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Atmospheres to Bar Converter
↔ Convert bar to atm instead

Common Conversions

atm bar
0.1 0.101
0.25 0.253
0.5 0.507
0.75 0.76
0.9869 1
1 1.01325
2 2.0265
5 5.066
10 10.133
50 50.663
100 101.325

Why this conversion matters in chemistry

Atm and bar are close cousins — 1 atm is 1.01325 bar, a 1.3% difference. In introductory chemistry that gap usually vanishes in rounding. But IUPAC redefined standard pressure from 1 atm to 1 bar in 1982, which means thermodynamic data from earlier eras is pinned to a slightly different reference than modern tables. A ΔG° reported at the 1-atm standard differs from a 1-bar value by about RT ln(1.01325), roughly 33 J/mol at 298 K — small in isolation, but large enough to notice in multi-step Hess's-law calculations or when a paper expects agreement with published formation enthalpies to a fraction of a kJ/mol.

Formula

bar = atm × 1.01325

Worked Examples

1 atm = 1.01325 bar

Sea-level atmospheric pressure, just barely above 1 bar. The gap that causes all the 1 atm versus 1 bar confusion.

0.9869 atm = 1 bar

Modern IUPAC standard pressure, exactly 100 kPa. The reference for post-1982 ΔG° and ΔH° tables.

5 atm = 5.066 bar

A moderate elevated pressure, typical of pressurized reaction vessels in catalytic hydrogenation or autoclave work.

0.5 atm = 0.507 bar

Reduced pressure for low-pressure distillation — sufficient to drop a volatile solvent's boiling point by 10 to 20 degrees.

Frequently Asked Questions

Is 1 atm equal to 1 bar?
Close, but not identical. 1 atm is 1.01325 bar — a 1.3% difference. For most teaching-lab calculations you can treat them as interchangeable, but the gap does matter in precise thermodynamic work. IUPAC switched standard pressure to 1 bar in 1982.
Why did IUPAC switch from atm to bar?
The bar is defined as exactly 100,000 Pa, which makes unit arithmetic across the SI system clean — no awkward 101,325 factor to carry. The atmosphere, on the other hand, is a historical artifact tied to the pressure that supports 760 mm of mercury at sea level, which is no longer the kind of definition modern SI wants to build on.
Does the difference affect my calculations?
Rarely for coursework. For precise thermodynamic data — standard formation enthalpies, Gibbs energies, Keq values at the fourth decimal — check which standard state your table uses before mixing it with another source. Combining 1-atm and 1-bar referenced values without correcting introduces a systematic offset that accumulates across multi-step calculations.
How do I convert atm to bar?
Multiply by 1.01325. So 5 atm is 5.066 bar, 10 atm is 10.133 bar. The factor is close enough to 1 that quick mental estimates can treat atm and bar as equivalent, which is fine for any calculation where rounding to 1% is acceptable.