): 760.1
Vapor pressure (kPa): 101.337
Vapor pressure (atm): 1.0001
Boils at sea-level pressure?: Yes, at or above 1 atm

  1. Constants (Water): A = 8.07131, B = 1730.63, C = 233.426
  2. log₁₀P = 8.07131 − 1730.63 ÷ (233.426 + 100) = 2.88086
  3. P = 10^2.88086 = 760.086 mmHg
  4. P = 760.086 × 0.133322 = 101.3365 kPa

The fitted constants return almost exactly one atmosphere at the normal boiling point, which is a quick check on the data set.

Water at 25 °C

Substance
Water
Temperature
25

Result Vapor pressure (mmHg): 23.7
Vapor pressure (kPa): 3.158
Vapor pressure (atm): 0.0312
Boils at sea-level pressure?: No, below 1 atm

  1. Constants (Water): A = 8.07131, B = 1730.63, C = 233.426
  2. log₁₀P = 8.07131 − 1730.63 ÷ (233.426 + 25) = 1.3745
  3. P = 10^1.3745 = 23.686 mmHg
  4. P = 23.686 × 0.133322 = 3.1579 kPa

About 3.2 kPa, the figure used to work out relative humidity and evaporation at room temperature.

Water at 150 °C (extrapolated)

Substance
Water
Temperature
150

Result Vapor pressure (mmHg): 3,611.7
Vapor pressure (kPa): 481.523
Vapor pressure (atm): 4.7523
Boils at sea-level pressure?: Yes, at or above 1 atm

  1. Constants (Water): A = 8.07131, B = 1730.63, C = 233.426
  2. log₁₀P = 8.07131 − 1730.63 ÷ (233.426 + 150) = 3.55771
  3. P = 10^3.55771 = 3611.721 mmHg
  4. P = 3611.721 × 0.133322 = 481.5232 kPa

The constants are fitted only to 100 °C, so the tool flags the result as an extrapolation.

How it is calculated

log10(P) = A − B ÷ (C + T)

P
vapor pressure in mmHg
T
temperature in °C
A, B, C
empirical Antoine constants for the substance

The Antoine equation is a three-parameter refinement of the Clausius–Clapeyron relation, fitted to measured vapor-pressure data. The preset constants are the common mmHg and °C sets: water for 1–100 °C, ethanol for −57–80 °C, benzene for 8–103 °C and acetone for −13–55 °C. The pressure is converted with 1 mmHg = 0.133322 kPa and 1 atm = 760 mmHg. Custom constants must use the same units, or the result will be wrong.

When not to use it

  • Near the critical point, where Antoine fits break down.
  • For mixtures; apply Raoult's law to each component instead.
  • With constants published for kPa and kelvin unless converted first.

Common mistakes

  • Mixing constants fitted for kelvin with a Celsius temperature.
  • Using natural log constants in a base-10 equation.
  • Trusting a preset far outside its fitted temperature span.

Frequently asked questions

Why does water boil at 100 °C?

A liquid boils when its vapor pressure equals the surrounding pressure. Water's vapor pressure reaches 760 mmHg, one standard atmosphere, at 100 °C, so that is its normal boiling point. At altitude the air pressure is lower and water boils at a lower temperature.

What units do the Antoine constants use here?

All presets and the custom fields assume pressure in mmHg and temperature in degrees Celsius, with a base-10 logarithm. NIST publishes many constants for bar and kelvin; those need converting before you enter them.

How is this different from the Clausius–Clapeyron equation?

Clausius–Clapeyron assumes a constant heat of vaporization and gives ln P linear in 1/T. Antoine adds a third constant, C, which absorbs much of the curvature, so it tracks real data more closely over a moderate range.

Can I use this for a solution or mixture?

Not directly. For an ideal solution, the partial pressure of each component is its mole fraction times the pure-liquid vapor pressure from this page (Raoult's law), and the total is the sum of the partial pressures.

Why did I get a warning?

Each preset is fitted only over a set temperature span. Outside it the equation still returns a number, but it is an extrapolation and can drift by several percent, so the page flags it.

How does vapor pressure relate to humidity?

Relative humidity is the partial pressure of water vapor in the air divided by water's saturation vapor pressure at that temperature. At 25 °C the saturation value is about 23.7 mmHg, so air holding 11.9 mmHg of water vapor is at about 50% relative humidity.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 27). Vapor Pressure Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/vapor-pressure-calculator/

MLA

Tools Directory Editorial Team. “Vapor Pressure Calculator.” Tools Directory, 27 Oct. 2026, https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/vapor-pressure-calculator/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Vapor Pressure Calculator.” Tools Directory. Last modified October 27, 2026. Accessed October 27, 2026. https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/vapor-pressure-calculator/.

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