Bar to atm converter, bar against one standard atmosphere.

To convert bar to atm, divide by 1.01325. So 1 bar is 0.986923 atm, 5 bar is 4.9346 atm and 10 bar is 9.869 atm. A standard atmosphere is defined as 101,325 pascals, while a bar is 100,000 pascals, so an atmosphere is 1.325 percent bigger.

How to use the converter

1. Type the amount in the Bars (bar) box. A decimal such as 2.5 or a fraction such as 3/4 works.
2. The big result shows Standard atmospheres (atm). Use Decimals in the side column to fix the number of places.
3. The cards under the result give the same pressure in other units.
4. Type in the second box, or press the swap button, to convert standard atmospheres to bars instead.
5. Click a row in Common values to load it. Copy result, Copy link, Copy table and Print chart keep the answer. Start again resets the box.

Why two nearly equal units

The bar was built to be close to normal air pressure at sea level, in round metric numbers. The atmosphere was fixed earlier, at 101,325 Pa. Both are used for the same kind of job, so a 1 bar reading and a 1 atm reading differ by 1.3 percent, which only matters for careful work.

Bar to atm, a table

Worked examples

• A reading of 3 bar. 3 ÷ 1.01325 = 2.9608 atm.
• A diver at 4 bar of water pressure. 3.948 atm.
• A tank at 200 bar. 197.4 atm.
• One atmosphere in bar. Press the swap button and type 1: exactly 1.01325 bar.
• Half an atmosphere. 0.5 atm = 0.50663 bar.

Gauge or absolute pressure

A tyre gauge, a pressure washer dial and most boiler gauges show gauge pressure, which is the pressure above the air around them. Absolute pressure counts from a perfect vacuum, so it is the gauge reading plus the air pressure. At one standard atmosphere, 101.325 kPa, 1 bar gauge is 2.01325 bar absolute. The Gauge or absolute box above the tables does the sum in either direction. Air pressure changes with weather and height, so one standard atmosphere is only an average. The standard atmosphere is from NIST SP 811 Appendix B.8, checked 24 Oct 2026.

Where the numbers come from

NIST SP 811, Appendix B.8 lists the standard atmosphere as 1.013 25 E+05 Pa and the bar as 1.0 E+05 Pa. Checked 24 Oct 2026. The converter keeps the factors as exact fractions and rounds only the last digit shown. It is a unit converter, not a safety tool: for a tyre, boiler or tank, use the value printed on the equipment or in its manual.

Where atmospheres show up

The atmosphere is common in chemistry, gas laws and diving. A gas law problem that gives pressure in bar often wants atm in the formula constant. If a constant is given as 0.082057 L·atm per mole per kelvin, you need atm. A pressure of 2.5 bar becomes 2.4673 atm before it goes in. Keeping the conversion visible avoids a 1.3 percent error that is easy to miss.

Mistakes to avoid

Do not treat bar and atm as identical in a calculation that needs more than two figures. At 200 bar the gap is 197.4 atm against 200, a difference of 2.6 atm. Also keep gauge and absolute readings apart.

Related pages: Bar to psi, Atm to mmHg, Bar to kPa and Psi to bar.