Vapor Pressure of Water Calculator
ChemistryCalculate the vapor pressure of water at any temperature from 0°C to 374°C using the Antoine equation. Returns results in mmHg, atm, kPa, and bar.
Vapor Pressure (mmHg)
23.756
Vapor Pressure (atm)
0.031
Vapor Pressure (kPa)
3.167
Vapor Pressure (bar)
0.032
What is a Water VP?
The Vapor Pressure of Water Calculator computes the saturation vapour pressure of pure water at any temperature from 0°C to 374°C (the critical point) using the Antoine equation and, for temperatures above 150°C, the Clausius-Clapeyron equation. Results are returned simultaneously in mmHg, atm, kPa, and bar.
Water vapour pressure is the single most important physical parameter in atmospheric science, industrial steam processes, food processing, pharmaceutical drying, and air conditioning. It determines whether water evaporates or condenses at a given temperature and humidity, sets the boiling point under any external pressure, and governs steam turbine operating conditions worldwide.
The Antoine equation — log₁₀(P) = A − B/(C + T) — is a three-parameter empirical fit to the liquid-vapour coexistence curve. It is more accurate than the two-parameter Clausius-Clapeyron equation because it implicitly accounts for the temperature dependence of the enthalpy of vaporisation (ΔHvap decreases from 44.0 kJ/mol at 25°C to 40.7 kJ/mol at 100°C to approximately 0 at the critical point). For the general case of computing vapour pressure at a different temperature given a known reference point, use the Vapor Pressure Calculator.
How to use this Water VP calculator
- Enter the water temperature in °C in the Temperature field. Slide from 0°C (near freezing) to 374°C (critical point).
- Read Vapor Pressure (mmHg) as the primary result. Common reference points: 0°C → 4.58 mmHg; 25°C → 23.8 mmHg; 37°C → 47.1 mmHg; 100°C → 760 mmHg.
- For humidity calculations: if air at your temperature contains water vapour at partial pressure P_actual, then RH = (P_actual / VP at that T) × 100%.
- For boiling point at reduced pressure: if local atmospheric pressure is known, find the temperature on this calculator where vapour pressure equals local pressure — that is the local boiling point. (Or use the Boiling Point at Altitude Calculator directly.)
- Use the kPa or bar output for steam engineering calculations where equipment specifications use those units.
Formula & Methodology
Antoine equation (0–60°C and 60–150°C):log₁₀(P_mmHg) = A − B / (C + T_°C)| Range | A | B | C | |---|---|---|---| | 0–60°C | 8.10765 | 1750.286 | 235.0 | | 60–150°C | 8.14019 | 1810.94 | 244.485 | Clausius-Clapeyron (above 150°C, from 100°C reference):P₂ = 760 mmHg × exp[(ΔHvap/R) × (1/373.15 − 1/T_K)] ΔHvap = 40,700 J/mol, R = 8.314 J/(mol·K)Unit conversions:P_kPa = P_mmHg × 0.133322 P_atm = P_mmHg / 760 P_bar = P_kPa / 100Worked example — vapour pressure at body temperature (37°C): T = 37°C, using Antoine (0–60°C range): A=8.10765, B=1750.286, C=235.0log₁₀(P) = 8.10765 − 1750.286 / (235.0 + 37) = 8.10765 − 1750.286 / 272.0 = 8.10765 − 6.4349 = 1.6728 P = 10^1.6728 = 47.1 mmHg P_kPa = 47.1 × 0.133322 = 6.28 kPa P_atm = 47.1 / 760 = 0.0619 atmAt body temperature 37°C, water vapour pressure is 47.1 mmHg. In the lungs (total pressure ≈ 760 mmHg at sea level), water vapour occupies 47.1/760 = 6.2% of the gas mixture — a fact critical for calculating inspired oxygen partial pressure in respiratory physiology, hyperbaric medicine, and high-altitude acclimatisation studies.
Frequently Asked Questions
The vapor pressure of water is the pressure exerted by water molecules in the gas phase when in equilibrium with liquid water at a given temperature in a sealed container. It rises steeply with temperature: at 0°C, it is 4.58 mmHg; at 25°C, 23.8 mmHg; at 37°C (body temperature), 47.1 mmHg; at 100°C, 760 mmHg (1 atm — the normal boiling point); at 374°C (critical point), about 217 atm.
The Antoine equation is log₁₀(P) = A − B/(C + T), where P is vapour pressure in mmHg and T is temperature in °C, with constants A, B, C that are empirically fitted to experimental data. For water between 0–60°C: A=8.10765, B=1750.286, C=235.0. Between 60–150°C: A=8.14019, B=1810.94, C=244.485. The Antoine equation is more accurate than the Clausius-Clapeyron equation over a wide temperature range because it accounts for the temperature dependence of ΔHvap.
Relative humidity (RH) is the ratio of the actual partial pressure of water vapour in the air to the saturation vapour pressure at that temperature: RH = (P_actual / P_sat) × 100%. The saturation vapour pressure P_sat is exactly what this calculator computes. When RH reaches 100%, the air is saturated — any cooling causes condensation (dew or fog). In the Indian monsoon season, relative humidity regularly exceeds 90% in coastal cities like Mumbai and Chennai, because P_actual is close to P_sat at the prevailing temperature.
The dew point is the temperature at which a sample of moist air, when cooled at constant pressure and humidity, becomes saturated — the actual vapour pressure equals the saturation vapour pressure at that lower temperature. If air at 30°C has a relative humidity of 70%, the actual water vapour pressure is 0.70 × 31.8 mmHg = 22.3 mmHg. The dew point is the temperature at which saturation vapour pressure equals 22.3 mmHg — from this calculator's reverse lookup, about 24°C. Below the dew point, water condenses.
Enter the water temperature in °C in the Temperature field (range 0–374°C). The calculator applies the Antoine equation for the relevant temperature range and returns vapor pressure in mmHg, atm, kPa, and bar simultaneously. No additional inputs are needed — the calculation is specific to pure water only.
Water vapour pressure rises exponentially with temperature, becoming very steep near the boiling point. At 90°C: 525.8 mmHg. At 95°C: 634.0 mmHg. At 99°C: 733.0 mmHg. At 100°C: 760.0 mmHg. At 101°C: 787.6 mmHg. At 105°C: 906.1 mmHg. The steep rise explains why boiling point is so sensitive to pressure near 100°C — a small pressure change (e.g., altitude change) produces a meaningful boiling point shift, which is computed by the [Boiling Point at Altitude Calculator](/boiling-point-at-altitude-calculator/).
In steam boilers, turbines, and heat exchangers, the saturation vapour pressure of water determines the operating conditions. At a given steam pressure, the saturation temperature (boiling point) is uniquely fixed by the vapour pressure curve. Boiler pressure and steam temperature are directly linked through this relationship. Power plants like Mundra Ultra Mega Power Plant (Gujarat) and thermal stations across India operate steam at 150–250 bar (8–13 MPa), corresponding to saturation temperatures of 342–400°C. Engineers use steam tables (which are tabulations of vapour pressure data) constantly.
The critical point of water is the end of the liquid-vapour coexistence curve: at 374.14°C and 217.75 atm (22.064 MPa), liquid and vapour become indistinguishable — their densities become equal. Above the critical temperature, water is a supercritical fluid regardless of pressure. This calculator covers up to 374°C using the Clausius-Clapeyron equation above 150°C. Near the critical point, the Clausius-Clapeyron approximation becomes less accurate as ΔHvap approaches zero, but provides reasonable estimates up to 370°C.
Yes — extensively. In pharmaceutical tablet coating and powder drying, the psychrometric conditions (temperature and humidity) are controlled using vapor pressure data. Spray dryers at major API manufacturers (Dr. Reddy's, Sun Pharma) operate using steam tables to ensure complete solvent removal at the outlet air humidity. In agriculture, crop protection sprays are affected by high humidity (high water vapour pressure): fungicide and pesticide efficacy is reduced when relative humidity exceeds 85%, because fungal spore germination is promoted and spray drying is impeded.
The Antoine equation used in this calculator matches steam table values to within 0.1–0.5% for temperatures between 0–150°C. Above 150°C, the Clausius-Clapeyron approximation (with constant ΔHvap = 40.7 kJ/mol) is used, which introduces errors up to 2–5% near the critical point. For precision engineering applications (steam turbine design, heat exchanger calculations), use the full IAPWS-IF97 steam tables rather than the Antoine or Clausius-Clapeyron approximations. For educational, lab planning, and everyday applications, the results from this calculator are accurate enough.