PSI Calculator

Q: Why is one input box gray and locked?

The gray box is the answer box. When you pick what to solve for , that field turns gray and read-only so the calculator can fill it in. Change the dropdown and a different box becomes the output.

Q: How do I find the area of a round piston or pipe?

Use A = π × d² ÷ 4 , where d is the inside diameter. A 3 inch bore gives 7.07 in². The Hydraulic tab does this for you when you type the bore size.

Q: What does "Fill now to hit target later" mean on the Tire tab?

It is the pressure to put in your tires right now , at today's temperature, so they land on your target pressure when the weather changes. Handy before a cold night or a hot day.

Q: Why does my air hose lose so much pressure?

Three things steal psi: long hose , narrow hose , and fittings . Diameter matters most. Going from 1/4 inch to 3/8 inch can cut the drop by a lot. Doubling the flow raises the drop even more than double.

Q: What is the difference between psi and CFM?

PSI is push. CFM is how much air flows per minute. A tool needs both. Plenty of psi with too little CFM makes the tool slow down or stall while you use it.

PSI Calculator

Updated Aug 17, 2026 By Jehan Wadia

Introduction

PSI means "pounds per square inch." It tells you how much pressure is pushing on a surface. The math is simple: pressure equals force divided by area (P = F ÷ A). Push harder, and pressure goes up. Spread that push over a bigger area, and pressure goes down.

This PSI calculator does that math for you. Type in any two values, and it finds the third. You can solve for pressure, force, or area, and pick the units you like — psi, bar, kPa, MPa, atm, and more.

The tool has six modes:

Basic PSI — find pressure, force, or area.
Unit Converter — switch between psi, bar, kPa, Pa, atm, mmHg, and others. For non-pressure units, try the Unit Converter Calculator.
Tire Pressure — check your tires and see how cold or hot weather changes them.
Hydraulic — find the pressure a cylinder needs to lift a load.
Air Compressor — find pressure lost in a hose and what to set your regulator to.
Force & Area — get force or area when you already know the pressure. See also our Force Calculator and Area Calculator.

Every answer comes with step-by-step work, so you can see how the numbers were found. Charts and warnings help you spot values that look too high or too low. Use it for tires, air tools, hydraulic presses, or class homework.

How to use our PSI Calculator

Enter a few known values — like force and area, tire pressure, or hose size — and the PSI calculator gives you the pressure in psi, bar, kPa, and more, plus step-by-step math and charts.

Unit system: Pick Imperial (psi, lbf, in) or Metric (bar, N, mm). Every value you already typed changes to the new units. For length and weight swaps, the Metric Conversion Calculator helps too.

Tab choice: Pick the tab you need: Basic PSI, Unit Converter, Tire Pressure, Hydraulic, Air Compressor, or Force & Area.

Basic PSI tab

Solve for: Choose Pressure, Force, or Area. That box turns gray and shows the answer.

Quick preset: Pick a sample job, like a hydraulic press or espresso puck, to fill the boxes fast.

Force (F): Type the push or load, then pick the unit (lbf, N, kN, kgf, or tons).

Area (A): Type the surface the force presses on, then pick in², ft², cm², mm², or m². For round faces, the Circle Area Calculator gives you that number fast.

Pressure (P): Type the pressure if you are solving for force or area, then pick psi, bar, kPa, MPa, atm, and more.

Unit Converter tab

Any pressure box: Type a number in psi, bar, mbar, Pa, kPa, MPa, atm, mmHg, inHg, inH₂O, or kgf/cm². All the other boxes update right away.

Tire Pressure tab

Vehicle preset: Pick car, SUV, truck, motorcycle, road bike, or mountain bike to fill the target pressure. Cyclists can fine-tune numbers with the Bike Tire Pressure Calculator.

Current tire pressure: Type what your gauge reads now, in psi, bar, or kPa.

Recommended pressure: Type the number from the sticker on the driver's door. If you changed wheel or tire sizes, check the Tire Size Calculator first.

Current temperature: Type the air temperature now in °F, °C, or K. Need a quick swap? Use the Celsius to Fahrenheit Calculator.

Expected temperature: Type the temperature you expect later. The tool shows how much the pressure will rise or fall.

Hydraulic tab

Force / load: Type the load the cylinder must move, then pick the unit.

Cylinder bore diameter: Type the inside width of the cylinder in inches, mm, or cm.

Rod diameter: Type the rod width. This is only used for the rod-end area.

Working side: Pick cap end (full bore) for pushing or rod end (annular) for pulling.

Stroke length: Type how far the rod moves. Optional.

Extend time: Type the seconds it should take. With stroke length, this gives flow rate and rod speed. Compare pump numbers with the Flow Rate Calculator and the Pump Power Calculator.

Show pressure in: Pick psi, bar, kPa, MPa, or kgf/cm² for the answer.

Air Compressor tab

Tool preset: Pick a tool like a nailer, impact wrench, or spray gun to fill pressure and air flow.

Pressure required at the tool: Type the psi, bar, or kPa the tool needs.

Air flow required: Type the CFM, L/min, or m³/h the tool uses. The CFM Calculator helps size air movement for bigger jobs.

Hose length: Type how long the hose run is in feet or meters.

Hose inner diameter: Type the inside width of the hose in inches or mm. A 3/8" hose is 0.375 in.

Fittings / quick-connects: Type how many couplers are in the line. Each one adds pressure loss. For liquid lines, see the Friction Loss Calculator and the Pipe Flow Calculator.

Force & Area tab

Solve for: Choose Force (F = P × A) or Area (A = F ÷ P).

Pressure (P): Type the known pressure in psi, kPa, bar, or MPa.

Area (A): Type the area if you are solving for force.

Force (F): Type the force if you are solving for area.

Press Calculate to see your answer, the worked steps, and a chart. Press Reset to start over, or Copy result to save it.

What Is PSI?

PSI stands for pounds per square inch. It tells you how much force pushes on one square inch of space. It is the most used pressure unit in the United States for tires, air tools, hoses, pumps, and water lines.

The PSI Formula

Pressure is force spread over an area:

P = F ÷ A

P = pressure in psi
F = force in pounds (lbf)
A = area in square inches (in²)

You can flip the formula to find the other parts. F = P × A finds force. A = F ÷ P finds area. So 500 pounds pressing on 2.5 square inches makes 200 psi. The same 500 pounds on a smaller area makes a much higher pressure. That is why a sharp point can push into wood while a flat hand cannot. If you need the force itself from mass and acceleration, start with the Acceleration Calculator or the Mass Calculator.

Common Pressure Units

Other countries use bar, kilopascals, or megapascals. Here are quick conversions from 1 psi:

1 psi = 6,894.76 pascals (Pa)
1 psi = 6.895 kilopascals (kPa)
1 psi = 0.0689 bar
1 psi = 0.068 atmospheres (atm)
1 bar = 14.5 psi
1 atm = 14.7 psi (normal air pressure at sea level)

Gauge Pressure vs. Absolute Pressure

Most gauges show gauge pressure (psig). That means zero on the dial equals the air around you. Absolute pressure (psia) starts at a true vacuum. To get absolute, add 14.7 psi to the gauge reading. Gas laws, like the ones used for tire temperature changes, need absolute pressure to work right — see the Ideal Gas Law Calculator for the full P-V-T picture, or the Pressure Altitude Calculator when elevation matters.

Tire Pressure and Temperature

Air pressure changes when the temperature changes, even if no air leaks out. A good rule: tire pressure moves about 1 psi for every 10 °F. Cold mornings make tires read low. Highway driving heats them up and makes them read high. Always check tires when they are cold, and use the number on the sticker inside the driver's door, not the number on the tire wall. Our dedicated Tire Pressure Calculator covers load and season tweaks in more detail.

Typical PSI Ranges

Passenger car tires: 30–35 psi
Pickup trucks: 35–45 psi
Road bike tires: 80–130 psi
Mountain bike tires: 22–35 psi
Home water lines: 40–60 psi (see the Hydrostatic Pressure Calculator for pressure from water height)
Air tools: 70–90 psi
Hydraulic systems: 1,000–3,000 psi

Hydraulic and Air Systems

In a hydraulic cylinder, pressure works on the piston face. A bigger bore gives more area, so it needs less pressure to lift the same load. Push on the rod side and the rod takes up part of the area, so that side needs more pressure for the same force. When you turn that push into rotation, the Torque Calculator and Horsepower Calculator show what the system delivers.

With air tools, pressure drops as air travels down a hose. Long hoses, narrow hoses, and extra quick-connect fittings all steal psi. To fix it, set the regulator a bit higher, use a shorter hose, or move up to a wider hose such as 1/2 inch. Flow behavior in the line follows the same rules used in the Reynolds Number Calculator.

Why PSI Matters

Right pressure means better safety, better gas mileage, longer tire life, and tools that work at full power. Underinflated tires also cost you at the pump — check the difference with the Gas Mileage Calculator. Too little pressure causes soft tires, weak tools, and slow presses. Too much pressure can burst hoses, blow seals, or crack parts. Always stay inside the pressure rating printed on your equipment, and when you are fastening that equipment together, use the Bolt Torque Calculator to get clamp loads right.

Formulas used

Pressure from force and area

P = \frac{F}{A}

Pressure unit conversion (via pascals)

V_{target} = \frac{V_{source} \times k_{source}}{k_{target}}

Tire pressure vs. temperature (Gay-Lussac, absolute units)

P_{2} = \left(P_{1} + P_{atm}\right)\frac{T_{2}}{T_{1}} - P_{atm}

Fill pressure now to reach target at expected temperature

P_{fill} = \left(P_{rec} + P_{atm}\right)\frac{T_{1}}{T_{2}} - P_{atm}

Hydraulic cylinder areas (cap end and rod end)

A_{cap} = \frac{\pi}{4}D^2 \qquad A_{rod} = \frac{\pi}{4}\left(D^2 - d^2\right)

Required hydraulic flow rate and rod speed

Q = \frac{A \times S}{t} \qquad v = \frac{S}{t}

Compressed-air pressure drop and required supply pressure

\Delta p = \frac{7.57\, Q^{1.85} L_{eq} \times 10^{4}}{d^{5}\, p_{abs}} \qquad P_{supply} = P_{tool} + \Delta p

Equivalent hose length with fittings and air velocity

L_{eq} = L + n \times 150\, d \qquad v = \frac{Q}{\left(\frac{P_{tool}+P_{atm}}{P_{atm}}\right)\frac{\pi}{4}d^{2}}

Frequently asked questions

Why is one input box gray and locked?

The gray box is the answer box. When you pick what to solve for, that field turns gray and read-only so the calculator can fill it in. Change the dropdown and a different box becomes the output.

How do I find the area of a round piston or pipe?

Use A = π × d² ÷ 4, where d is the inside diameter. A 3 inch bore gives 7.07 in². The Hydraulic tab does this for you when you type the bore size.

Why does the rod end need more pressure than the cap end?

The rod takes up part of the piston face. That leaves a smaller ring, or annular, area for the oil to push on. Less area means more pressure is needed for the same force. That is why pulling takes more psi than pushing.

What does the error "Rod diameter must be smaller than the bore diameter" mean?

The rod slides inside the cylinder, so it must be thinner than the bore. Check that both numbers use the same unit. Mixing inches and mm is the most common cause.

What does "Fill now to hit target later" mean on the Tire tab?

It is the pressure to put in your tires right now, at today's temperature, so they land on your target pressure when the weather changes. Handy before a cold night or a hot day.

How does the tool decide if my tire is optimal, low, or high?

It compares your reading to the recommended pressure. If the gap is under about 2 psi (or 3% of the target, whichever is larger), it shows Optimal. Bigger gaps show Under-inflated or Over-inflated.

Why does the Tire tab add 14.7 psi to my numbers in the steps?

Gas laws only work with absolute pressure. Your gauge reads zero at normal air pressure, so the tool adds about 101,325 Pa (14.7 psi), does the temperature math, then takes it back out. That is why the answer is more exact than the "1 psi per 10 °F" rule.

Why does my air hose lose so much pressure?

Three things steal psi: long hose, narrow hose, and fittings. Diameter matters most. Going from 1/4 inch to 3/8 inch can cut the drop by a lot. Doubling the flow raises the drop even more than double.

How much pressure does each quick-connect cost me?

The tool treats each fitting like extra hose. It adds about 150 times the hose diameter in length per fitting. For a 3/8 inch hose that is roughly 4.7 feet of hose per coupler.

What is the difference between psi and CFM?

PSI is push. CFM is how much air flows per minute. A tool needs both. Plenty of psi with too little CFM makes the tool slow down or stall while you use it.

Why is my hose drop bigger when I raise the CFM?

Pressure loss grows faster than flow. The math uses flow raised to the power 1.85. So a small jump in CFM makes a big jump in lost psi. Use a wider hose when your tool needs lots of air.

Does the Hydraulic tab include pump or seal losses?

No. It gives the ideal pressure from force and area. Real systems lose some pressure to seal friction, hose drag, and valves. Add a safety margin of about 10% to 20% when sizing a pump.

What units does the flow rate result use?

The Hydraulic tab shows flow in GPM and L/min, and rod speed in in/s and mm/s. You need both stroke length and extend time filled in to see them.

What is inH₂O used for?

Inches of water measures very small pressures. It is used for furnace ducts, gas lines, fans, and filters. 1 psi equals about 27.7 inH₂O.

What is mmHg or Torr?

Millimeters of mercury, also called Torr. It is used for blood pressure, vacuum work, and weather. 1 psi equals about 51.7 mmHg, and normal air pressure is 760 mmHg.

Can I enter a vacuum or negative pressure?

No. The tool needs values above zero. For vacuum work, enter the pressure below atmosphere as a positive number, then subtract it from 14.7 psi to get absolute pressure.

Why do I see a warning about high pressure?

The tool flags results that are outside normal ranges. Over 5,000 psi it reminds you to check part ratings. Over 10,000 psi it warns you that special high-pressure gear is needed. Very low values usually mean the wrong area unit was picked.

Does switching to Metric change my answers?

No, the physics stays the same. The button just converts every number you typed into the matching metric unit. Values like 100 psi become 6.89 bar, and the results follow.

Can I save or share my results?

Yes. Click Copy result to put the answer on your clipboard, or Print results on the Basic tab to make a paper or PDF copy with the steps.

Why do bike tires use so much more psi than car tires?

Bike tires are tiny, so their contact patch is small. To hold up a rider on that small area, the air pressure must be much higher. Car tires are wide, so 35 psi is plenty.

Does altitude change my psi reading?

Your gauge reads pressure above the outside air, so a higher elevation makes a gauge read slightly high compared to sea level. The change is small, about 0.5 psi per 1,000 feet, and tire targets do not need changing.

Is this calculator good for school homework?

Yes. Every tab shows the full worked steps with the unit conversions, the formula, and the final answer. You can copy the steps to check your own work.

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