Understanding Pressure Units: psi, Bar and Pascal

Pressure measures how much force is applied over a given area, and it shows up under several different unit names depending on the context: psi on a tire gauge, bar on a weather map, pascals in physics, atmospheres in chemistry.

What Is Pressure?

Pressure is force distributed over an area: the same force pressing on a smaller area produces higher pressure than it would spread across a larger one.

Common Pressure Units

• Pascal (Pa): the SI unit of pressure, one newton per square meter.
• Bar: a metric unit commonly used for weather and industrial pressure, equal to 100,000 Pa.
• psi (pounds per square inch): common in the US for tire pressure and plumbing.
• Atmosphere (atm): based on average sea-level air pressure, used in chemistry and physics.

Worked Example: Tire Pressure

Gauge Pressure vs Absolute Pressure

Most everyday pressure readings, like a tire gauge, measure gauge pressure: pressure relative to normal atmospheric pressure, not relative to a true vacuum. Absolute pressure includes atmospheric pressure in the total. A tire reading “0 psi” on a gauge isn’t actually a vacuum, it’s just at normal atmospheric pressure; its absolute pressure is around 14.7 psi.

Common Mistakes

• Confusing gauge and absolute pressure. Most consumer gauges read gauge pressure, but some scientific and industrial contexts require absolute pressure, and mixing the two up produces a result off by roughly one atmosphere.
• Assuming psi and bar are close enough to round casually. They differ by roughly a factor of 14.5, easy to get wrong if you drop the conversion factor entirely.
• Ignoring temperature effects. Gas pressure changes with temperature, so a pressure reading taken cold versus warm (like a tire in winter versus after driving) can differ meaningfully even with no air added or removed.

Use the Pressure Converter

The Pressure Converter converts between psi, bar, kPa, atm, and more instantly.