Vapor Pressure of Water Calculator
Enter a temperature and instantly get water's saturation vapor pressure, useful for humidity, meteorology, distillation, and lab calculations.
💧 What is the Vapor Pressure of Water?
The vapor pressure of water is the pressure exerted by water vapor when it is in equilibrium with liquid water at a given temperature, the point where water molecules are escaping into the air and condensing back at exactly the same rate. It rises sharply and non-linearly with temperature: water's vapor pressure at 100°C is more than 160 times higher than at 0°C, which is why warm air can hold dramatically more moisture than cold air.
This single number underlies a surprising number of everyday and technical calculations. Meteorologists use it to compute relative humidity and dew point. HVAC and greenhouse engineers use it to size dehumidification and humidification systems. Chemists use it to plan vacuum distillations and rotary evaporations, since a lower system pressure lets water (or an aqueous mixture) boil off at a lower, gentler temperature. It also explains why water boils at exactly 100°C at sea level: that is the temperature where water's vapor pressure reaches 101.325 kPa, standard atmospheric pressure, and no higher pressure remains to hold the liquid together.
A common source of confusion is mixing up saturation vapor pressure with the actual, real-world water vapor pressure in the air. Saturation vapor pressure is the maximum possible value at a given temperature, what this calculator returns. The air is rarely saturated; relative humidity describes what fraction of that maximum is actually present, so 50% relative humidity at 25°C means the true water vapor partial pressure is about half of the 3.17 kPa saturation value this calculator gives for 25°C.
This calculator is deliberately simpler than a general vapor pressure tool: since water is the single most looked-up substance for vapor pressure, it uses a formula purpose-built and precisely fitted for water, so you only need to enter a temperature, no enthalpy of vaporization or reference point lookup required. For any other liquid, or if you already have measured (pressure, temperature) data points, use the general Vapor Pressure Calculator instead.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Room Temperature (25°C)
What is the saturation vapor pressure of water at 25°C, common room temperature?
Example 2 — Boiling Point (100°C)
What is water's vapor pressure at 100°C, its normal boiling point at sea level?
Example 3 — Human Body Temperature (37°C)
What is water's vapor pressure at 37°C, normal human body temperature, relevant to breath and respiratory humidity calculations?
❓ Frequently Asked Questions
🔗 Related Calculators
What is the vapor pressure of water at room temperature?
At 25°C, water's saturation vapor pressure is approximately 3.17 kPa (23.77 mmHg, or 31.69 hPa). This is the maximum partial pressure water vapor can exert in equilibrium with liquid water at that temperature; actual vapor pressure in a room is this value multiplied by the relative humidity.
What formula does this calculator use?
It uses the Buck equation (Arden Buck, 1981, revised 1996): es = 6.1121 x e^[(18.678 - T/234.5)(T / (257.14 + T))], where es is saturation vapor pressure in hPa and T is temperature in Celsius. It is one of the most accurate simple empirical formulas for water vapor pressure, within about 0.05% of measured values across its valid range.
How is this different from the general Vapor Pressure Calculator?
The general Vapor Pressure Calculator uses the Clausius-Clapeyron equation, which needs an enthalpy of vaporization (ΔHvap) and a reference pressure-temperature point that you supply, and it works for any pure substance. This calculator is purpose-built for water only, using a formula fitted specifically to water's real behavior, so you only enter a temperature and nothing else.
Why does water boil at 100°C?
A liquid boils when its vapor pressure equals the surrounding external pressure. At 100°C, water's saturation vapor pressure is almost exactly 101.325 kPa (1 standard atmosphere), the typical sea-level air pressure, so 100°C is where water boils under normal atmospheric conditions. At higher altitude, air pressure is lower, so water boils below 100°C.
What is the difference between saturation vapor pressure and relative humidity?
Saturation vapor pressure is the maximum water vapor pressure possible at a given temperature. Relative humidity is the ratio of the actual water vapor pressure in the air to that maximum, expressed as a percentage. Multiply saturation vapor pressure by relative humidity (as a decimal) to get the actual partial pressure of water vapor present.
Is this the same as the vapor pressure over ice?
No. This calculator computes vapor pressure over liquid water. Below 0°C, water can exist as supercooled liquid (common in clouds) or as ice, and ice has a slightly lower saturation vapor pressure than supercooled liquid water at the same temperature, which is why ice crystals grow at the expense of supercooled droplets in mixed-phase clouds (the Wegener-Bergeron-Findeisen process).
How accurate is the Buck equation for water vapor pressure?
The Buck (1996) equation is accurate to within about 0.05% of measured values between -40°C and 50°C, making it one of the most trusted simple closed-form formulas in meteorology and atmospheric science. Above 50°C up to boiling, it remains a good approximation, typically within a percent or so, though dedicated steam-table correlations are more precise very close to 100°C.
What units can this calculator show the result in?
It shows the result in four units simultaneously: kilopascals (kPa), millimeters of mercury (mmHg, also called torr), hectopascals (hPa, the standard meteorological unit, numerically equal to millibars), and standard atmospheres (atm).
Why do I need water's vapor pressure for humidity calculations?
Relative humidity, dew point, and wet-bulb temperature calculations all require the saturation vapor pressure of water at the air temperature as a starting point. Meteorologists, HVAC engineers, and greenhouse or lab climate-control systems all rely on an accurate saturation vapor pressure value as an input to these downstream calculations.
Can I use this for vacuum distillation planning in a lab?
Yes. If you need to know at what temperature water (or an aqueous solution, approximately) will boil under a reduced pressure in a rotary evaporator or vacuum distillation setup, this calculator's underlying curve shows the pressure-temperature relationship; find the temperature where the plotted vapor pressure matches your system's vacuum level.