Vapour Pressure of Water Calculator

About the Vapour Pressure of Water Calculator

The vapour pressure of water is the equilibrium partial pressure of water vapour above a flat surface of liquid water at a given temperature. It is one of the most frequently needed physical properties in meteorology, HVAC engineering, chemical processing, food science, and environmental monitoring.

Several empirical equations approximate this relationship with different levels of accuracy. The Buck equation (1981) is accurate to within 0.05% from −80 °C to 50 °C. The Magnus-Tetens equation is simpler and widely used in meteorology. The Goff-Gratch equation, adopted by the WMO, offers the highest accuracy across the full range from −100 °C to +100 °C. For engineering applications, the Antoine equation parameterized for water is also common.

This calculator computes saturation vapour pressure using all four equations for comparison, derives dew point from relative humidity, and provides a comprehensive steam table reference from −40 °C to 100 °C.

When This Page Helps

Accurate water vapour pressure data is critical for weather forecasting, humidity control in data centers, greenhouses, and cleanrooms, drying process design, HVAC load calculations, and food preservation science. A reliable conversion-ready value also helps when reconciling readings from mixed unit systems used in laboratory reports, instrumentation panels, and international technical references.

How to Use the Inputs

1. Enter the temperature in °C (or select from common presets).
2. View the saturation vapour pressure calculated by four different equations.
3. Optionally enter relative humidity to calculate the actual vapour pressure and dew point.
4. Enter actual vapour pressure to calculate relative humidity and dew point.
5. Compare results across equations and review the steam table reference.
6. Use the psychrometric section for wet-bulb/dry-bulb conversions.
7. Check the meteorological applications table for common humidity scenarios.

Example Calculation

Tips & Best Practices

• For sub-zero temperatures, use the saturation pressure over ice, not liquid water.
• Most weather stations report pressure in hPa (= mbar). 1 kPa = 10 hPa.
• A dew point spread < 3 °C means fog or low clouds are likely.
• ASHRAE standard psychrometric charts are based on the Hyland-Wexler equations, very close to Buck.
• Hair hygrometers and capacitive RH sensors are calibrated against these equations.
• The August-Roche-Magnus equation is the simplified form most often cited in textbooks.

Comparison of Vapour Pressure Equations

The four most common equations for the saturation vapour pressure of water are:

1. **Buck (1981)** — Best overall accuracy for meteorological applications (±0.05% from −80 to +50 °C). 2. **Magnus-Tetens** — Simple, widely taught, adequate for most purposes (±0.4% from −40 to +50 °C). 3. **Tetens (1930)** — Original formulation, slightly less accurate than modified Magnus. 4. **Goff-Gratch (1946)** — WMO reference standard, most complex but highest accuracy.

For temperatures above 100 °C, steam tables from IAPWS-IF97 should be used instead.

Psychrometric Applications

HVAC engineers use the vapour pressure of water in psychrometric calculations that relate dry-bulb temperature, wet-bulb temperature, dew point, relative humidity, humidity ratio, enthalpy, and specific volume. The psychrometric chart is a graphical tool that encodes these relationships.

Health and Comfort

Human comfort depends on both temperature and humidity. The dew point is a better indicator of mugginess than relative humidity: dew points below 10 °C feel dry, 10-16 °C comfortable, 16-20 °C humid, and above 20 °C oppressive. Heat index calculations rely on accurate vapour pressure data.

Frequently Asked Questions

• What is the difference between vapour pressure and partial pressure?

  Vapour pressure is the equilibrium pressure of the vapour above its liquid. Partial pressure is the actual pressure contributed by water vapour in the atmosphere. RH = (partial pressure / saturation pressure) × 100%.

• Which equation is most accurate for water vapour pressure?

  The Goff-Gratch equation (WMO standard) is most accurate across the widest range. The Buck equation is nearly as accurate and simpler to compute. For most practical purposes, all four equations agree within 0.5%.

• How do I calculate dew point from temperature and RH?

  First compute actual vapour pressure e_a = (RH/100) × e_s(T). Then invert the Magnus equation: T_d = 237.3 × ln(e_a/6.1078) / (17.27 − ln(e_a/6.1078)).

• What is the dew point?

  The dew point is the temperature at which air becomes saturated (100% RH) at constant pressure. Below the dew point, water condenses. It indicates the comfort level: dew points above 20 °C feel oppressive.

• How does altitude affect vapour pressure?

  The saturation vapour pressure depends only on temperature, not altitude. However, the total atmospheric pressure decreases with altitude, so the mole fraction of water vapour changes.

• What is the wet-bulb temperature?

  The wet-bulb temperature is the lowest temperature achievable by evaporative cooling. It is always between the dew point and dry-bulb temperature and is used in HVAC psychrometric calculations.