Pressure has more units in daily use than any other physical quantity: tires speak psi or bar depending on the continent, weather maps speak hectopascals, blood pressure monitors speak millimeters of mercury, and divers speak atmospheres. They all measure the same thing, force spread over area, and they all connect through the pascal. This guide lines them up with the conversions that actually come up, and our free pressure converter handles every pair.
In this guide
The pascal and its big siblings
The SI unit is the pascal: one newton of force per square meter. It is a tiny unit, roughly the pressure of a sheet of paper resting on a table, which is why real-world figures arrive in multiples: the kilopascal (kPa, thousands) and the bar, defined as exactly 100,000 Pa and chosen to sit close to atmospheric pressure. The actual standard atmosphere is 101,325 Pa = 1.013 bar = 14.7 psi, so “1 bar” and “1 atmosphere” differ by about 1.3%, close enough to interchange in conversation and far enough apart to matter in engineering. The pound per square inch, psi, is the imperial member of the family: 1 bar = 14.50 psi.
The conversion table
| Unit | In pascals | Everyday context |
|---|---|---|
| 1 pascal (Pa) | 1 | a sheet of paper on a table |
| 1 hectopascal (hPa) | 100 | weather maps; identical to the millibar |
| 1 kilopascal (kPa) | 1,000 | tire pressure in metric countries |
| 1 psi | 6,894.8 | tire pressure in the US |
| 1 mmHg | 133.3 | blood pressure |
| 1 bar | 100,000 | tire pressure in Europe, scuba |
| 1 atmosphere (atm) | 101,325 | sea-level air pressure |
The two worth memorizing are boxed by daily life: 1 bar ≈ 14.5 psi and 1 atm ≈ 1 bar. Everything else in the table is reachable from those two and a power of ten.
Tires: psi, bar, kPa
The classic crossover problem: the door sticker says one unit, the pump dial says another. A common passenger-car pressure of 32 psi is 2.21 bar or 221 kPa; a European spec of 2.4 bar is 34.8 psi. The fast mental bridge: psi to bar, divide by 14.5 (or just remember 30 psi ≈ 2.1 bar, 36 psi ≈ 2.5 bar and interpolate); kPa is simply bar times 100. Two practical notes that outrank the conversion itself: pressures are specified for cold tires, and the sticker in the door jamb, not the number molded on the tire sidewall, is the spec to follow. The sidewall number is the tire’s maximum, not the car’s recommendation.
Weather and the disappearing millibar
Weather charts label sea-level pressure around 1013 hPa. The hectopascal exists for one historical reason: it is exactly equal to the older millibar, so when meteorology went SI it could change the label without changing a single number. Reading the map: high-pressure systems run roughly 1020 to 1040 hPa and bring settled weather, deep low-pressure systems can fall below 980 and bring storms; the famous numbers attached to hurricanes are central pressures in this same unit. In US forecasts you may still meet inches of mercury, where the same standard atmosphere reads 29.92 inHg.
Blood pressure and diving
- Blood pressure stays loyal to the oldest unit in the family, the millimeter of mercury, because a column of mercury is literally how it was first measured. A reading of 120/80 mmHg is 16.0/10.7 kPa, and exactly nobody says it that way; mmHg survives because changing medical convention is harder than changing units. Health decisions, as always, belong to measurements and clinicians, not conversions.
- Diving thinks in atmospheres and bars because water is conveniently consistent: every 10 meters of depth adds about 1 bar. A diver at 30 meters sits under about 4 atm total (one of air plus three of water), which is the entire mathematical reason dive tables and ascent rates exist.
Like length and weight, all these factors are fixed definitions rather than measurements, part of the same story told in our unit conversion pillar.
Frequently asked questions
Is PSI gauge or absolute pressure?
Tire gauges read pressure above atmospheric, called gauge pressure (sometimes written psig). Absolute pressure (psia) adds the atmosphere’s 14.7 psi on top. A “32 psi” tire actually contains about 46.7 psi absolute; gauges subtract the atmosphere because that is the part doing the pushing against it.
Why does my ear pop on a plane if cabins are pressurized?
Pressurized does not mean sea-level: cabins are held at the equivalent of roughly 1,800 to 2,400 m of altitude, around 0.75 to 0.8 bar. Your ears equalize across that change during climb and descent, hence the pops.
What is a torr?
For practical purposes, a millimeter of mercury: the two differ by less than a millionth. Torr appears in vacuum engineering and lab work; mmHg in medicine. Same column of mercury, two professional dialects.
Why did 100,000 Pa get its own name?
Convenience: the bar lands within 1.3% of real atmospheric pressure while being a clean power of ten of the pascal. Engineers got a unit that feels like “one atmosphere” without the awkward 101,325.