How to Calculate Pressure
Pressure is defined as the force applied per unit area. It's one of the most practical physics concepts, appearing in weather forecasting, pneumatic systems, deep-sea diving, and blood pressure measurements.
The Formula
Pressure = Force / Area
P = F / A
SI unit: Pascal (Pa) = 1 Newton per square meter (N/m²)
Step-by-Step Example
A 100 kg person stands on a flat surface with shoe contact area of 400 cm² (0.04 m²):
Force = mass × gravity = 100 × 9.81 = 981 N Pressure = 981 / 0.04 = 24,525 Pa ≈ 24.5 kPa
Common Pressure Units and Conversions
| Unit | Equivalent |
|---|---|
| 1 atm | 101,325 Pa = 14.696 PSI = 1.01325 bar |
| 1 bar | 100,000 Pa = 14.504 PSI |
| 1 PSI | 6,894.76 Pa = 0.0689 bar |
| 1 kPa | 1,000 Pa = 0.145 PSI |
Pascal's Law
Pascal's Law states that pressure applied to a confined fluid is transmitted equally in all directions. This is the principle behind hydraulic systems:
F₂ = F₁ × (A₂ / A₁)
A car jack with a 5 cm² input piston and 500 cm² output piston: Force multiplication = 500/5 = 100×. A 50 N input force lifts 5,000 N.
Absolute vs. Gauge Pressure
- Absolute pressure: Measured relative to a perfect vacuum
- Gauge pressure: Measured relative to atmospheric pressure
Tire pressure is gauge pressure: a reading of "35 PSI" means 35 PSI above atmospheric, or 49.7 PSI absolute.
Use our pressure calculator for any force and area.