What pressure is
Pressure is how concentrated a force is. The same push spread over a large area makes a low pressure; squeezed onto a small area it makes a high one. That's why you can lie on a bed of many nails but not stand on one, and why snowshoes, tank treads and wide tires spread weight out.
The SI unit is the pascal (Pa), one newton per square meter. Because a pascal is tiny, everyday pressures are usually given in kilopascals (kPa), bar, atmospheres (atm) or, in the US, pounds per square inch (psi).
The formula
F = P × A
A = F ÷ P
1 atm = 101,325 Pa 1 bar = 100,000 Pa 1 psi = 6,894.757 Pa
Convert areas carefully: 1 cm² = 0.0001 m², 1 mm² = 0.000001 m², 1 in² = 0.00064516 m² and 1 ft² = 0.09290304 m². Squaring the unit squares the conversion factor, which is the most common source of mistakes.
Example: 1,000 N on 25 cm²
A force of 1,000 N (roughly the weight of a 102 kg mass) presses on an area of 25 cm² (the default values).
| Step | Result |
|---|---|
| Area in m²: 25 × 0.0001 | 0.0025 m² |
| P = 1,000 ÷ 0.0025 | 400,000 Pa |
| In kPa: 400,000 ÷ 1,000 | 400 kPa |
| In psi: 400,000 ÷ 6,894.757 | 58.02 psi |
| In bar: 400,000 ÷ 100,000 | 4 bar |
| In atm: 400,000 ÷ 101,325 | 3.948 atm |
Everyday pressures
| Situation | Approximate pressure |
|---|---|
| Sea-level air (standard atmosphere) | 101.3 kPa, 14.7 psi |
| Car tire (gauge) | 210–250 kPa, 30–36 psi |
| Road bike tire (gauge) | 550–700 kPa, 80–100 psi |
Common mistakes
- Converting area like length. 1 cm = 0.01 m, but 1 cm² = 0.0001 m². Forgetting to square gives answers 100 times too small.
- Using mass instead of force. A 70 kg person presses with about 686 N (70 × 9.80665), not 70 N.
- Mixing gauge and absolute pressure. Tire and pump gauges read pressure above the air around them. Add 101.325 kPa (14.7 psi) for absolute pressure.
- Using the whole object's area. Only the contact area counts: the footprint of a tire, not its whole surface.
Frequently asked questions
What is the formula for pressure?
Pressure is force divided by the area it acts on: P = F ÷ A. With force in newtons and area in square meters, the answer is in pascals (Pa). One pascal is one newton per square meter, a very small pressure.
How do I convert psi to kPa or bar?
1 psi is 6,894.757 Pa, or about 6.895 kPa. 1 bar is exactly 100,000 Pa (100 kPa), which is about 14.5 psi. So a tire at 32 psi is about 221 kPa or 2.21 bar.
What is 1 atmosphere in other units?
Standard atmospheric pressure is defined as exactly 101,325 Pa. That's 101.325 kPa, 1.01325 bar or about 14.70 psi. It's roughly the air pressure at sea level.
Why do sharp knives cut better?
A sharp edge has a tiny contact area, so the same push creates a much higher pressure. Halving the area doubles the pressure. The same idea explains why stiletto heels dent floors and why snowshoes keep you from sinking.
Is tire pressure absolute or gauge pressure?
Tire gauges read gauge pressure, the pressure above the surrounding air. Absolute pressure is gauge pressure plus atmospheric pressure, so a tire at 32 psi gauge is about 46.7 psi absolute at sea level. This calculator works with whatever force you enter; it doesn't add atmospheric pressure.
Rates and figures last checked October 2026.
This calculator gives estimates for planning. Check current rates and product labels before you buy, list or file.