What Is PSI?
PSI (pounds per square inch) is a unit of pressure widely used in the United States and UK for measuring gas pressure, hydraulic systems, tire pressure, and material strength. One PSI equals one pound-force applied over one square inch of area. It is part of the Imperial/US customary system and relates to the SI unit Pascal by the factor 1 PSI = 6894.757 Pa.
PSI comes in two variants: psia (absolute) measures pressure relative to perfect vacuum, while psig (gauge) measures pressure relative to atmospheric pressure. Standard atmospheric pressure is 14.696 psia or 0 psig. Most everyday pressure readings (tire pressure, blood pressure cuffs, compressor gauges) use psig unless stated otherwise. Understanding this distinction is critical for accurate engineering calculations and safety.
PSI Formula
Pressure Conversion Table
| PSI | kPa | Bar | atm | mmHg |
|---|---|---|---|---|
| 1 | 6.895 | 0.0689 | 0.0680 | 51.72 |
| 14.7 | 101.3 | 1.013 | 1.000 | 760 |
| 30 | 206.8 | 2.069 | 2.041 | 1551 |
| 100 | 689.5 | 6.895 | 6.805 | 5172 |
| 3000 | 20,684 | 206.8 | 204.1 | 155,145 |
Common Applications
- Tire pressure: Passenger cars typically 30-35 psi, trucks 80-100 psi, bicycles 80-130 psi.
- Hydraulic systems: Industrial hydraulics operate at 1000-5000 psi, aircraft at up to 5000 psi.
- Scuba tanks: Filled to 2250-3500 psi depending on tank type.
- Water pressure: Residential water systems typically 40-80 psi.
Frequently Asked Questions
What is the difference between PSI and PSIA?
PSI typically means psig (gauge pressure) measured relative to atmospheric pressure. PSIA (absolute) includes atmospheric pressure. At sea level, PSIA = psig + 14.696. This matters for gas law calculations (Boyle's law, ideal gas law) which require absolute pressure.
How much PSI can the human body withstand?
Normal atmospheric pressure is 14.7 psi. Humans can tolerate pressure changes of a few psi in everyday activities. Scuba divers experience about 14.7 additional psi per 33 feet of water depth. Rapid pressure changes of 5+ psi can cause barotrauma. Material failure in pressurized systems is far more dangerous than the pressure itself.
Why is PSI still used instead of Pascals?
PSI remains dominant in US engineering, manufacturing, and consumer products due to established standards, equipment calibration, safety regulations, and industry familiarity. Many pressure vessels, pipes, and fittings are rated in PSI. However, scientific work and international engineering increasingly use kPa or bar.