Pressure Converter (bar, psi, MPa)
Convert between bar, psi, MPa, kPa, atm and mmHg instantly — all values shown at once.
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How It Works
Every value passes through the pascal (Pa), the SI unit of pressure — force per unit area. Enter a number in any of six units (Pa, kPa, bar, psi, atm, mmHg) and the tool converts it to all the others simultaneously using fixed ratios: 1 bar = 100,000 Pa = 14.5038 psi = 0.986923 atm; 1 atm = 101,325 Pa exactly, by definition — the historical average air pressure at sea level; 1 mmHg (Torr) = 133.322 Pa, from the height of a column of mercury a given pressure supports. So a tire spec of 32 psi converts to roughly 2.21 bar or 220.6 kPa — the same physical pressure, three common ways to write it down.
Every one of these conversions moves between absolute-pressure reference points — 0 Pa always means a perfect vacuum on both sides of the conversion. That matters because most everyday pressure readings you'll type in are actually gauge pressure — measured relative to the surrounding atmosphere, not to a vacuum. A tire gauge reading "32 psi" means 32 psi above atmospheric pressure (itself about 14.7 psi absolute), so the tire's absolute internal pressure is closer to 46.7 psi. This converter scales the number you type — it doesn't know or ask whether that number is gauge or absolute, so keeping track of which one you're working with stays on you.
What to Know
- By field: tire and pneumatic tool pressures are usually psi or bar (32 psi ≈ 2.2 bar); hydraulic systems quote bar or MPa (200 bar = 20 MPa ≈ 2,900 psi); weather reports use hPa, numerically identical to millibar (1 hPa = 1 mbar); medicine and vacuum gauges use mmHg.
- The psi/bar mix-up: psi and bar differ by roughly 14.5×, and both are common in the same industries (US vs. European spec sheets) — misreading "200" as bar when the source meant psi is a genuine, expensive real-world error in hydraulics and pneumatics.
- Gauge vs. absolute, concretely: a vacuum pump spec of "0.1 bar absolute" is a near-total vacuum; a tire spec of "0.1 bar gauge" (≈1.45 psi) is nearly flat — same number, opposite meaning, depending on which reference the manufacturer used.
- Standard atmosphere is a fixed constant: 101,325 Pa is not a measurement that varies day to day — it's a defined reference value used to build every other pressure unit's conversion factor, distinct from your local, weather-dependent barometric pressure.
- Precision: the six units convert through pascals with full floating-point precision; results round for display but the underlying ratios (14.5038 psi/bar, 133.322 Pa/mmHg) are standard published constants, not rough approximations.
Frequently Asked Questions
Is psi the same as psig?
Not quite — psig explicitly means gauge pressure (relative to the surrounding atmosphere, what a tire gauge or pressure washer display shows), while psia means absolute pressure (gauge plus roughly 14.7 psi of atmospheric pressure). Plain "psi" is ambiguous in casual use and usually means gauge unless a datasheet says otherwise. This converter scales magnitudes only — whether your number is gauge or absolute stays your own bookkeeping.
What is the practical difference between bar and atm?
Almost none: 1 atm = 1.01325 bar, a 1.3% difference. The bar was defined as a round 100,000 Pa for convenience; the atmosphere is the historical reference for average sea-level air pressure. In most non-scientific contexts you can treat them as interchangeable.
Why does gauge vs. absolute pressure matter so much?
Because the same numeral means a different physical state depending on the reference. "0 psi gauge" is normal atmospheric pressure — the tire is flat but not evacuated. "0 psi absolute" is a total vacuum. Vacuum pumps, altitude/aviation specs, and some scientific instruments are always quoted absolute; tire gauges, pneumatic tools, and most consumer pressure gauges are always gauge. Converting the number without tracking which reference it uses is the single most common pressure-unit mistake.
Why is standard atmosphere defined as exactly 101,325 Pa?
It's a fixed reference constant, not a live measurement — chosen in the 19th century to approximate mean sea-level air pressure, and later frozen as an exact defined value so every other pressure unit (psi, mmHg, atm itself) has one stable conversion factor to build from. Your actual local barometric pressure varies with weather and altitude and is rarely exactly 101,325 Pa.
My spec sheet gives psi but my gauge reads bar — which do I trust?
Neither is more "correct" — they describe the same physical pressure in different units, so convert one to match the other rather than guessing. Use this tool to turn the spec's psi value into bar (or vice versa) and compare like-for-like; the ~14.5× ratio between them is exactly why mixing the two without converting causes real equipment damage.
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Pressure Converter (bar, psi, MPa)
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