Atmospheres to kPa:
multiply by 101.325

How many kPa are in an atmosphere?

One standard atmosphere is exactly 101.325 kPa. The value is fixed by definition and is meant to represent the average air pressure at sea level, which is why it is used as a reference in science and engineering.

The atmosphere is a large unit compared with the kilopascal, so converting atm to kPa gives a number about a hundred times bigger. An atmosphere is also 1.01325 bar and about 14.696 psi.

The atm to kPa formula

A second example goes the other way. A weather report giving a pressure of 91.19 kPa corresponds to 91.19 ÷ 101.325 = 0.9 atm, which is below the sea-level standard, as you would find at a high-altitude location or in a low-pressure system.

Chart: atm to kPa

To use the chart for other values, add rows. A pressure of 12 atm is 10 atm plus 2 atm, which is 1,013.25 + 202.65 = 1,215.9 kPa.

Where atm to kPa is used

• Weather: air pressure is reported in kPa in some countries, such as Canada, and in hPa or millibars in many others.
• Chemistry and physics classes: gas law problems often give pressure in atm and ask for an answer in kPa.
• Diving: pressure underwater rises by roughly one atmosphere for every 10 metres of depth, which dive tables often express in kPa or bar.
• Engineering: vessel and pipe ratings given in atm must be matched to kPa on data sheets.

Estimating and checking your answer

Because 101.325 is close to 100, you can estimate by moving the decimal point two places and adding about 1.3 percent. For 5 atm, 500 plus 1.3 percent is about 506.6, and the exact figure is 506.625 kPa. For the reverse, divide the kPa figure by 100 and take away about 1.3 percent.

As a second check, one half of an atmosphere is 50.6625 kPa, and 0.1 atm is 10.1325 kPa. Remembering these two values makes most conversions quick.

A short worked check with a gas cylinder

Suppose a chemistry problem states a gas pressure of 3 atm and asks for kPa. Multiplying gives 3 × 101.325 = 303.975 kPa. A rough check with 3 × 100 gives 300, which is close, and the small extra comes from the 1.3 percent difference between 101.325 and 100. If the final answer were 30.4 or 3,040, you would know the decimal point had moved.

Common mistakes with atm and kPa

• Using 100 instead of 101.325. It is a 1.3 percent error, which becomes 13 kPa on a 10 atm pressure.
• Confusing atm with bar. They are different units, with 1 bar equal to exactly 100 kPa.
• Mixing up the standard atmosphere with the technical atmosphere. The technical atmosphere is a separate older unit equal to about 98.07 kPa.
• Forgetting gauge versus absolute pressure. A gauge reading of 1 atm is the pressure above the surrounding air, so absolute pressure is about 2 atm.