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Bar to Atmospheres Converter
↔ Convert atm to bar instead

Common Conversions

bar atm
0.1 0.0987
0.5 0.4935
1 0.9869
1.01325 1
2 1.974
5 4.935
10 9.869
50 49.35
100 98.69
200 197.4
500 493.5

Why this conversion matters in chemistry

Bar and atm sit close to each other — 1 bar is 0.9869 atm — and for a lot of calculations you can treat them as interchangeable. The gap becomes important once you're reading across thermodynamic tables from different eras: IUPAC changed standard pressure from 1 atm to 1 bar in 1982, so older reference values of ΔH° and ΔG° assume a slightly higher standard state than newer ones. A high-pressure hydrogenation run logged at 50 bar is 49.35 atm, and while the difference rarely shifts turnover numbers, it does matter for Henry's-law solubility calculations and for comparing reaction rates between papers. Whichever unit you use, match it to the gas constant in your equation.

Formula

atm = bar / 1.01325

Worked Examples

1 bar = 0.9869 atm

IUPAC standard pressure (100 kPa) is slightly less than 1 atm

1.01325 bar = 1 atm

Standard atmospheric pressure defined as exactly 101.325 kPa

5 bar = 4.935 atm

Low-pressure chromatography territory — FPLC and gravity-fed preparative columns operate in this range, well below the 100+ bar regime of analytical HPLC.

200 bar = 197.4 atm

Pressure inside a full compressed hydrogen gas cylinder

Frequently Asked Questions

How do I convert bar to atm?
Divide by 1.01325. So 5 bar is 4.935 atm, 200 bar is 197.4 atm. Because the factor is close to 1, the two numbers look almost identical — which is exactly what makes the conversion easy to forget at the precision you sometimes need it.
Is 1 bar the same as 1 atm?
Close but not the same. 1 bar is 0.9869 atm — a 1.3% difference. For quick mental arithmetic you can treat them as equivalent; for thermodynamic work you can't, because IUPAC uses 1 bar as the standard state while older reference tables are pinned to 1 atm.
Which should I use, bar or atm?
IUPAC has recommended bar (or Pa / kPa) since 1982, and most modern papers follow that convention. Plenty of teaching textbooks still use atm, though, especially when the ideal gas constant is written as R = 0.08206 L·atm/(mol·K). Whichever unit you pick, match it to the R value in your equation.
Does this affect standard formation enthalpies?
Slightly. Tables from before 1982 use 1 atm standard state; newer IUPAC data uses 1 bar. The difference in standard Gibbs energy works out to around 0.1 kJ/mol for a gaseous species, which is usually lost in the rounding. It does accumulate in multi-step Hess's-law calculations where signs reinforce, so it's worth knowing which convention your source uses.