Pressure in Engineering — And Why It Gets Confusing
Pressure seems simple: force per unit area. But between psi, bar, atm, Pascals, inches of mercury, feet of water head, and absolute vs gauge — it’s one of the most unit-confused concepts in engineering.
The Basics
P = F / A
1 psi = 1 pound of force distributed over 1 square inch. That’s it. Everything else is just conversion factors.
Common Units (and When You’ll See Them)
| Unit | Where You’ll See It | = 1 atm |
|---|---|---|
| psi | US mechanical, hydraulics, tires | 14.696 |
| bar | European industry, scuba | 1.01325 |
| Pa / kPa / MPa | SI standard, structural engineering | 101,325 Pa |
| atm | Chemistry, diving, weather | 1.000 |
| inHg | Aviation, weather, HVAC | 29.92 |
| ft H₂O | Pumps, water systems | 33.9 |
Gauge vs. Absolute
Gauge pressure (psig) is relative to atmospheric pressure. Your tire reads 35 psig — that’s 35 psi above atmospheric.
Absolute pressure (psia) includes atmospheric. That same tire is at 35 + 14.7 = 49.7 psia.
This matters critically in thermodynamics and vacuum systems. A “perfect vacuum” is 0 psia = -14.7 psig.
Bernoulli’s Principle: Pressure vs. Velocity
Here’s where pressure gets really interesting in fluids. Bernoulli’s equation states that along a streamline of incompressible flow:
P + ½ρv² + ρgh = constant
In words: static pressure + dynamic pressure + hydrostatic pressure = constant.
The implication: when fluid velocity increases, pressure decreases (and vice versa). This is why:
- Airplane wings generate lift — faster air over the curved top surface = lower pressure above the wing
- A garden hose nozzle increases velocity — the constriction trades pressure for speed
- Venturi meters measure flow — measure the pressure drop at a constriction to calculate velocity
- Carburetors mix fuel — the venturi effect draws fuel into the airstream
Example: Pipe Flow
Water flows through a 2″ pipe at 5 ft/s and enters a 1″ restriction. What happens?
- By continuity (A₁v₁ = A₂v₂): v₂ = v₁ × (2/1)² = 5 × 4 = 20 ft/s
- Velocity quadrupled, so dynamic pressure increased by 16×
- That energy came from static pressure — so pressure DROPS at the restriction
- ΔP = ½ρ(v₂² – v₁²) = ½ × 1.94 × (400 – 25) = 364 lbf/ft² ≈ 2.5 psi drop
Hydrostatic Pressure
Still water creates pressure proportional to depth:
P = ρgh
For water: every 2.31 feet of depth adds 1 psi. A 100-foot water tower provides 43 psi at its base — no pump needed.
Key Takeaways
- Pressure = force / area — that’s all it is
- Always clarify gauge vs absolute when it matters (thermodynamics, vacuum, altitude)
- In moving fluids, pressure and velocity trade off (Bernoulli) — faster flow = lower pressure
- Use our unit converter if the pressure units are driving you crazy — that’s what it’s there for