Bar, barg, bara and mbar — European pressure notation
1 bar = 100 000 Pa exactly. It is not an SI unit but it is accepted for use with SI, and it dominates European datasheets.
The bar wins on convenience: one bar is close enough to one atmosphere that engineers use them interchangeably in rough calculations, but 100 kPa exactly is a round SI-friendly number. What confuses newcomers is the g/a suffix that turns bar into a full absolute or gauge specification.
The exact factors
1 bar = 100 kPa = 14.5038 psi = 750.06 mmHg = 0.98692 atm > 1 mbar = 100 Pa = 1 hPa > barg = gauge, referenced to local atmosphere > bara = absolute, referenced to vacuum
Where each is used
Bar: European industrial specs, hydraulic ratings, automotive turbo. barg / bara: chemical process, gas systems, cryogenic vessels. mbar: meteorology (as hPa), leak-testing, low-pressure vacuum. Pneumatic tyres in Europe: 2.0–2.5 bar (gauge, implicit).
Worked example — nitrogen cylinder
A 200 bar cylinder is 200 barg → 201 bara → 2915 psig → 2930 psia. Miss the g/a in a manifold design and relief valve sizing is off by one atmosphere — usually harmless, occasionally a citation on a pressure vessel audit.
Recap
1 bar = 14.5 psi. Always ask 'g or a?' unless the datasheet says. Mbar and hPa are identical.
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FAQ
Is 1 bar the same as 1 atm?+
Close but not identical. 1 atm = 1.01325 bar. In practical work they are interchangeable; in cryogenic and chemical thermodynamics they are not.
Why do weather charts use hPa?+
hPa (hectopascal) equals mbar exactly. Meteorology switched notation to align with SI without changing the numeric values.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.