Vapor Pressure of Water Calculator
Table of contents
What is vapor pressure? Vapor pressure definitionFactors influencing vapor pressureVapor pressure formulasVapor pressure of waterHow to use the vapor pressure of water calculatorFAQsThe vapor pressure of water calculator is a handy tool that can help in determining the vapor pressure of water and ice. Just type in the temperature, and the pressure will appear in no time — don't hesitate. Give it a go! If you're unsure what vapor pressure is, keep scrolling. You'll find the definition, five different vapor pressure formulas, and details about the most oftenused one — Antoine equation.
What is vapor pressure? Vapor pressure definition
Vapor pressure is the pressure exerted by a vapor that is in thermodynamic equilibrium with its condensed phases (solid or liquid) in a closed system at a given temperature. The equilibrium — in other words, steady state — between evaporation and condensation occurs when:
The rate of evaporation of the liquid = Rate of condensation of the gas
Vapor pressure is one of the fluid characteristics: it's a measure of the tendency of a material to change into the gaseous/vapor state. The vapor pressure of a liquid can be measured in many ways, e.g., by a manometer connected to the flask with the measured liquid.
Factors influencing vapor pressure
There are two factors that influence the vapor pressure:

Temperature
The higher the temperature is, the more molecules have enough energy to escape from the liquid or solid, which leads to higher vapor pressure values.
temperature of a liquid increases ($T\uparrow$) → kinetic energy of its molecules increases ($E_{\mathrm{k}}\uparrow$) → number of molecules transitioning into a vapor increases → vapor pressure increases ($P\uparrow$)
At lower temperatures, fewer molecules have sufficient energy.

Substance nature (types of molecules)
The vapor pressure will be relatively low for substances with stronger intermolecular forces. On the contrary, the vapor pressure is relatively high for relatively weak forces.
The important thing to mention is the fact that the surface area of liquid/solid substance in contact with the gas doesn't affect the vapor pressure. So it doesn't matter if we put our liquid into a wide flask or a thin graduated cylinder — the vapor pressure remains the same.
Vapor pressure formulas
There are many different formulas, thanks to which you can calculate the vapor pressure of water. The most wellknown and established is the Antoine equation, but other methods also exist (and they perform better in typical conditions). In our calculator, you'll find implemented:
1. Simple formula:
where vapor pressure is expressed in $\mathrm{mmHg}$ and temperature in kelvin.
2. Antoine formula:
The temperature $T$ is expressed in degrees Celsius and the vapor pressure $P$ is in $\mathrm{mmHg}$. Jump to the next section to read more about the constants in the Antoine formula.
3. Magnus formula, also known as AugustRocheMagnus or MagnusTetens equation:
where $T$ is expressed in $\degree\mathrm{C}$ and $P$ in $\mathrm{kPa}$.
4. Tetens formula:
where $T$ is expressed in $\degree\mathrm{C}$ and $P$ in $\mathrm{kPa}$.
5. Buck formula, also known as Arden Buck equation:
where $T$ is expressed in $\degree\mathrm{C}$ and $P$ in $\mathrm{kPa}$.
You can also use another equation, called the GoffGratch formula, but as it's more complicated (and approximately as accurate as the Buck formula), we didn't implement it in our vapor pressure of water calculator. The table below shows the comparison of the accuracies between different formulas for several temperatures from $0\ \degree\mathrm{C}$  $100\ \degree\mathrm{C}$ range ($32\ \degree\mathrm{F}$  $212\ \degree\mathrm{F}$). The reference values come from with the vapor pressure of water (all pressures are given in $\mathrm{kPa}$).
T [°C]  T [F]  P (Lide Table)  P (Simple)  P(Antoine)  P (Magnus)  P (Tetens)  P (Buck) 

0  32  0.6113  0.6593 (+7.85%)  0.6056 (0.93%)  0.6109 (0.06%)  0.6108 (0.09%)  0.6112 (0.01%) 
20  68  2.3388  2.3755 (+1.57%)  2.3296 (0.39%)  2.3334 (0.23%)  2.3382 (+0.05%)  2.3383 (0.02%) 
35  95  5.6267  5.5696 (1.01%)  5.6090 (0.31%)  5.6176 (0.16%)  5.6225 (+0.04%)  5.6268 (+0.00%) 
50  122  12.344  12.065 (2.26%)  12.306 (0.31%)  12.361 (+0.13%)  12.336 (+0.08%)  12.349 (+0.04%) 
75  167  38.563  37.738 (2.14%)  38.463 (0.26%)  39.000 (+1.13%)  38.646 (+0.40%)  38.595 (+0.08%) 
100  212  101.32  101.31 (0.01%)  101.34 (+0.02%)  104.077 (+2.72%)  102.21 (+1.10%)  101.31 (0.01%) 
As you can notice, the Antoine equation is reasonably accurate for higher temperatures, but the low ones are calculated with quite a big error. The Tetens equation works well for $0\ \degree\mathrm{C}$  $50\ \degree\mathrm{C}$ range, but Buck beats all of them for every checked value. The values start to differ significantly for temperatures higher than $100\ \degree\mathrm{C}$, and the Antoine equation is usually the most accurate one.
Antoine equation
The Antoine equation is derived from the Clausius–Clapeyron relation (the one we used in our boiling point calculator). It's a semiempirical formula describing the association between vapor pressure and temperature. It works for many substances, although you need to know the coefficients. There are usually two sets of parameters used for a single component:
 One for describing the vapor pressure curve up to the normal boiling point. For water, it's the range $0\ \degree\mathrm{C}$  $100\ \degree\mathrm{C}$ or $32\ \degree\mathrm{F}$  $212\ \degree\mathrm{F}$.
So the Antoine equation is:
 The second for the range from the normal boiling point to the critical point ($100\ \degree\mathrm{C}$  $374\ \degree\mathrm{C}$  or $212\ \degree\mathrm{F}$  $705\ \degree\mathrm{F}$  for water)
So the formula looks as follows:
The Antoine equation is sometimes simplified (omitting the C coefficient) or extended by three additional terms, which can increase the flexibility of the equation.
🙋 Before heading into the next section, be sure to master the conversions between the various pressure measurement units: our pressure conversion tool is a comprehensive guide to do so. You can also try our temperature conversion tool for an easier mnemonic exercise!
Vapor pressure of water
The vapor pressure of water is the pressure at which water vapor is in thermodynamic equilibrium with its condensed state. The water will condense if we raise the pressure and keep the temperature.
Have a look at this handy vapor pressure for water table to find the pressure for different temperatures quickly:
T [°C]  T [°F]  P [kPa]  P [torr]  P [atm] 

0  32  0.6113  4.5851  0.0060 
5  41  0.8726  6.5450  0.0086 
10  50  1.2281  9.2115  0.0121 
15  59  1.7056  12.7931  0.0168 
20  68  2.3388  17.5424  0.0231 
25  77  3.1690  23.7695  0.0313 
30  86  4.2455  31.8439  0.0419 
35  95  5.6267  42.2037  0.0555 
40  104  7.3814  55.3651  0.0728 
45  113  9.5898  71.9294  0.0946 
50  122  12.3440  92.5876  0.1218 
55  131  15.7520  118.1497  0.1555 
60  140  19.9320  149.5023  0.1967 
65  149  25.0220  187.6804  0.2469 
70  158  31.1760  233.8392  0.3077 
75  167  38.5630  289.2463  0.3806 
80  176  47.3730  355.3267  0.4675 
85  185  57.8150  433.6482  0.5706 
90  194  70.1170  525.9208  0.6920 
95  203  84.5290  634.0196  0.8342 
100  212  101.3200  759.9625  1.0000 
Two formulas have a version for vapor pressure of water over ice (so for temperatures below $0\ \degree\mathrm{C}$). Type negative temperatures into the calculator, and the vapor pressure will be determined according to Buck and Teten's formulas.
🙋 For a more generic tool, visit our vapor pressure calculator!
How to use the vapor pressure of water calculator
Now as you know what vapor pressure is and you heard about different vapor pressure formulas, it's high time for a practical demonstration. This calculator is one of the easiest to use, as you need to enter only one value, so you shouldn't have any problems with using it! But just in case, we're showing the example:

Enter the temperature. Assume we want to calculate the vapor pressure of water in $86\ \degree\mathrm{F}$ ($30\ \degree\mathrm{C}$).

Poof! The vapor pressure of water calculator found the pressure according to five formulas. The most often used is the Antoine equation ($4.232\ \mathrm{kPa}$), but the Buck formula ($4.245\ \mathrm{kPa}$) is usually the most accurate one for temperature ranges we typically look for.

If you want to get the result in a different pressure unit, simply click on the unit name and choose the one you need: $\mathrm{Pa}$, $\mathrm{hPa}$, $\mathrm{torr}$, $\mathrm{mmHg}$ or any other unit.
What is vapor pressure?
The vapor pressure of water is the point of equilibrium between the number of water molecules moving between the liquid phase and the gas phase in a closed container. At this point, there are as many molecules leaving the liquid and entering the gas phase as there are molecules leaving the gas phase and entering the liquid phase.
Does vapor pressure increase with temperature?
Yes, vapor pressure increases with temperature as the molecules receive more energy to escape from the liquid phase and transition to the gas phase. Note that a closed container is required, as otherwise, the molecules in the gas phase will fly away.
How can I calculate the vapor pressure of water at 80°C degrees?
The vapor pressure of water at 80 °C will be 47.27 kPa (Antoine formula) or 46.19 kPa (simple formula).
To find the vapor pressure of water:

Use one of the popular approximations, e.g., Antoine formula:
P_{Antoine} = 10^{A−B/(C+T)} = 10^{8.14019−1810.94/(244.485+T)}

Enter T = 80 °C in Celsius degrees: 10^{8.14019−1810.94/(244.485+80)}.

Compute 10^{1.6746} = 47.27 kPa.

Compare with the simplified formula:
P_{simple} = e^{20.386−5132/(T+273.15)} = e^{20.386−5132/(80+273.15)} = 46.19 kPa
Can a vapor pressure of water be zero?
No, vapor pressure can't be zero when the temperature is above absolute zero. Note that many objects have resided for eons in the vacuum of space, whose temperature is not absolute zero, but have not evaporated because they have nonzero vapor pressure (e.g., asteroids).
Why is the vapor pressure of water so important?
The vapor pressure of water is crucial for life forms on Earth, as its value is high enough to allow for the process of evaporation but low enough to also allow for the existence of liquid and solid water.