What vapor pressure is and why it matters
Vapor pressure is the pressure exerted by water vapor when it is in equilibrium with liquid water at a given temperature. In plain terms: it is the force that water molecules exert as they evaporate from a surface. The warmer the water, the higher the vapor pressure, because more molecules have enough energy to escape into the air.
You encounter vapor pressure in everyday situations. It is why wet clothes dry faster on a hot day than a cold one, why a pot of water boils at lower temperatures at high altitudes, and why humidity feels more oppressive in summer. For homeowners, understanding vapor pressure matters when dealing with condensation in basements, attics, or crawl spaces—places where water vapor can accumulate and cause mold or wood rot.
Vapor pressure is measured in units called pascals (Pa), millimeters of mercury (mmHg), or atmospheres (atm). The exact value depends entirely on temperature. At 0°C, water vapor pressure is about 611 Pa. At 100°C (the boiling point at sea level), it reaches 101,325 Pa—one full atmosphere of pressure.
Key Takeaways
- Vapor pressure increases with temperature and can be estimated using the Antoine equation or looked up in standard reference tables.
- The Antoine equation—log₁₀(P) = A − B/(C + T)—is the most practical method for calculating vapor pressure across a wide temperature range.
- For water, the Antoine constants are A = 8.07131, B = 1730.63, and C = 233.426 when temperature is in degrees Celsius and pressure is in mmHg.
- You can convert the result to other pressure units (pascals or atmospheres) using standard conversion factors.
- For quick reference without calculation, vapor pressure tables are available in engineering handbooks and online databases.
The Antoine equation: the practical calculation method
The Antoine equation is the standard formula used by engineers and scientists to calculate vapor pressure across a practical range of temperatures. It is accurate, relatively straightforward, and works well for water between roughly 0°C and 100°C—the range most homeowners encounter.
The equation is written as:
log₁₀(P) = A − B/(C + T)
In this formula, P is the vapor pressure in millimeters of mercury (mmHg), T is the temperature in degrees Celsius, and A, B, and C are constants specific to water. For water, these constants are:
- A = 8.07131
- B = 1730.63
- C = 233.426
To use the equation, plug in your temperature, solve for the logarithm, then convert back to find P. The result will be in mmHg, which you can then convert to other units if needed.
Step-by-step calculation at a specific temperature
Let us work through an example: calculating the vapor pressure of water at 25°C (a typical room temperature).
Step 1: Set up the equation with your values. You have T = 25°C, and the Antoine constants for water. Plug them in:
log₁₀(P) = 8.07131 − 1730.63/(233.426 + 25)
Step 2: Solve the denominator. Add 233.426 + 25 = 258.426.
Step 3: Divide B by the denominator. Divide 1730.63 by 258.426 = 6.6968.
Step 4: Subtract from A. Calculate 8.07131 − 6.6968 = 1.37451.
Step 5: Convert from logarithm to pressure. You now have log₁₀(P) = 1.37451. To find P, raise 10 to this power: 10^1.37451 = 23.66 mmHg.
At 25°C, the vapor pressure of water is approximately 23.66 mmHg. This is the pressure exerted by water vapor in equilibrium with liquid water at room temperature.
Converting vapor pressure to other units
The Antoine equation gives you the answer in millimeters of mercury, but you may need the result in pascals (Pa) or atmospheres (atm) depending on what you are calculating. The conversion factors are straightforward.
From mmHg to pascals: Multiply by 133.322. Using the 25°C example above, 23.66 mmHg × 133.322 = 3,155 Pa (or about 3.16 kPa).
From mmHg to atmospheres: Divide by 760. So 23.66 mmHg ÷ 760 = 0.0311 atm.
From pascals to mmHg: Divide by 133.322.
From atmospheres to mmHg: Multiply by 760.
Keep a conversion reference handy if you work with vapor pressure regularly. Most engineering calculators and spreadsheet programs can handle these conversions automatically.
Using vapor pressure tables instead of calculating
If you do not want to work through the Antoine equation every time, vapor pressure tables are readily available in engineering handbooks, chemistry textbooks, and online databases. These tables list the vapor pressure of water at standard temperature intervals—usually every 1°C or 5°C—across the full range from freezing to boiling.
To use a table, find your temperature in the left column and read across to the pressure column. The values in these tables are calculated using the Antoine equation or similar methods, so they are reliable and accurate. For most household purposes—estimating condensation risk, understanding humidity, or troubleshooting moisture problems—a table lookup is faster and just as useful as doing the math yourself.
Standard reference sources include the NIST Chemistry WebBook (maintained by the U.S. National Institute of Standards and Technology), engineering handbooks like Perry's Chemical Engineers' Handbook, and many online calculators that do the Antoine equation for you if you enter the temperature.
Why vapor pressure changes with temperature
Vapor pressure rises with temperature because heat gives water molecules more kinetic energy. At higher temperatures, more molecules have enough energy to break free from the liquid surface and enter the gas phase. This is why a sealed container of warm water has higher pressure inside than the same container filled with cold water.
This relationship is not linear—vapor pressure does not straightforward double when temperature doubles. Instead, it increases exponentially. This is why the Antoine equation uses a logarithmic form: it captures this curved, accelerating relationship accurately across a wide temperature range.
At sea level, water boils when its vapor pressure equals atmospheric pressure (101,325 Pa or 760 mmHg). At higher altitudes, where atmospheric pressure is lower, water boils at a lower temperature because it needs less vapor pressure to escape. This is why cooking times are longer in the mountains—the water is actually cooler.
Practical applications in your home
Understanding vapor pressure helps explain moisture problems in basements, attics, and crawl spaces. Warm air holds more water vapor than cold air. When warm, humid air contacts a cold surface, the temperature drops, the vapor pressure drops below the saturation point, and water condenses into liquid droplets. This is why you see condensation on windows on cold mornings and why basements stay damp even when there is no active leak.
If you are dealing with condensation or mold, knowing the vapor pressure at your indoor temperature tells you how much moisture the air can hold. Reducing temperature or increasing ventilation lowers the relative humidity and reduces condensation risk. A dehumidifier works by cooling air below the dew point, forcing water vapor to condense and removing it from the space.
For most homeowners, you do not need to calculate vapor pressure yourself. But understanding what it is and how it changes with temperature helps you understand why moisture problems happen and what solutions actually work.
Frequently Asked Questions
What is the difference between vapor pressure and relative humidity?
Vapor pressure is the actual pressure exerted by water vapor in the air at a given temperature. Relative humidity is the ratio of the actual vapor pressure to the maximum vapor pressure the air can hold at that temperature, expressed as a percentage. At 25°C, if the vapor pressure is 11.8 mmHg and the maximum possible is 23.66 mmHg, the relative humidity is 50 percent.
Why does water boil at a lower temperature at high altitude?
At high altitude, atmospheric pressure is lower. Water boils when its vapor pressure equals the atmospheric pressure pushing down on the surface. Since the atmospheric pressure is lower in the mountains, water reaches that pressure point at a lower temperature—often 95°C or less instead of 100°C at sea level.
Can I use the Antoine equation for temperatures below 0°C or above 100°C?
The Antoine equation constants given here are calibrated for water between roughly 0°C and 100°C. Outside that range, the equation becomes less accurate. For temperatures below freezing, you would need different constants, and the behavior of ice vapor (sublimation) differs from liquid water vapor.
Do I need to know vapor pressure to fix a condensation problem?
Not necessarily. Most condensation problems are solved by reducing humidity (with a dehumidifier or ventilation), improving air circulation, or insulating cold surfaces. Understanding vapor pressure helps explain why the problem exists, but fixing it usually comes down to temperature and humidity control rather than calculating pressure values.
Where can I find a vapor pressure calculator online?
The NIST Chemistry WebBook has an interactive tool, and many engineering sites offer Antoine equation calculators where you enter the temperature and get the vapor pressure when ready. Search for "water vapor pressure calculator" to find several options that do the math for you.