What Is PSI? Pound-Force Per Square Inch Explained

5 min read•June 1, 2026•Reviewed: June 1, 2026

✓Reviewed by Apex Conversion Editorial Team · Last reviewed June 1, 2026

PSI stands for pounds per square inch — the unit of pressure most commonly used in the United States for everyday tasks. Tire inflation, water supply pressure, compressed air tools, and hydraulic equipment are all rated in PSI in the US. Understanding what PSI means and how it relates to metric pressure units (kPa and bar) is essential for working with international specifications, European equipment, or any context that mixes measurement systems.

One PSI is the pressure exerted by one pound of force distributed over one square inch of surface area. It is a larger unit than the metric pascal: 1 PSI = 6.895 kPa = 6,894.76 Pa.

Definition and Formula

PSI is defined as: Pressure (PSI) = Force (pounds-force) ÷ Area (square inches). One pound-force is the gravitational force on a one-pound mass at Earth's surface. One square inch is a 1-inch by 1-inch area. In SI terms, 1 PSI = 6,894.76 pascals (Pa) = 6.89476 kPa. Standard atmospheric pressure at sea level is 14.696 PSI (101.325 kPa).

Gauge pressure vs absolute pressure: when you inflate a tire to '32 PSI,' that is gauge pressure — measured relative to the surrounding atmosphere, not a vacuum. The absolute pressure inside the tire is about 32 + 14.7 = 46.7 PSI. Most pressure gauges and specifications use gauge pressure (often written 'PSIG'), while vacuum-referenced absolute pressures are written 'PSIA'.

PSI Conversion Formulas

PSI → kPa:  kPa = PSI × 6.89476
kPa → PSI:  PSI = kPa ÷ 6.89476
PSI → bar:  bar = PSI ÷ 14.5038
bar → PSI:  PSI = bar × 14.5038
PSI → atm:  atm = PSI ÷ 14.696
atm → PSI:  PSI = atm × 14.696

Common tire pressure:
  30 PSI = 206.8 kPa = 2.07 bar
  35 PSI = 241.3 kPa = 2.41 bar

Where You Encounter PSI

Tire pressure is the most common everyday use. Passenger car tires typically call for 30–35 PSI; truck tires can require 80–100 PSI. Residential water pressure runs 40–80 PSI — below 40 PSI causes weak shower flow, above 80 PSI risks pipe damage. Workshop air compressors operate at 90–150 PSI. Hydraulic systems range from a few hundred PSI in automotive power steering to tens of thousands of PSI in industrial presses.

Fire hoses deliver water at 100–200 PSI at the nozzle. Scuba tanks are pressurized to 3,000 PSI (about 207 bar). A typical espresso machine forces water through grounds at 9 bar ≈ 130 PSI — hence 'nine-bar pressure' as a quality standard in espresso culture. Natural gas home delivery pressure is much lower: roughly 0.25 PSI at the meter.

PSI Quick Reference

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Car tire pressure: check the sticker inside your driver's door, not the tire sidewall — the sidewall shows maximum rated pressure, not the recommended operating pressure.
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Multiply PSI by 7 for a quick kPa estimate (30 PSI × 7 = 210 kPa; actual is 206.8 — within 1.5%).
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Always measure tire pressure when cold: tires heat up during driving, raising pressure by 4–8 PSI, which will give a falsely high reading.

Frequently Asked Questions

Is PSI used outside the United States?

PSI is primarily used in the US, UK, and Canada, and in some international industrial specifications. Most other countries use kPa, bar, or MPa. European automotive specs list tire pressure in bar (e.g., 2.3 bar = 33.4 PSI). Scientific contexts worldwide always use pascal-derived units.

What is the difference between PSI and PSIG?

PSIG (PSI gauge) is measured relative to atmospheric pressure. PSIA (PSI absolute) is measured from a perfect vacuum. In everyday use, 'PSI' almost always means gauge pressure. Atmospheric pressure at sea level is 14.696 PSIA = 0 PSIG.

What household water pressure is considered normal?

40–60 PSI is considered normal for residential water supply. Most building codes require a minimum of 20 PSI at fixtures. Pressure above 80 PSI can damage appliances and pipes and typically requires a pressure-reducing valve.

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