Pressure Units Explained: PSI, Bar, Pascal and Why Gauge Is Not Absolute

Pressure has more competing units in daily use than almost any other quantity, and it has a trap the others lack: the same physical pressure has two legitimate numerical values depending on the reference point you measure from.

The units

• Pascal (Pa) — the SI unit, one newton per square metre. It is a very small unit; atmospheric pressure is about 101,325 Pa.
• Bar — 100,000 Pa, chosen to be close to atmospheric pressure. Common in European engineering and diving.
• PSI — pounds-force per square inch, 6,894.76 Pa. Standard in the US for tyres, hydraulics and compressed air.
• Atmosphere (atm) — defined as exactly 101,325 Pa.
• Torr / mmHg — about 133.3 Pa, from mercury-column manometers. Still standard in medicine and vacuum work.
• inHg — inches of mercury, used in aviation altimetry and US weather reporting.
• hPa / millibar — identical to each other, standard in meteorology.

Handy anchors: 1 bar ≈ 14.5 PSI, and standard atmospheric pressure ≈ 14.7 PSI ≈ 1013 hPa.

Gauge versus absolute

This is the part that matters more than any conversion factor.

Absolute pressure is measured from a perfect vacuum. Gauge pressure is measured from ambient atmospheric pressure.

The relationship is straightforward:

absolute = gauge + atmospheric

Notation matters: psig is gauge, psia is absolute. In metric, "bar g" and "bar a" serve the same purpose. Where a document does not specify, gauge is usually implied for pressures above atmospheric, but "usually" is doing dangerous work in that sentence.

Which one you need depends on the physics. Gas law calculations require absolute pressure — using gauge in the ideal gas law produces badly wrong answers. Structural calculations usually want gauge, since that is the net load on a vessel wall.

Why weather uses hectopascals

A hectopascal is exactly one millibar, so the switch from millibars was purely cosmetic and meteorologists changed nothing but the label.

Standard sea-level pressure is 1013.25 hPa. Below about 1000 hPa suggests unsettled weather; above 1020 hPa suggests settled conditions. Strong hurricanes drop below 950 hPa, and the lowest reliably recorded sea-level pressure, in Typhoon Tip in 1979, was 870 hPa.

Aviation uses inches of mercury in North America and hectopascals nearly everywhere else, which means pilots crossing certain boundaries change altimeter units mid-flight.

Blood pressure and mmHg

Medicine has kept mercury millimetres despite mercury sphygmomanometers largely disappearing, because the entire clinical evidence base — every threshold, every trial — is expressed in mmHg. Changing units would mean re-anchoring a century of medical literature for no clinical gain.

A reading of 120/80 mmHg is about 16.0/10.7 kPa, a number no clinician uses.

Diving: where errors are dangerous

Water is dense enough that pressure rises fast: roughly 1 bar for every 10 metres of seawater.

• Surface: 1 bar absolute
• 10 m: 2 bar absolute
• 30 m: 4 bar absolute

At 30 metres a diver breathes gas four times as dense as at the surface, which is why air consumption rises steeply with depth and why gas planning uses absolute pressure throughout.

Tyre pressure specifics

Always inflate to the figure on the door placard, not the maximum moulded into the sidewall — the sidewall number is a limit, not a recommendation.

Check cold. Driving heats tyres and raises pressure by several PSI. And pressure falls with ambient temperature at roughly 1 PSI per 10 °F, which is why warning lights appear on the first cold morning of autumn.

Vacuum

Below atmospheric, conventions multiply. Vacuum may be quoted as absolute pressure in torr or pascals, or as "inches of mercury vacuum" measured downward from atmospheric. A reading of "25 inHg vacuum" means about 5 inHg absolute. Always establish the reference before converting.

Our pressure converter handles all of these units, but it cannot know your reference point — establishing gauge versus absolute is on you.