Vapor Pressure of Water Calculator
Calculate the saturation vapour pressure of water at any temperature from 0 to 100°C using the NIST Antoine equation. Furthermore, results are shown in mmHg, kPa, atm, and bar alongside the absolute humidity (saturation water content in g/m³) — essential for HVAC, meteorology, and chemistry.
How to use the Vapor Pressure of Water Calculator
Type the temperature in °C, K, or °F. Furthermore, valid range is 0–100°C (32–212°F, 273–373 K). Above 100°C, consult steam tables.
Vapour pressure appears in four units plus absolute humidity at saturation. Moreover, absolute humidity at saturation = P°×M_w/(R×T) in g/m³.
Absolute humidity at saturation is the maximum water vapour content of air at that temperature. Furthermore, actual humidity = RH% × saturation humidity.
Dew point is the temperature at which current water vapour pressure equals the saturation vapour pressure. Furthermore, at 50% RH and 25°C: P_actual = 0.5 × 23.8 = 11.9 mmHg → dew point ≈ 13.8°C.
P° (water) at the process temperature feeds into Raoult's law for aqueous mixture vapour pressures and Henry's law for gas solubility. Moreover, cooling towers and evaporative coolers use water vapour pressure extensively.
Variants, options and when to use each
| Temperature | P° (mmHg) | P° (kPa) | Abs. humidity (g/m³) |
|---|---|---|---|
| 0°C | 4.58 | 0.611 | 4.85 |
| 10°C | 9.21 | 1.228 | 9.40 |
| 20°C | 17.5 | 2.338 | 17.3 |
| 25°C | 23.8 | 3.167 | 23.0 |
| 37°C | 46.6 | 6.211 | 44.0 |
| 100°C | 760 | 101.3 | 597 |
The formula explained
T°C = temperature in Celsius
AH = absolute humidity at saturation (g/m³)
M_w = 18.015 g/mol (molar mass of water)
R = 8.314 J/mol·K
The Antoine equation with NIST constants A=8.07131, B=1730.63, C=233.426 gives log₁₀(P/mmHg) as a function of temperature. Furthermore, the constants are valid over 1–100°C and reproduce the NIST steam tables to within 0.1%. Moreover, absolute humidity AH = P°×M_w/(R×T) derives from the ideal gas law — treating water vapour as an ideal gas.
Worked example — dew point from relative humidity
Air at 25°C, RH = 60%. Find the dew point. Furthermore, P°(25°C) = 23.8 mmHg.
| Step | Calculation | Result |
|---|---|---|
| Partial pressure of water | 0.60 × 23.8 | 14.28 mmHg |
| Dew point: find T where P° = 14.28 | Inverse Antoine | Tdp ≈ 16.5°C |
| Condensation risk? | Surface below 16.5°C | Condensation will form |
What is the saturation vapour pressure of water?
The saturation vapour pressure of water P° is the partial pressure of water vapour in thermodynamic equilibrium with liquid water at a given temperature. Furthermore, it increases rapidly with temperature — roughly doubling every 10°C near room temperature. At 100°C, P° = 760 mmHg = 1 atm, which is the definition of the normal boiling point.Relative humidity RH = (actual water vapour pressure / P°) × 100%. Moreover, when RH = 100%, the air is saturated — no more water can evaporate, and any cooling causes condensation (fog, dew, or frost). The dew point is the temperature at which the actual water vapour pressure equals P° — cooling below this temperature causes condensation.
Absolute humidity (g/m³) measures the actual mass of water vapour per unit volume of air. Additionally, it is directly proportional to water vapour pressure through the ideal gas law: AH = P × M_w/(RT). At saturation, AH = P° × M_w/(RT) — this is the maximum water vapour content of air at temperature T.
Who uses this calculator?
Meteorologists calculate dew point, relative humidity, and fog formation from water vapour pressure. Furthermore, HVAC engineers size cooling coils using the saturation vapour pressure at different temperatures for dehumidification calculations. Food scientists control water activity in dried foods using water vapour pressure relationships. Moreover, industrial drying engineers (freeze dryers, spray dryers, tunnel dryers) use P° to calculate evaporation driving force and outlet air humidity.
Historical context and related concepts
The relationship between temperature and vapour pressure of water was measured by Dalton (1801) and systematically tabulated throughout the 19th century. Furthermore, the thermodynamic basis was provided by Clausius and Clapeyron (1834–1850). NIST has maintained steam tables since the 1930s — the current IAPWS-IF97 formulation covers 0–800°C to extremely high precision. Moreover, the Antoine equation remains the standard simple formula for educational and routine engineering calculations.
Why water vapour pressure calculations underpin HVAC, meteorology, and food safety
Air conditioning systems must remove sufficient water vapour from air to maintain comfortable humidity levels. Furthermore, the cooling coil must be below the dew point of the incoming air — the amount of condensate removed determines the dehumidification capacity. A 1°C error in dew point calculation corresponds to significant moisture load errors in large commercial HVAC systems. Moreover, food water activity (aw = P/P°) directly controls microbial growth — products above aw = 0.85 support most bacteria.Water vapour pressure in climate science and global warming
Water vapour is the most abundant greenhouse gas — responsible for about 50% of the greenhouse effect. Furthermore, as CO₂ warms the atmosphere, higher temperatures allow more water vapour (following the Antoine equation), amplifying warming — the water vapour feedback. A 1°C temperature increase increases P° by ~7%, allowing ~7% more atmospheric water vapour. Moreover, this positive feedback roughly doubles the warming from CO₂ alone — the key amplification mechanism in climate models.
Frequently asked questions
Related tools
Vapor Pressure Calculator
Antoine equation for 6 solvents including water. Furthermore, this calculator is the water-specific version.
→Raoult's Law Calculator
Mixture vapour pressure. Moreover, water P° from Antoine feeds into Raoult's law for aqueous solutions.
→Henry's Law Calculator
Dissolved gas at equilibrium. Furthermore, water vapour pressure determines atmospheric water absorption.
→Boiling Point Elevation Calculator
Solutes raise boiling point by reducing water vapour pressure. Moreover, boiling point elevation = vapour pressure depression effect.
→Significant Figures Calculator
Round vapour pressure to appropriate precision. Furthermore, Antoine equation gives 4 significant figures.
→Combined Gas Law Calculator
Gas laws apply to water vapour. Moreover, water vapour partial pressure follows ideal gas law at ambient conditions.
→