Vapor pressure is the pressure water exerts as it evaporates into the air above it
Water molecules at the surface of a liquid are always escaping into the air as vapor. At the same time, some vapor molecules are condensing back into the liquid. When these two processes reach balance, the vapor above the water exerts a measurable pressure — this is vapor pressure. The higher the temperature, the faster molecules escape, so vapor pressure rises as water gets warmer. At sea level, water's vapor pressure reaches atmospheric pressure (14.7 pounds per square inch) at 100°C (212°F), which is why that is water's boiling point.
Finding vapor pressure means either measuring it directly with equipment, reading it from a reference table, or calculating it using a mathematical formula. For most practical purposes — cooking, weather prediction, or understanding how water behaves in pipes — a reference table is the fastest and most reliable method. The choice depends on what temperature you need and how precise your answer must be.
Key Takeaways
- Vapor pressure tables published by the National Institute of Standards and Technology (NIST) list water's vapor pressure at every temperature from freezing to boiling, and are free to read.
- The Antoine equation is a three-part formula that calculates vapor pressure for any temperature between 0°C and 100°C, and requires only a calculator.
- A mercury barometer or electronic pressure gauge can measure vapor pressure directly in a sealed container, but requires laboratory equipment most people do not own.
- Vapor pressure increases exponentially with temperature, so a 10-degree rise does not double the pressure — it roughly triples it.
Using a vapor pressure reference table
The simplest method is to look up the temperature you need in a published table. The NIST Chemistry WebBook maintains a free database of water's vapor pressure at temperatures from 0°C to 374°C. You enter the temperature, and the table returns the pressure in pascals, bars, atmospheres, or pounds per square inch — whichever unit you need. This table is based on decades of laboratory measurements and is the standard reference used by engineers and scientists.
For temperatures between 0°C and 100°C (the range most people encounter), simpler printed tables exist in chemistry textbooks and online. These tables typically list vapor pressure at 5-degree or 10-degree intervals. If your exact temperature falls between two listed values, you can estimate by drawing a straight line between the two nearest points — a method called linear interpolation. For example, if the table shows 20°C at 2.34 kPa and 25°C at 3.17 kPa, the value at 22°C would be roughly 2.71 kPa.
Calculating vapor pressure with the Antoine equation
The Antoine equation is a formula that predicts vapor pressure for any temperature without needing a table. It looks like this: log₁₀(P) = A − B/(C + T), where P is vapor pressure in millimeters of mercury, T is temperature in degrees Celsius, and A, B, and C are constants specific to water. For water between 0°C and 60°C, the constants are A = 8.07131, B = 1730.63, and C = 233.426. For 60°C to 100°C, they are A = 8.14019, B = 1810.94, and C = 244.485.
To use the equation, plug in your temperature and the correct constants, then solve for P. For example, at 25°C using the first set of constants: log₁₀(P) = 8.07131 − 1730.63/(233.426 + 25) = 8.07131 − 6.87738 = 1.19393. Taking the antilog (10 raised to this power) gives P = 15.6 millimeters of mercury, or about 2.08 kPa. A scientific calculator or spreadsheet makes this calculation quick. The Antoine equation is accurate to within 1 to 2 percent across the temperature range, which is sufficient for most applications.
Measuring vapor pressure directly with equipment
In a laboratory setting, vapor pressure can be measured by sealing a known volume of water in a container, heating it to a set temperature, and measuring the pressure of the gas space above the liquid using a pressure gauge. A mercury barometer or electronic pressure transducer connected to the sealed container will show the vapor pressure directly. This method is accurate but requires specialized glassware, a thermometer, a heat source, and a way to measure pressure — equipment most households and small businesses do not have.
Dynamic methods also exist, in which a gas is bubbled through water at a known temperature and the pressure is measured as the gas becomes saturated with water vapor. These methods are used in research and industrial settings but are too complex for casual measurement. For practical purposes, a reference table or the Antoine equation will give you the answer you need without the equipment or informed.
Why vapor pressure matters in everyday situations
Understanding vapor pressure explains why wet clothes dry faster on a hot day than a cold one — higher temperature means higher vapor pressure, so water evaporates more readily. It also explains why a pressure cooker works: by trapping steam, it raises the pressure above the water, which raises the boiling point, allowing food to cook faster at higher temperatures. In air conditioning and dehumidification, vapor pressure determines how much moisture the air can hold at a given temperature.
In plumbing and heating systems, vapor pressure becomes important when water is heated in a closed pipe. If the temperature rises above what the pipe can safely contain, the vapor pressure can build until the pipe fails. This is why expansion tanks are installed in hot water systems — they give the expanding water and steam room to go without building dangerous pressure.
Common mistakes when finding vapor pressure
The most frequent error is confusing vapor pressure with atmospheric pressure or gauge pressure. Vapor pressure is the pressure exerted by the water vapor itself, not the total pressure in the space above the water. At sea level, atmospheric pressure is 101.325 kPa, but water's vapor pressure at 20°C is only 2.34 kPa — much lower. The two are independent quantities.
Another mistake is assuming vapor pressure increases linearly with temperature. It does not. The relationship is exponential, so the vapor pressure at 50°C is not straightforward double that at 25°C — it is roughly triple. Using a table or the Antoine equation avoids this error. A third mistake is using the wrong constants in the Antoine equation for your temperature range. The constants change at 60°C, so using the low-temperature constants at 80°C will give a significantly wrong answer.
Vapor pressure tables for quick reference
Below is a short reference table of water's vapor pressure at common temperatures. For a complete table covering every degree from 0°C to 100°C, consult the NIST Chemistry WebBook or a chemistry reference handbook.
| Temperature (°C) | Vapor Pressure (kPa) | Vapor Pressure (mmHg) |
|---|---|---|
| 0 | 0.61 | 4.6 |
| 10 | 1.23 | 9.2 |
| 20 | 2.34 | 17.5 |
| 30 | 4.25 | 31.8 |
| 40 | 7.38 | 55.3 |
| 50 | 12.35 | 92.5 |
| 60 | 19.95 | 149.4 |
| 70 | 31.19 | 233.7 |
| 80 | 47.39 | 355.1 |
| 90 | 70.18 | 526.0 |
| 100 | 101.33 | 760.0 |
Frequently Asked Questions
What is the difference between vapor pressure and partial pressure?
Vapor pressure is the pressure exerted by water vapor when it is in equilibrium with liquid water — meaning evaporation and condensation are balanced. Partial pressure is the pressure any gas contributes to the total pressure in a mixture, whether or not it is in equilibrium with a liquid. In humid air, the partial pressure of water vapor is usually less than the vapor pressure at that temperature, because the air is not yet saturated.
Does vapor pressure depend on the amount of water?
No. Vapor pressure depends only on temperature and the identity of the liquid. A cup of water and a swimming pool at the same temperature have the same vapor pressure above them. What changes is how much total vapor is present — a larger surface area produces more vapor molecules, but the pressure they exert remains the same.
Can vapor pressure exceed atmospheric pressure?
In an open container, no — water boils when its vapor pressure equals atmospheric pressure, and the liquid turns to steam. In a sealed container, vapor pressure can exceed atmospheric pressure if the temperature is raised high enough. This is how a pressure cooker works: steam builds up inside, raising the pressure and the boiling point.
Why does the Antoine equation have two sets of constants?
The Antoine equation is an approximation, and no single set of constants fits water's behavior perfectly across a wide temperature range. Using two sets — one for 0°C to 60°C and another for 60°C to 100°C — keeps the error small throughout the range. If you use the wrong set, your answer will be noticeably wrong.
Is vapor pressure the same as saturation pressure?
Yes, they are the same thing. Saturation pressure is the vapor pressure at which air becomes saturated with water vapor and condensation begins. Both terms refer to the same measurement: the pressure exerted by water vapor in equilibrium with liquid water at a given temperature.