Pressure Converter
Convert one pressure reading into 16 units at once, from psi, bar and kPa to inHg, mmHg and inches of water, and switch between gauge and absolute pressure using your local atmosphere.
Every unit goes through pascals · 1 psi = 6,894.757 Pa · 1 bar = 100,000 Pa
Which Unit Is Used Where
Most pressure confusion comes from reading a spec written for one country or trade with a gauge made for another.
| Unit | Used for | Typical value |
|---|---|---|
| psi | US tyres, compressors, water mains | Car tyre 32–35 psi |
| bar | European tyres, scuba tanks, espresso machines | Scuba tank 200 bar (2,901 psi) |
| kPa | Canadian and Australian tyres, engineering | Tyre placard 240 kPa (34.8 psi) |
| MPa | Hydraulics, material strength, pipe ratings | Excavator hydraulics 21 MPa (210 bar) |
| ksi | Steel and concrete strength in the US | A36 steel yield 36 ksi (248 MPa) |
| hPa | Weather maps and forecasts | Standard sea level 1,013.25 hPa |
| inHg | US weather reports, aircraft altimeters | Altimeter setting 29.92 inHg |
| mmHg | Blood pressure | 120/80 mmHg = 16.0/10.7 kPa |
| inH₂O | HVAC duct and filter pressure drop | Furnace filter 0.5 inH₂O (125 Pa) |
| cmH₂O | CPAP machines and ventilators | CPAP 10 cmH₂O (9.8 hPa) |
| kgf/cm² | Older Asian and European gauges | 2.2 kgf/cm² = 2.16 bar |
| psf | US building codes: wind and snow loads | 30 psf snow load (1.44 kPa) |
Worked Example: a European Tyre Placard at a US Air Pump
The door-pillar sticker on an imported car says 2.4 bar, but the gas station pump only shows psi. (This is the Load Example.)
Step 1: Bar to pascals
Step 2: Pascals to psi
Set the pump to 35 psi. Both the placard and the pump are gauge pressures, so no atmosphere adjustment is needed. A quick shortcut: 1 bar ≈ 14.5 psi.
Gauge vs Absolute Pressure
Gauge pressure counts from the surrounding air, so an empty tyre reads 0 psig. Absolute pressure counts from a perfect vacuum, so the same tyre is at about 14.7 psia at sea level. Converting units never changes which reference a number uses. Only adding or subtracting the atmosphere does that.
- Gauge: tyres, compressors, hydraulics, water pressure, blood pressure. Often written psig or barg.
- Absolute: weather and altimeters, vacuum work, gas-law calculations, engine manifold (MAP) sensors. Written psia or bara.
- Differential: the difference between two points, such as across a filter. It stays the same whatever the reference, so just convert it.
Why a Tyre Reads Higher in the Mountains
You set a tyre to 32 psig at sea level and drive to 5,000 ft, where the air outside is about 12.2 psi instead of 14.7. The air inside the tyre hasn't changed, but the gauge measures against thinner air outside.
Absolute pressure inside the tyre
Gauge reading at 5,000 ft
The gauge reads about 2.5 psi higher, before any change from temperature. Select "Gauge" above and enter 84.3 kPa as the atmosphere to try it.
How Each Unit Is Defined
- Pascal: one newton per square metre, the SI unit. Small: a sheet of paper resting on a table presses with about 1 Pa.
- Bar: exactly 100,000 Pa, chosen to be close to sea-level air pressure. 1 atm is 1.01325 bar.
- Standard atmosphere: exactly 101,325 Pa, a fixed reference value rather than today's air pressure.
- psi: one pound-force on one square inch, 6,894.757 Pa, fixed by the exact definitions of the pound and the inch.
- kgf/cm² (the "technical atmosphere"): 98,066.5 Pa, 2% less than a bar. Old gauges marked "kg" mean this.
- Torr and mmHg: a torr is 1/760 atm; a mmHg is defined from a mercury column and is larger by about one part in seven million. For vacuum work, see the torr to atm converter.
- inHg, inH₂O, cmH₂O: the height of a liquid column. Values change slightly with the liquid's temperature, so this page uses conventional figures (mercury at 0 °C, water at 4 °C for inH₂O).
Frequently asked questions
How many psi is 1 bar?
14.5038 psi. Going the other way, 1 psi is 0.0689 bar, so 30 psi is about 2.07 bar.
Is 1 bar the same as 1 atmosphere?
Close, but not the same: 1 atm is 1.01325 bar. The 1.3% gap rarely matters for tyres but does in lab and gas calculations.
What do the "g" and "a" in psig and psia mean?
Gauge and absolute. psig is measured from the surrounding air; psia from a vacuum. At sea level, psia ≈ psig + 14.7.
Why can I enter a negative pressure?
A gauge or differential reading can be negative, meaning below the surrounding air or lower on one side. Absolute pressure can't, so negative values are refused when "Absolute" is selected.
Sources
- NIST Special Publication 811: conversion factors for psi, atm, torr, mmHg, inHg, inH₂O and kgf/cm².
- BIPM SI Brochure: definition of the pascal; bar and mmHg as non-SI units.
- National Weather Service: station pressure: how atmospheric pressure falls with altitude.
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