What does PSI stand for? Nine times out of ten, the letters mean pounds per square inch, the standard US unit of pressure. It tells you how many pounds of force push on each square inch of a surface. You see it on tire sidewalls, air compressors, pressure washers, water heaters and gauges of every kind.
A tire at 35 PSI has air inside pushing outward with 35 pounds of force on every square inch. The same letters also name the Greek letter psi and a few terms in science and psychology, covered further down.
Quick facts
Stands for
Pounds per square inchType
Initialism, used as a unit symbolHow to say it
P S IWritten as
PSI or psi, both common; also lbf/in²Metric match
1 psi is about 6.9 kilopascalsSea level air
About 14.7 psiVariants
psig (gauge), psia (absolute)
What does PSI stand for in pressure?
The three letters break down this way:
- P for pounds, meaning pounds of force
- S for square
- I for inch
Pressure is force spread over an area. If you press 10 pounds of weight evenly over 1 square inch, that is 10 psi. Spread the same 10 pounds over 10 square inches and it drops to 1 psi. That is why a sharp heel can dent a floor that a flat shoe never would: the same weight sits on a much smaller area.
PSI in tires and cars
Tire pressure is where most people meet the unit. The right number for your car is printed on a sticker inside the driver's door frame or in the owner's manual. The number molded into the tire sidewall is the maximum the tire can hold, not the target.
On a door frame sticker: "Front 35 PSI, Rear 33 PSI, cold." Fill the front tires to 35 and the rear to 33, measured before driving, when the tires are cool.
Tire pressure drops a little as the air gets colder, which is why a warning light often comes on during the first cold mornings of fall. Low pressure wears tires faster and hurts gas mileage, so a quick check once a month helps.
PSI around the house and the shop
- Water pressure: home plumbing often runs somewhere around 40 to 80 psi. Very high readings can strain pipes and fixtures.
- Air compressors: tools list the psi they need, and the compressor must keep up.
- Pressure washers: ratings in the thousands of psi show how hard the spray hits.
- Bike tires: narrow road tires run at much higher psi than wide mountain tires.
- Sports balls: footballs, basketballs and soccer balls each have a set inflation range in psi.
For other measurements used around machines and homes, see RPM and BTU.
PSIG vs PSIA, and metric conversions
Gauges usually read psig, pounds per square inch gauge, which is pressure above the air around you. A flat tire reads 0 psig even though air at sea level is pressing on it at about 14.7 psi. Psia, absolute pressure, counts from a perfect vacuum, so psia equals psig plus the air pressure around you.
| Unit | About equal to |
|---|---|
| 1 psi | 6.895 kilopascals (kPa) |
| 1 bar | 14.5 psi |
| 1 standard atmosphere | 14.7 psi |
| 100 kPa | 14.5 psi |
Car manuals outside the US often list tire pressure in kPa or bar, so the same tire might read 240 kPa, 2.4 bar or about 35 psi.
Other meanings of PSI
| Field | Meaning | Example |
|---|---|---|
| Greek alphabet | Psi, the letter Ψ | Used in math, physics and as a symbol of psychology |
| Parapsychology | Psi, a label for claimed mind powers | "Psi research" on telepathy |
| Air quality (some countries) | Pollutant Standards Index | A daily haze reading |
The Greek letter is said "sigh" or "psy," not as three letters. So if someone says "P S I," they almost always mean pressure.
How to write psi
Both PSI and psi are accepted. Technical writers often prefer lowercase psi as a unit symbol, while signs and labels use capitals. Do not add an s for plural: write "32 psi," not "32 psis." Leave a space after the number, as in 35 psi. Because each letter is read aloud, psi is an initialism. The famous acronyms hub collects more everyday short forms.
Questions people ask
What does PSI stand for on a tire?
It means pounds per square inch, the pressure of the air inside the tire. The target pressure for your car is on the driver's door frame sticker, while the sidewall number is the tire's maximum.
What does PSI stand for in pressure washers?
The same thing, pounds per square inch. A higher psi rating means the water stream hits harder, which helps on tough stains but can damage soft wood or paint.
Is PSI capitalized?
Either form is fine. Labels and signs often use PSI, while technical writing often uses lowercase psi as a unit symbol.
How many psi is one bar?
One bar is about 14.5 psi. One standard atmosphere, the air pressure at sea level, is about 14.7 psi.
What is the difference between psi and psig?
Psig is gauge pressure, measured above the surrounding air. Plain psi is often used to mean psig in daily life, while psia counts from a vacuum.