Pressure Converter Guide: PSI, Bar, ATM, and Pascal

A tyre placard in the United States reads 35 psi, a European compressor gauge shows 2.4 bar, a weather report gives 1,013 hectopascals, and a blood pressure cuff reports 120 over 80 millimetres of mercury. All four describe force spread over an area, and moving between them takes one multiplication each.

The Base Unit and the Practical Ones

The SI unit is the pascal, one newton per square metre. It is tiny, so real work uses multiples: the kilopascal for engineering, the hectopascal for meteorology, and the megapascal for material strength. The bar was defined as exactly 100,000 pascals to sit close to atmospheric pressure, which is why 1 bar and 1 atmosphere are similar but not identical.

• 1 bar = 100,000 Pa = 100 kPa = 14.504 psi
• 1 atmosphere = 101,325 Pa = 1.01325 bar = 14.696 psi
• 1 psi = 6,894.76 Pa = 0.0689 bar
• 1 mmHg (torr) = 133.322 Pa, and 760 mmHg = 1 atm
• 1 inch of mercury = 3,386.4 Pa = 33.86 hPa

Conversions You Will Actually Use

A 35 psi tyre is 35 x 0.0689 = 2.41 bar, or 241 kPa. A 2.4 bar recommendation is 34.8 psi, close enough that the two placards agree. Standard sea-level pressure of 1,013 hPa is 29.92 inches of mercury, the number on an aviation altimeter setting. A blood pressure of 120 mmHg is 16.0 kPa, which is how it is reported in a few countries.

For rough field work, remember that bar to psi is roughly times 14.5, and psi to bar is roughly divide by 14.5. Since a 1% error on a tyre is about 0.35 psi, that shortcut is well within gauge accuracy.

Absolute, Gauge, and Differential

This distinction causes more confusion than any conversion factor. Gauge pressure, written psig or barg, measures above the surrounding atmosphere, and it is what a tyre gauge or a compressor dial reads. Absolute pressure, psia or bara, measures from a vacuum, so absolute equals gauge plus about 14.7 psi at sea level. A tyre at 35 psig holds 49.7 psia. Vacuum systems and thermodynamic calculations need absolute values; using gauge figures in the gas laws produces answers that are badly wrong at low pressures.

Differential pressure is the difference between two points, used for filter monitoring and airflow measurement. A clogged filter might show 250 Pa across it against 50 Pa when clean.

Everyday Reference Points

Temperature Changes Tyre Pressure

Gas pressure rises with temperature, roughly 1 psi for every 10 degrees Fahrenheit, or about 0.07 bar per 5 degrees Celsius. A tyre set to 35 psi on a 25 degree afternoon reads near 32 psi on a 5 degree morning, which is why winter triggers so many low-pressure warnings. Always check pressures cold, before driving, because a warm tyre after a motorway run can read 3 to 4 psi high and tempt you into letting air out.

Weather and Altitude

Meteorologists use hectopascals, which are numerically identical to the older millibars, so 1,013 hPa and 1,013 mb are the same reading. Pressure falls roughly 1 hPa for every 8 metres of altitude near sea level, which is how a barometric altimeter works and why a phone barometer can detect climbing a flight of stairs. Weather maps report pressure corrected to sea level so that stations at different heights can be compared.

Material Strength Uses Megapascals

Concrete compressive strength is quoted in MPa, with typical structural mixes at 25 to 40 MPa. Since 1 MPa equals 145 psi, a 30 MPa mix is about 4,350 psi. Steel yield strengths run several hundred MPa. The same unit describes pressure and stress because both are force per unit area, which is why a materials datasheet and a hydraulic spec can share notation.

Common Errors

Treating bar and atmospheres as identical introduces a 1.3% error, which matters in a calibration lab and not on a tyre. Mixing gauge and absolute in a gas law calculation is far worse. And confusing kPa with psi by a factor of about 7 has real safety consequences on pressure vessels, so always carry the unit next to the number rather than trusting memory.

Convert Pressure Units Free

Our pressure converter on SimpleCounter switches between psi, bar, kPa, MPa, atmospheres, mmHg, and inches of mercury in one field, so you can set a tyre from a foreign placard or read a European gauge without hunting for a factor. It is free, needs no signup, and works on a phone in the garage.