Vapor Pressure of Water Calculator

What Is the Vapor Pressure of Water?

Vapor pressure is the pressure exerted by water vapor when it is in equilibrium with its liquid phase at a given temperature. As temperature increases, more water molecules escape into the gas phase, raising the vapor pressure. This calculator uses the Antoine equation with standard constants to determine the saturation vapor pressure of water over a temperature range of 1°C to 100°C.

Understanding vapor pressure is essential in fields like meteorology, chemical engineering, HVAC design, and distillation. It directly affects boiling point, evaporation rates, and humidity calculations.

How the Calculation Works

The calculator applies the Antoine equation, a widely accepted empirical formula for estimating vapor pressure of pure substances:

log₁₀(P) = A − B / (C + T)

Where:

• P = vapor pressure (mmHg)
• T = temperature (°C)
• A, B, C = Antoine constants specific to water

For water between 1°C and 100°C, the standard constants are:

• A = 8.07131
• B = 1730.63
• C = 233.426

The result is then converted to other common units (atm, kPa, bar, psi, torr) for practical use.

How to Use the Calculator

1. Enter the temperature in degrees Celsius (between 1 and 100).
2. Select the desired output unit from the dropdown menu.
3. Click "Calculate" to see the vapor pressure result.

The tool automatically validates the temperature range and provides clear feedback if an out-of-range value is entered.

Example Calculation

If you enter a temperature of 25°C, the calculator returns a vapor pressure of approximately 23.76 mmHg (or 0.0313 atm, 3.17 kPa). This matches standard reference values for water at room temperature.

At 100°C, the vapor pressure equals 760 mmHg (1 atm), which is the boiling point of water at sea level.

Understanding Your Results

The vapor pressure value represents the pressure at which water vapor and liquid water are in equilibrium at the given temperature. Higher vapor pressure means water evaporates more readily. Key points to remember:

• Vapor pressure increases exponentially with temperature, not linearly.
• At 100°C, vapor pressure equals standard atmospheric pressure, causing water to boil.
• At lower temperatures, vapor pressure is much smaller, which is why evaporation is slower.

Common Mistakes to Avoid

• Using temperatures outside the valid range. The Antoine constants are calibrated for 1°C to 100°C. Results outside this range are unreliable.
• Confusing vapor pressure with atmospheric pressure. Vapor pressure is a property of water, not the surrounding air pressure.
• Assuming linear behavior. Vapor pressure does not increase proportionally with temperature; small temperature changes can cause significant pressure changes.

Practical Use Cases

• Boiling point determination: Find the temperature at which water boils at different altitudes by comparing vapor pressure to local atmospheric pressure.
• Humidity calculations: Relative humidity is the ratio of actual water vapor pressure to saturation vapor pressure at a given temperature.
• Process engineering: Design distillation columns, evaporators, and drying equipment using accurate vapor pressure data.
• Weather forecasting: Understand dew point, fog formation, and evaporation rates.

Limitations

This calculator uses the Antoine equation with constants optimized for pure water between 1°C and 100°C. It does not account for:

• Dissolved substances (salt, sugar, etc.) that alter vapor pressure
• Pressure effects from non-standard atmospheric conditions
• Supercooled water or ice (below 0°C)
• Steam above 100°C

For applications involving solutions or extreme conditions, consult specialized thermodynamic data or experimental measurements.