How to Convert Pressure Units (psi, bar, Pa, atm)

Exact and conventional anchors:

• 1 Pa = 1 N/m²
• 1 bar = 10⁵ Pa = 100 kPa (exact)
• 1 atm = 101325 Pa (exact, standard atmosphere)
• 1 psi = 6894.757293168 Pa (conventional; 1 lbf/in²)
• 1 torr = 1/760 atm ≈ 133.322 Pa

So 1 atm = 1.01325 bar = 14.6959 psi. 1 bar ≈ 14.5038 psi. If a compressor nameplate says 8 bar and you treat it as 8 atm, you are 1.3 percent high before any gauge error.

Four names you will actually see

Pascals and kilopascals. Weather maps in SI countries often show hPa (hectopascals). 1 hPa = 1 mbar = 100 Pa. Standard sea-level pressure is 1013.25 hPa, which is 1 atm, not 1 bar (1000 hPa).

Bar. Process industry default in much of Europe. 1 bar is a round 100 kPa, slightly below 1 atm. Easy mental math, slightly wrong if you needed the atmosphere standard.

psi. US mechanical work. Tire placards, hydraulic specs, HVAC. There is no “metric psi.” Convert to kPa or bar when the rest of the sheet is SI.

atm. Chemists and older steam tables. Convenient because many constants were tabulated at 1 atm. Not a license to set 1 atm = 1 bar = 10⁵ Pa in the same sentence.

Pressure converter holds these together. It runs in the browser; no account.

Gauge versus absolute

A tire at 32 psi on a shop gauge is 32 psig. Absolute pressure in the carcass is about 32 + 14.7 = 46.7 psia at sea level (using 1 atm ≈ 14.7 psi). On a high mountain the added atmosphere is smaller; the gauge still reads difference from local air.

Ideal-gas homework wants absolute T and usually absolute P. Feeding 32 psi into P₁V₁/T₁ as if it were absolute understates the pressure by about 30 percent. Weather “1008 hPa” is already absolute (it is the atmospheric column). A vacuum of “25 torr” is an absolute pressure above zero, not a gauge 25 torr below 760 unless the instrument says so.

Write 32 psig or 2.21 bar(g) when you know. The converter will not infer it from the number 32.

One numeric check: 32 psi

32 psi × 6894.757 ≈ 220632 Pa ≈ 220.6 kPa.

32 / 14.5038 ≈ 2.206 bar.

32 / 14.6959 ≈ 2.177 atm.

Those last two differ because bar ≠ atm. For a tire, bar is what European placards use; 2.2 bar is the usual rounding of 32 psi, not 2.2 atm.

Confirm 32 psi → kPa on the pressure converter. You want about 220.6. If you get 32, the target unit did not change. If you get 2.2 and you asked for kPa, you landed on bar and misread the label.

When you do not have a pressure yet

P = F / A. A 500 N force on 25 cm² (0.0025 m²) is 200000 Pa = 2 bar. That is pressure calculator, then optionally the converter. The formula write-up is how to calculate pressure from force and area. Gas-law work that needs kelvin as well is ideal gas.

Manometer mm of water is a different density and g problem (ρgh). Do not run mmH₂O through a torr factor and expect lab-grade agreement.

Blood-pressure mmHg uses the same millimetre-of-mercury factor as torr (~133.3 Pa). 120/80 mmHg is a clinical pair, not two industrial gauge readings you convert to bar for a compressor. Vacuum shops talk in torr or mbar below atmosphere; 1 × 10⁻³ torr is a good backing-pump vacuum, not “almost 1 atm.” Convert the number, then put it back in the sentence it came from.

Copy the number, the unit, and gauge/absolute if you know it. 2.21 in a shared sheet will become psi by Friday. The unit converters hub is the index if you arrived from mass or temperature and picked the wrong family.