Convert between bar and atmospheres (atm) with bidirectional conversion, PSI/kPa/mmHg equivalents, reference table, and preset values.
The bar and atmosphere (atm) are two closely related pressure units commonly used in science, engineering, and meteorology. One bar equals exactly 100,000 Pascals, while one standard atmosphere equals 101,325 Pascals — a difference of just 1.325%. Despite their near-equivalence, the distinction matters in precise scientific and engineering work.
This bidirectional converter lets you convert between bar and atm instantly. It also displays the equivalent values in Pascals, kilopascals, PSI, and millimeters of mercury, which is useful when you need to compare a metric pressure spec with a chemistry reference or a US gauge reading. The conversion table covers a wide range of values for quick reference, so you can sanity-check both small and large pressures without leaving the page.
The bar is widely used in European industry, weather forecasting (as millibars or hectopascals), and scuba diving. The atmosphere is the standard in chemistry for defining standard conditions (STP). Understanding the relationship between these two units is essential for anyone working across disciplines or international standards, especially when validating cross-border technical documents and equipment labels, or when you want to translate a lab value into something that matches a field gauge.
Bar and atm are close enough in value that people sometimes swap them casually, but the 1.3% gap still matters in precise lab, engineering, and standards work. This converter gives the exact relationship and shows the nearby units that usually appear beside them, so you can spot when a value was rounded too aggressively.
It is useful when you need to move between chemistry-style atm references and engineering-style bar references without manually carrying the 1.01325 factor through every comparison. It also helps when you are checking pressure logs, comparing vendor specifications, or translating values for documentation that mixes SI and older scientific units.
Bar to ATM: atm = bar ÷ 1.01325 ATM to Bar: bar = atm × 1.01325 Bar to Pa: Pa = bar × 100,000 Bar to kPa: kPa = bar × 100 Bar to psi: psi = bar × 14.5038
Result: 0.98692 atm
1 bar = 100,000 Pa. Since 1 atm = 101,325 Pa, we get 100,000 ÷ 101,325 = 0.98692 atm. The bar is slightly less than one atmosphere.
The bar was introduced in 1909 by British meteorologist Napier Shaw. It is defined as exactly 100,000 Pascals (10⁵ Pa). The atmosphere was defined as the average pressure at sea level, standardized to 101,325 Pa. While the two are close, the bar has a cleaner relationship to SI units.
In 1982, the International Union of Pure and Applied Chemistry (IUPAC) changed the standard pressure for reporting thermodynamic data from 1 atm to 1 bar (100 kPa). This means standard enthalpy, entropy, and Gibbs energy values in modern references use 1 bar, not 1 atm.
In Europe, tire pressure, gas cylinder ratings, and hydraulic system specifications use bar. In the US, the same measurements use psi. Scuba diving worldwide uses bar for tank pressure. Meteorology uses millibars (mbar) or the equivalent hectopascals (hPa).
No, but they are very close. 1 bar = 0.98692 atm, and 1 atm = 1.01325 bar. The difference is about 1.3%, which is small for rough estimates but important in precise calculations or when values are used in formulas.
Divide the bar value by 1.01325. For example, 5 bar ÷ 1.01325 = 4.9346 atm. If you are doing a quick estimate, multiplying by 0.987 gives nearly the same answer.
IUPAC changed the standard from 1 atm to 1 bar in 1982. Modern chemistry usually uses 1 bar (100 kPa) as standard pressure, though many textbooks and legacy references still use 1 atm (101.325 kPa).
The bar was defined as exactly 100,000 Pa to be a convenient round number in the metric system. Standard atmospheric pressure happens to be 101,325 Pa, which is close to 100,000, so the two units ended up nearly aligned by coincidence.
A millibar (mbar) is one-thousandth of a bar, equal to 100 Pa or 1 hectopascal (hPa). Meteorology still uses this scale because typical weather pressures land neatly around 1000 hPa at sea level.
Multiply the bar value by 14.5038. For example, 2 bar × 14.5038 = 29.0076 psi, which is why the same pressure feels familiar in both metric and US engineering contexts.