Vapour Pressure of Water Calculator
Calculate water vapour pressure at any temperature using the Antoine equation. Free chemistry calculator for students and researchers with results in kPa, mmHg, atm, and bar.
About This Calculator
The Vapour Pressure of Water Calculator computes the equilibrium vapour pressure of water at any given temperature using the Antoine equation, one of the most widely used semi-empirical formulas in physical chemistry. This tool is essential for students, chemists, engineers, and researchers working with phase equilibria, distillation, evaporation, and thermodynamic systems involving water.
The calculator uses the Antoine equation in the form log₁₀(P) = A − B / (C + T), where P is the vapour pressure in mmHg and T is the temperature in degrees Celsius. For water in the 1–100 °C range, the constants are A = 8.07131, B = 1730.63, and C = 233.426. The result is also converted to kilopascals (kPa), atmospheres (atm), and bars (bar) for international convenience.
Regional Context
Worldwide: Vapour pressure is a fundamental thermodynamic property independent of region. Temperature units are in degrees Celsius (°C), and pressure is reported in kPa (SI unit), mmHg (common in chemistry), atm (standard atmosphere), and bar (meteorology).
Education: This calculator is used globally in chemistry and physics classrooms. The Antoine constants for water are standardized across all textbooks and references worldwide.
Applications: Vapour pressure calculations are critical in distillation column design, steam engineering, humidity and dew point analysis, meteorology, food processing, and desalination plant operations across all countries.
How the Calculation Works
The Antoine equation is a simplified form of the Clausius-Clapeyron relation. For a given temperature T in °C, the vapour pressure in mmHg is: P = 108.07131 − 1730.63/(233.426 + T). The calculator then multiplies by 0.133322 to convert to kPa, divides by 101.325 for atm, and divides by 100 for bar. A pressure-temperature curve is generated from 30 °C below to 30 °C above the target temperature for visual analysis.
At 100 °C (the normal boiling point of water at sea level), the vapour pressure equals 101.32 kPa — exactly matching standard atmospheric pressure. Below this temperature, the vapour pressure is lower; above it, the vapour pressure exceeds atmospheric pressure, and water exists as a vapour.
Frequently Asked Questions
How does the vapour pressure of water calculator work?
Enter the temperature in degrees Celsius and the calculator uses the Antoine equation (log₁₀(P) = A − B / (C + T)) with water-specific constants A = 8.07131, B = 1730.63, and C = 233.426 to compute the vapour pressure. Results are displayed in kPa, mmHg, atm, and bar.
What is the Antoine equation for water?
The Antoine equation for water uses constants A = 8.07131, B = 1730.63, and C = 233.426 with temperature T in °C, giving vapour pressure in mmHg: P = 10^(8.07131 − 1730.63 / (233.426 + T)). This is valid for the temperature range of 1–100 °C.
How accurate is the Antoine equation for water vapour pressure?
The Antoine equation provides accuracy within about 0.3–1% for temperatures between 1–100 °C when compared to experimental Lide table values. The Buck formula is slightly more accurate overall, but the Antoine equation is the most widely used standard in chemistry for vapor pressure calculations.
What is vapour pressure of water at 25 °C?
At 25 °C (77 °F), the vapour pressure of water is approximately 3.17 kPa (23.8 mmHg, 0.031 atm). This is the equilibrium pressure of water vapour above liquid water at room temperature.
What is the boiling point of water and its vapour pressure?
At 100 °C (212 °F), the vapour pressure of water equals standard atmospheric pressure at sea level — 101.32 kPa (760 mmHg, 1 atm). This is why water boils at 100 °C at sea level; boiling occurs when the vapour pressure equals the surrounding atmospheric pressure.
Why does water evaporate at room temperature if the vapour pressure is low?
Water evaporates at room temperature because the equilibrium vapour pressure (3.17 kPa at 25 °C) is non-zero. In an open container, water molecules with sufficient kinetic energy escape into the air until the partial pressure of water vapour in the air reaches the equilibrium value, after which condensation balances evaporation.
How does altitude affect the vapour pressure of water?
The vapour pressure of water itself depends only on temperature, not altitude. However, at higher altitudes with lower atmospheric pressure, water reaches its boiling point at a lower temperature since boiling occurs when vapour pressure equals the ambient pressure. For example, at 3000 m elevation (≈70 kPa), water boils at about 90 °C.
Does the vapour pressure of water change with different measurement units?
No, the physical vapour pressure is the same regardless of the unit. This calculator converts the same value into kPa, mmHg (torr), atm, and bar for convenience. 1 atm = 101.325 kPa = 760 mmHg = 1.01325 bar.