Water Vapor Saturation Pressure Calculator + 3 Charts (°F, °C, K)

At what pressure (kPa, PSI, mmHg, bar, atm, torr) and temperature (°F, °C, K) does water boil?

This is exactly what the water saturation pressure calculators determine. We will show you how exactly to determine the saturation pressure of water at 3 temperature scales (°F, °C, K).

Namely, to turn water into vapor at a lower temperature, we need a low pressure, and visa versa. To determine is vapor saturation pressure of water at different temperatures, we created 3 toolboxes you can freely use that make this conversion:

Water Vapor Temperature (°F, °C, K) ⇒ Pressure (kPa, PSI, mmHg, Bar, atm, torr)

Quick Example: Vapor pressure of water at 20°C is 2.37 kPa, 0.34 PSI, 17.81 mmHg, 0.02 Bar, 0.02 atm, or 17.81 torr (from 2nd °C calculator and chart).

Since we have 3 different temperature units (Fahrenheit °F, Celsius °C, and Kelvin K), we created 3 separate parts here. Here’s how this article is structured:

• 1st Part: Water Saturation Pressure For Fahrenheit (°F) – Calculator, Example, Chart. Using this 1st calculator, you insert temperature in °F, and get the vapor pressure of water in terms of kPa, PSI, mmHg, Bar, atm, torr. We look at the 68°F example specifically. On top of that, we calculated water saturation pressure at 32°F to 200°F, and summarized all the results in the 1st chart.
• 2nd Part: Water Saturation Pressure For Celsius (°C) – Calculator, Example, Chart. Here, you insert temperature in Celsius (°C) and get saturation pressures. Example: Vapor pressure of water at 25°C and 20°C. We also created a water vapor pressure chart ranging from water vapor pressure chart 1°C to 150°C.
• 3rd Part: Water Saturation Pressure For Kelvins (K) – Calculator, Example, Chart. We did the same thing with the Kelvin temperature scale, including a 273K to 400K vapor pressure chart.

Note: We will also look at vapor saturation pressure formulas. Once you see how complex they are, you quickly realize how useful this 3 online calculators + 3 charts are because they do all the math automatically (instead of having to use a hand calculator every time).

Let’s start with the 1st part (°F), and then proceed with the 2nd part (°C) and 3rd part (K). Here is the 1st vapor pressure calculator for the Fahrenheit temperature scale:

To illustrate how this Fahrenheit water vapor pressure calculator works, let’s solve the following example:

What is the saturation pressure of water at 68°F? We simply input ’68’ in the temperature field, and we get the result in this form (1st screenshot):

We can see that vapor saturation pressure at 68°F is equal to 2.37 kPa, 0.34 PSI, 17.81 mmHg, 0.02 Bar, 0.02 atm, or 17.81 torr (exact to 2 decimal numbers). As you can see, this 1st calculator, as well as the 2nd and 3rd calculators down below, will determine the saturation pressure in 6 different pressure units.

For example, if you need to know the water saturation pressure at 68°F in mmHg, you can quickly see that water boils at 17.81 mmHg pressure (at 68°F).

Now, before we check the 1st °F water vapor pressure chart, let’s have a quick look at the math behind this:

Saturation Pressure Formula For Fahrenheit (°F)

Since the relationship between vapor saturation pressure and temperature is not exactly linear, we have to use approximation formulas. Namely, there are 6 different approximation formulas you can check here. The simplest and most useful, for most intents and purposes, is this August equation:

P_sat (mmHg) = e^{20.386 – 5132/T (K)}

If you think this saturation pressure formula is a bit complex, don’t panic. The other 5 formulas are even more complex (that’s why it’s so useful to have an online calculator with temperature-saturated pressure charts, so you don’t have to calculate by hand).

In the equation above, P_sat stands for saturation pressure (in mmHg), and T stands for temperature (in Kelvin’s K). Now, to figure out how much pressure we need for vapor saturation at a certain Fahrenheit temperature, we need to express temperature in F like this:

P_sat (mmHg) = e^{20.386 – 5132/((T (°F) – 32) × 5/9 + 273.15)}

This is a snake of an equation, but let’s (just for fun) look at how many mmHg (millimeters of mercury) of pressure we need to get water vapor saturation at 100°F. We insert 100°F in the equation above like this (with the result):

P_sat (mmHg) = e^{20.386 – 5132/((100°F– 32) × 5/9 + 273.15)} = 48.45 mmHg

We can see, after a lot of manual calculations, that the vapor saturation pressure at 100°F is 48.45 mmHg (you can check this result with the 1st calculator). Now, we can convert this mmHg pressure into other pressure units (kPa or kilo-Pascals, PSI or Pounds Per Square Inch, Bar, atmosphere, and torr).

We calculated vapor pressures at temperatures ranging from 32°F to 200°F, expressed them in 6 different pressure units, and summarized all the results in this 1st Fahrenheit vapor saturation pressures chart:

Water Vapor Saturation Pressure Fahrenheit Chart (32°F – 200°F)

Here, we see quite clearly that water turns to vapor at increasing pressure if we increase the temperature. Here are some outtakes:

• At 32°F, the vapor pressure is very low. We see that we need only 0.66 kPa, 0.10 PSI, 4.94 mmHg, 0.01 Bar, 0.01 atm, or 4.94 torr pressure to boil water at 32°F.
• At 68°F, we will see water reach the saturation level if the pressure equals 2.37 kPa, 0.34 PSI, or 17.81 mmHg.
• At higher temperatures (say 150°F), we need more pressure for the saturation criteria to be met. Specifically, we see 150°F water vapor saturation pressure at 25.01 kPa.

Alright, this was 1st part where we look at the vapor pressures at temperatures, expressed in degrees Fahrenheit (°F). Let’s have a look at these pressures when the temperature is expressed in degrees Celsius (°C). Here is the 2nd calculator for Celsius:

Let’s say that we want to know the vapor pressure of water at 25°C. You just insert ’25’ in the degrees Celsius field, and you get this result (2nd screenshot):

We see that water at 25°C reaches vapor saturation pressure when the pressure equals 3.18 kPa. You can check the pressures expressed in other units as well; 25°C saturated water vapor pressure is equal to 0.46 PSI, 23.88 mmHg, 0.03 Bar, 0.03 atm, or 23.88 torr.

Let’s have a look at the formula that enables us to calculate the vapor saturation pressure in degrees Celsius:

Vapor Saturation Pressure Formula For Celsius Temperature Scale (°C)

We use the same general August formula we used in the 1st part, but, in this case, we have to convert K (Kelvins) to °C (degrees Celsius). The new equation looks this this:

P_sat (mmHg) = e^{20.386 – 5132/(T (°C) + 273.15)}

Quick Example: What is the vapor pressure of water in mmHg at 20°C? We insert 20°C in the equation like this (with the result):

P_sat (mmHg) = e^{20.386 – 5132/(20°C + 273.15)} = 17.81 mmHg

We see that water vapor pressure at 20°C equals 17.81 mmHg (use the 2nd calculator to confirm this). Using relationships between pressure units, we can see that 17.81 mmHg is equal to 2.37 kPa, 0.34 PSI, 0.02 Bar, 0.02 atm, or 17.81 torr.

Here is the full 2nd chart with water vapor saturation pressures for temperatures ranging from 1°C to 150°C:

Water Vapor Saturation Pressure Celsius Chart (1°C – 150°C)

Here are a few outtakes:

• Everybody who uses degrees Celsius (°C) knows that water boils at 100°C at everyday pressure. We can see that the vapor saturation pressure at 100°C is exactly 1 atm (1 atmospheric pressure).  We see that saturation pressure at 100°C is 101.27 kPa, or 14.69 PSI, in line with the standard everyday pressure (1 atm).
• At 1°C, we see that we need a very low pressure in order to achieve water vapor saturation conditions. Specifically, the pressure should be 0.71 kPa, 0.10 PSI, 5.29 mmHg, or 0.01 atm (this is quite close to a vacuum).
• If you want to boil water at above standard 100°C (say 120°C), you need more than 1 atm of pressure. We can see that, at 120°C, the water vapor saturation conditions are met at 203.87 kPa, 29.57 PSI, or 1529.14 mmHg pressure.

Finally, let’s have a look at the Kelvin temperature scale, starting with the 3rd water vapor saturation pressure calculator:

Again, let’s look at one example:

What is the water vapor saturation pressure at 298K? We input ‘298’ in the Kelvins field, and get this result (3rd screenshot):

We see that the saturation pressure at 298K is 3.16 kPa or 0.46 PSI or 23.68 mmHg, and so on.

The formula for the calculation of water vapor saturation pressure is the basic August approximation formula that already uses Kelvins:

P_sat (mmHg) = e^{20.386 – 5132/T (K)}

Again, it’s so much easier to use the calculator, or the following 3rd vapor pressure chart in Kelvins (from 273K to 400K):

Water Vapor Saturation Pressure Kelvin Chart (273K – 400K)

Using these 3 calculators, formulas, and 3 charts, you are now fully equipped to determine the water vapor pressure at any temperature and in all 3 temperature units. If you need any help with these calculations, you can always turn to our comment section, give us a few numbers, and we can help out with the math. Thank you.