Pressure Units: PSI, Bar, Pascals, and Head

Overview

Pressure is force per unit area. In SI, the pascal (Pa) is one newton per square meter; kilopascals and megapascals are more practical on drawings. Elsewhere you will see pounds per square inch (psi), bar, atmospheres, inches of water column, and millimeters of mercury—each common in a niche.

Hydraulics adds another layer: pressure can be expressed as head—the height of a fluid column that would produce that pressure. Head depends on fluid density, so “feet of water” and “feet of oil” are not interchangeable. Gauge pressure is relative to atmosphere; absolute pressure includes atmospheric pressure. Confusing the two is a frequent source of compressor and vacuum errors.

Cylinder force, pipe rating, and relief-valve setpoints all depend on correct pressure units. A factor-of-14.5 mix-up between bar and psi is large enough to damage equipment or fail a safety review.

Practice conversions with Dockzio’s engineering unit calculator, explore force from pressure and area in the hydraulic force calculator, and keep pipe flow context in mind when pressure drop and velocity interact.

Step-by-step

1. 1. State whether pressure is gauge or absolute

   Mark values as psig/psía or barg/bara. Atmospheric pressure is about 14.7 psi or 101 kPa at sea level. Vacuum readings are often gauge values below atmosphere—know the instrument convention before converting.

2. 2. Convert using reliable factors

   Useful anchors: 1 bar ≈ 14.5 psi ≈ 100 kPa; 1 atm ≈ 14.7 psi ≈ 101.325 kPa; 1 psi ≈ 6.895 kPa. Memorize a few, verify the rest with a conversion tool when safety margins are thin.

3. 3. Relate pressure to hydraulic force

   Force = pressure × area (with consistent units). Cylinder bore area mistakes—using diameter instead of radius, or mixing inches and millimeters—propagate directly into wrong force estimates.

4. 4. Convert pressure to fluid head when needed

   Head = pressure ÷ (fluid density × gravity) in coherent units. For water near standard conditions, rules of thumb exist (for example, about 2.31 feet of water per psi), but density changes with temperature and fluid type.

5. 5. Connect pressure drop to flow context

   In piping, friction losses grow with velocity and length. A pressure unit conversion alone does not tell you whether a line is oversized; you still need flow, diameter, roughness, and fluid properties.

Common mistakes

• Mixing gauge and absolute. Using psig where an absolute formula expects psia (or the reverse) shifts results by roughly one atmosphere—enough to ruin vacuum and compression calculations.
• Assuming all ‘head’ is water head. Head is fluid-specific. Oil, glycol mixes, and hot water change the pressure equivalent of a given column height.
• Confusing bar and atm. They are close but not identical (1 bar = 100 kPa; 1 atm = 101.325 kPa). Precision work should not treat them as the same.
• Ignoring elevation and atmosphere. Site altitude changes ambient pressure. Equipment sized at sea-level atmosphere may behave differently in the mountains.