Understanding Pounds per Square Inch (PSI)
Pressure is defined as force distributed over an area: P = F / A. Pounds per square inch (psi) is the US customary unit of pressure, equal to one pound-force (lbf) acting on one square inch (in²) of surface. It is the standard unit for tire pressure, hydraulic and pneumatic systems, water and air pressure ratings, and pressure vessel specifications across the United States. This calculator applies the same pressure formula used in SI units (where pressure is measured in pascals, Pa = N/m²), just with US customary force and area units as the reference.
How the calculation works
Enter the force being applied and the area it acts over, choosing whichever units are convenient. The calculator first converts your force to pounds-force (lbf) and your area to square inches (in²), then divides force by area to get pressure in psi: psi = F(lbf) ÷ A(in²). From that psi value it also derives the equivalent pressure in kilopascals (× 6.894757), bar (× 0.0689476), and standard atmospheres (÷ 14.6959), so you can compare against equipment rated in metric units.
Common mistakes
- Mixing force and weight units: a "pound" of force (lbf) is not the same as a kilogram of mass — convert kgf to lbf (× 2.204623) before combining with other lbf values.
- Forgetting the area units match: psi is defined per square inch, not per square foot or square centimeter — always convert area to in² (or let the unit selector do it) before dividing.
- Confusing gauge and absolute pressure: a tire gauge reads psig (relative to atmosphere), while psia includes the roughly 14.7 psi of atmospheric pressure already pressing on the surface.
Real-world applications
- Tire inflation specs for cars, bicycles, and trucks are given in psi, typically 30-35 psi for passenger vehicles.
- Hydraulic systems, pressure washers, and air compressors are rated by maximum output pressure in psi.
- Plumbing and irrigation systems specify water line pressure in psi to size pipes, valves, and fittings correctly.
- Pressure vessels, boilers, and hoses list a maximum working pressure in psi that must never be exceeded for safe operation.