Article — Bar to ATM Converter
Bar to ATM Converter: Exact Pressure Conversion
One atmosphere equals exactly 1.01325 bar. One bar equals 0.98692 atmospheres. The factor comes from two fixed definitions: the standard atmosphere is 101 325 pascals, set by the General Conference on Weights and Measures in 1954, and the bar is 100 000 pascals exactly. Multiply atm by 1.01325 to get bar; divide bar by 1.01325 to go back. Both directions are exact, not measured.
The two units are close enough that engineers, divers, and meteorologists often treat them as interchangeable in casual use. The 1.3 percent gap rarely matters for tires or weather. It does matter for compressed-gas safety calculations, vapour-pressure tables, and any laboratory work that traces back to NIST or BIPM standards.
What is the bar to atm conversion?
The bar and the atmosphere are two non-SI metric units of pressure. Both can be derived from the pascal (the SI base unit), but they differ in origin. The atmosphere is anchored to a physical reference — sea-level air pressure on a standard day. The bar is anchored to mathematics — one bar is one hundred thousand pascals, a round number chosen for convenience.
Converting between them is dividing or multiplying by a fixed rational number. The factor 1.01325 has only five significant digits and terminates, which makes it one of the cleanest pressure conversions in metric engineering. Compare it with the bar-to-psi factor (14.5038), which is an irrational decimal.
The standard atmosphere was defined long before the pascal existed. The 10th General Conference on Weights and Measures fixed it at 101 325 Pa in 1954, choosing a value that matched the mean pressure at sea level on a 15 °C day. The bar was defined in 1909 by Vilhelm Bjerknes specifically because meteorologists wanted a round metric pressure unit.
The bar to atm formula
The forward and reverse formulas are reciprocals.
atm = bar ÷ 1.01325 bar = atm × 1.013251 bar = 0.98692 atm 1 atm = 1.01325 bar2 bar = 1.974 atm 10 bar = 9.869 atm200 bar = 197.4 atm (scuba tank, full)Examples: a car tire at 2.2 bar sits at 2.17 atm. A scuba tank rated to 200 bar holds gas at 197.4 atm. Sea-level atmospheric pressure is 1.01325 bar, often called “1 atmosphere” in everyday speech.
Why 1 atm = 1.01325 bar exactly
Both units share the pascal as their reference. The pascal, defined as one newton per square metre, is an SI derived unit that depends on the metre (defined by the speed of light), the kilogram (defined by Planck’s constant since 2019), and the second (defined by caesium-133). Anchored to fundamental physics, the pascal carries no uncertainty.
The standard atmosphere is defined as exactly 101 325 Pa. The bar is defined as exactly 100 000 Pa. Their ratio is 101 325 / 100 000 = 1.01325, which terminates after five decimals. Multiplying or dividing by it introduces no rounding beyond what your display precision allows.
The atmosphere is not actually equal to average sea-level pressure on Earth. Real sea-level pressure varies by altitude, latitude, and weather, ranging from about 870 mbar in deep low-pressure systems to 1085 mbar in strong high-pressure systems. The standard atmosphere is a reference value, not an observation.
Bar to atm reference table
The pressures you see in real applications:
- 1 bar = 0.987 atm = 14.50 psi = 100 kPa = 750.1 mmHg
- 1 atm = 1.01325 bar = 14.696 psi = 101.325 kPa = 760 mmHg
- 2.2 bar = 2.17 atm = 31.91 psi (typical car tire)
- 9 bar = 8.88 atm = 130.5 psi (espresso machine)
- 10 bar = 9.87 atm = 145 psi (boiler safety limit)
- 200 bar = 197.4 atm = 2901 psi (full scuba tank)
- 300 bar = 296.1 atm = 4351 psi (DOT-3AA cylinder)
- 0.5 bar = 0.493 atm (half-vacuum, vacuum chamber)
- 0.012 bar = 0.0118 atm (low Earth orbit, ISS altitude air pressure)
Bar vs atm vs psi vs kPa
The four units overlap in scope but separate by application. Pascal and kilopascal are strict SI; bar and atmosphere are accepted-for-use; psi is imperial. None of them are wrong; they reflect industries with different traditions.
The pascal is too small for most uses. Kilopascal and bar handle everyday engineering. The atmosphere is the natural unit for upper-atmosphere physics and planetary science. The psi rules in American shop floors.
Torricelli and the origin of the atmosphere
Evangelista Torricelli built the first barometer in 1643, filling a one-metre glass tube with mercury, sealing one end with his thumb, and inverting it into a mercury bath. The column stabilised at about 760 millimetres above the bath, and Torricelli concluded that atmospheric pressure was holding the mercury up.
The 760 mm of mercury became the working definition of one atmosphere for three centuries. The pascal era brought a more fundamental basis — 101 325 Pa — but the historical link is exact: 1 atm = 760 mmHg = 760 torr by definition.
Common bar to atm mistakes
The 1.3 percent gap between bar and atm is small for tires (2 bar vs 2.03 atm, an unmeasurable 0.4 psi difference) but compounds in high-pressure work. A 200-bar dive tank logged as “200 atm” understates the actual pressure by 2.65 atm, or 38.5 psi. Safety calculations and pressure-vessel certifications use the exact factor, not the round one.
The second mistake is mixing absolute and gauge pressure. Absolute pressure is measured from a perfect vacuum; gauge pressure is measured from local atmospheric pressure. A tire gauge reading 2 bar means 2 bar above ambient, so 3.01 bar absolute. A pure-bar-to-atm conversion preserves the meaning — either both absolute or both gauge — but mixing them creates a one-atmosphere error.
Bar to atm in scuba, tires, weather
Scuba divers use bar globally; American divers add psi for tank gauges. A 200-bar (197.4-atm, 2901-psi) tank gains 1 bar of ambient pressure for every 10 metres descended. At 30 m, the regulator delivers air at 4 bar absolute.
European cars spec tire pressure in bar. A label reading “2.2 bar front, 2.5 bar rear” converts to 31.9 psi and 36.3 psi. The atm equivalents are 2.17 and 2.47, close enough to ignore.
Weather services report atmospheric pressure in hectopascals (hPa). One hPa equals one millibar exactly, and 1013 hPa equals 1013 mbar equals 1.013 bar equals 1 atm. The hPa-mbar identity is why European weather maps and US maps with mbar labels look identical even though their units appear different.