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Vapor Pressure Calculator

Chemistry

Calculate vapor pressure at any temperature using the Clausius-Clapeyron equation. Enter a known vapor pressure and temperature pair, enthalpy of vaporisation, and find vapor pressure at a new temperature.

760 mmHg
mmHg
373 K
K
40.7 kJ/mol
kJ/mol
350 K
K

Vapor Pressure at T₂ (mmHg)

320.815
Vapor Pressure at T₂ (atm)
0.422
Vapor Pressure at T₂ (kPa)
42.772

This calculator computes your Vapor Pressure at T₂ (mmHg), Vapor Pressure at T₂ (atm), Vapor Pressure at T₂ (kPa) from the values you enter.

Inputs
Known Vapor Pressure (P₁)Temperature at P₁ (T₁)Enthalpy of Vaporisation (ΔHvap)New Temperature (T₂)
Outputs
Vapor Pressure at T₂ (mmHg)Vapor Pressure at T₂ (atm)Vapor Pressure at T₂ (kPa)

What is a Vapor Pressure?

The Vapor Pressure Calculator computes the vapor pressure of a liquid at any temperature using the Clausius-Clapeyron equation: ln(P₂/P₁) = (ΔHvap/R) × (1/T₁ − 1/T₂). By entering a reference vapor pressure at a known temperature, the enthalpy of vaporisation, and the target temperature, the calculator returns the vapor pressure in three units: mmHg, atm, and kPa.

Vapor pressure is the equilibrium pressure of a liquid's vapor above its surface at a given temperature. It governs volatility, boiling point, evaporation rate, and behaviour in mixtures. The Clausius-Clapeyron equation quantifies the steep, exponential increase in vapor pressure with temperature — a relationship rooted in Boltzmann statistics: as temperature rises, more molecules acquire the kinetic energy to escape the liquid phase.

For pure water, the reference point most commonly used is the normal boiling point (P₁ = 760 mmHg at T₁ = 373 K) with ΔHvap = 40.7 kJ/mol. But the equation works for any liquid given its ΔHvap and any known (P, T) pair. The inverse of this calculation — finding the temperature at which vapor pressure equals a target atmospheric pressure — gives the boiling point, handled by the Boiling Point at Altitude Calculator and Boiling Point Calculator.

How to use this Vapor Pressure calculator

  1. Find a reference vapor pressure for your liquid at a known temperature. Common references: water at 100°C (373 K) = 760 mmHg; water at 25°C (298 K) = 23.8 mmHg; ethanol at 78.4°C (351.6 K) = 760 mmHg. Enter P₁ and T₁.
  2. Find the enthalpy of vaporisation ΔHvap for your liquid from a chemical databook or NIST. Enter in kJ/mol in the Enthalpy of Vaporisation field.
  3. Enter the target temperature T₂ in Kelvin in the New Temperature field.
  4. Read Vapor Pressure at T₂ (mmHg) — compare to 760 mmHg to determine whether the liquid boils at T₂ under atmospheric pressure.
  5. Use the atm value as input to Raoult's law for mixture vapour pressure calculations.

Formula & Methodology

Clausius-Clapeyron equation:

ln(P₂/P₁) = (ΔHvap/R) × (1/T₁ − 1/T₂) P₂ = P₁ × exp[(ΔHvap/R) × (1/T₁ − 1/T₂)]

Where: ΔHvap in J/mol (multiply kJ/mol by 1,000), R = 8.314 J/(mol·K), T in Kelvin

Unit conversions:

P_atm = P_mmHg / 760 P_kPa = P_atm × 101.325

Worked example — vapor pressure of water at 60°C:

Reference: P₁ = 760 mmHg at T₁ = 373 K (100°C), ΔHvap = 40,700 J/mol, T₂ = 333 K (60°C)

exponent = (40,700 / 8.314) × (1/373 − 1/333)           = 4,895.6 × (0.002681 − 0.003003)           = 4,895.6 × (−0.000322)           = −1.576  P₂ = 760 × e^(−1.576)    = 760 × 0.2071    = 157.4 mmHg  P₂_atm = 157.4 / 760 = 0.207 atm P₂_kPa = 0.207 × 101.325 = 21.0 kPa

The measured vapor pressure of water at 60°C is 149.4 mmHg (from steam tables). The Clausius-Clapeyron approximation (assuming constant ΔHvap) gives 157.4 mmHg — a 5% overestimate, which is typical for the equation applied over a 40°C range where ΔHvap varies slightly with temperature.

Frequently Asked Questions

Vapor pressure is the pressure exerted by the vapor of a substance in equilibrium with its liquid (or solid) phase at a given temperature in a closed container. It represents the tendency of molecules to escape from the liquid surface into the gas phase. Higher vapor pressure means more volatile liquid — more molecules have enough kinetic energy to escape at that temperature. At the normal boiling point, the vapor pressure equals atmospheric pressure (760 mmHg or 1 atm).
The Clausius-Clapeyron equation relates vapor pressure to temperature: ln(P₂/P₁) = (ΔHvap/R) × (1/T₁ − 1/T₂), where P₁ and P₂ are vapor pressures at temperatures T₁ and T₂ (in Kelvin), ΔHvap is the enthalpy of vaporisation (J/mol), and R is the gas constant (8.314 J/mol·K). This exponential temperature dependence explains why vapor pressure rises sharply with temperature — liquids become much more volatile as they approach their boiling point.
Enthalpy of vaporisation (also called heat of vaporisation or latent heat of vaporisation) is the energy required to convert one mole of liquid to vapor at constant temperature and pressure. It reflects the energy needed to overcome intermolecular attractive forces. Common values: water = 40.7 kJ/mol (unusually high due to hydrogen bonding), ethanol = 38.6 kJ/mol, methanol = 35.3 kJ/mol, acetone = 31.4 kJ/mol, hexane = 28.9 kJ/mol. High ΔHvap means low vapor pressure at a given temperature.
Vapor pressure increases exponentially with temperature. The exponential relationship comes from the Boltzmann factor: at higher temperature, a larger fraction of molecules have kinetic energy exceeding the intermolecular binding energy. For water, vapor pressure roughly doubles for every 10°C rise near room temperature. At 100°C, vapor pressure equals 1 atm (760 mmHg). At 37°C (body temperature), vapor pressure of water is about 47 mmHg. At 20°C, it is about 17.5 mmHg.
The boiling point is the temperature at which a liquid's vapor pressure equals the external (atmospheric) pressure. The relationship is the inverse of this calculator's function: instead of finding vapor pressure at a new temperature, you find the temperature at which vapor pressure equals a target pressure. At higher altitude (lower atmospheric pressure), water boils at a lower temperature — e.g., at 3,000 m altitude in the Himalayas, atmospheric pressure is about 530 mmHg and water boils at approximately 90°C. Use the [Boiling Point at Altitude Calculator](/boiling-point-at-altitude-calculator/) for this specific calculation.
Enter a known vapor pressure P₁ at temperature T₁ in Kelvin (e.g., for water: P₁ = 760 mmHg at T₁ = 373 K). Enter the enthalpy of vaporisation ΔHvap in kJ/mol. Enter the new temperature T₂ in Kelvin. The calculator applies the Clausius-Clapeyron equation to return vapor pressure at T₂ in mmHg, atm, and kPa.
The three most common vapor pressure units are: mmHg (millimeters of mercury, also called torr — standard in laboratory chemistry); atm (atmospheres — 1 atm = 760 mmHg); kPa (kilopascals — 1 atm = 101.325 kPa). This calculator returns all three. To convert manually: mmHg ÷ 760 = atm; atm × 101.325 = kPa; mmHg ÷ 7.5006 = kPa.
Trouton's rule states that the entropy of vaporisation at the normal boiling point (ΔSvap = ΔHvap/Tb) is approximately constant at 85–90 J/mol·K for most non-polar liquids. This means ΔHvap ≈ 85 × T_b (K) for non-polar liquids. Water is a notable exception — its ΔSvap ≈ 109 J/mol·K is higher than predicted because liquid water has extensive hydrogen-bond structure that is largely destroyed upon vaporisation. Polar liquids and liquids with strong intermolecular forces generally deviate from Trouton's rule.
In high-altitude regions of India — Leh-Ladakh (3,500 m), Shimla (2,200 m), Darjeeling (2,100 m), Manali (2,050 m) — the reduced atmospheric pressure (about 65–80% of sea-level pressure) causes water to boil at temperatures of 86–93°C rather than 100°C. Foods that require boiling water above 90°C (e.g., pasta, pulses, rice) take significantly longer to cook or require a pressure cooker. This directly follows from the Clausius-Clapeyron equation applied in reverse: lower external pressure = lower boiling point.
Raoult's law states that the partial vapor pressure of a component in an ideal solution equals its mole fraction multiplied by its pure vapor pressure: P_A = x_A × P°_A, where P°_A is the vapor pressure of the pure component at the same temperature. For an ideal binary mixture, the total vapor pressure is P_total = x_A × P°_A + x_B × P°_B. Raoult's law uses the vapor pressures calculated here as the pure-component reference values. The Raoult's Law Calculator uses this relationship for mixture vapour pressure calculations.
Relative humidity (RH) is the ratio of the actual partial pressure of water vapor in the air to the saturation vapor pressure at that temperature, expressed as a percentage: RH = (P_water / P°_water) × 100%. At saturation (RH = 100%), the air holds the maximum water vapor possible at that temperature — the dew point. The saturation vapor pressure of water (P°_water) at any temperature is what this calculator computes from the Clausius-Clapeyron equation. Rising temperature increases P°_water, which is why warm air can hold more moisture.