Pressure unit definitions

The pascal is one newton of force per square meter. One kilopascal is 1,000 pascals, one bar is exactly 100,000 pascals, and one standard atmosphere is exactly 101,325 pascals.

psi and mmHg

The pound per square inch is widely used for tires and equipment in the United States. This tool uses 1 psi = 6,894.757293168 pascals. A conventional millimeter of mercury is approximately 133.322387415 pascals.

Gauge and absolute pressure

Unit conversion does not change the reference point. Gauge pressure is measured relative to local atmospheric pressure, while absolute pressure is measured relative to a vacuum. Confirm which kind a specification or instrument reports.

Work a tire-style example

To convert 35 psi, multiply by 6,894.757293168 Pa per psi. The result is about 241,316.5 Pa, or 241.3 kPa. Dividing 241.3 kPa by 6.894757 gives approximately 35 psi again. That reverse calculation is a useful transcription check, but it does not tell you whether 35 psi is appropriate. Tire placards generally state gauge pressure, while some technical calculations require absolute pressure.

Keep the measurement context

Record the unit, pressure reference, medium, temperature if relevant, and instrument precision with the result. Converting a gauge reading to another unit does not convert it to absolute pressure; that requires local atmospheric pressure. Likewise, mmHg and Torr may be treated differently in precise work even though ordinary tables often show similar values. Compare critical factors with the NIST pressure conversion table, then follow the equipment’s specified tolerance and procedure.

Do not use a unit conversion alone to set safety-critical, medical, diving, industrial, or pressure-vessel limits. Follow the applicable equipment documentation and standards.