Fluid Mechanics

Pressure vs Head Explained

Pressure is force per area; head is equivalent fluid column height. Learn how density connects them and when to use each.

TypeEngineering guide — concept explainer

Definition

Pressure is force per unit area (Pa, psi, bar). Head is the equivalent height of a fluid column that would exert that pressure (m, ft). They are related through the fluid density: P = ρgh. For a given pressure, a denser fluid has a shorter equivalent head, and a lighter fluid has a taller equivalent head.

Why it matters

Many engineering systems express measurements in both pressure and head. Pump curves use head; pressure gauges read pressure. Converting between them is a daily task for process engineers, and getting the density wrong is the most common source of error. Understanding the distinction also prevents confusion when comparing systems that handle different fluids.

Formula

Pressure from head

P = ρ × g × h

Head from pressure

h = P / (ρ × g)

Units involved

•P — pressure in pascals (Pa), kilopascals (kPa), bar, or psi
•h — head in metres (m) or feet (ft)
•ρ — fluid density in kg/m³ or lb/ft³
•g — gravitational acceleration, 9.80665 m/s²

Concept diagram

Worked example

A gauge reads 200 kPa. What is the equivalent head for water (998 kg/m³) and for a brine solution (1200 kg/m³)?

01P = 200,000 Pa
02For water: h = 200,000 / (998 × 9.80665) = 20.44 m
03For brine: h = 200,000 / (1200 × 9.80665) = 17.00 m

Result

200 kPa = 20.44 m of water head = 17.00 m of brine head

Common mistakes

•Assuming 1 m of head always equals 9.81 kPa — this is only true for water at standard density.
•Converting head to pressure without specifying the fluid density.
•Mixing absolute and gauge pressure without stating the reference — this guide does not cover absolute/gauge conversion.
•Using the wrong gravitational constant — standard gravity is 9.80665 m/s², not 10 m/s².

When to use the calculator

Use the Head to Pressure calculator when you need to convert between head and pressure for a specific fluid density. Enter the head (or pressure), the fluid density, and the calculator returns the other value with unit options.

Head to Pressure Calculator →

FAQ

Is 1 metre of head always equal to about 10 kPa?

Only for water at approximately 1000 kg/m³. For other fluids, multiply the head by the actual density and g. For example, 1 m of mercury (13,600 kg/m³) is about 133.3 kPa.

When should I use head instead of pressure?

Use head when working with pump curves, since head is independent of fluid density. Use pressure when specifying equipment pressure ratings, relief valve settings, or gauge readings.

How do I convert feet of water to psi?

At standard water density (62.4 lb/ft³): multiply feet of water by 0.4333 to get psi. For example, 100 ft of water ≈ 43.3 psi. Use the Head to Pressure calculator for exact values with different densities.

Does head change with altitude?

Head itself (as a height) does not change, but the relationship P = ρgh uses standard gravity. At significantly different altitudes, local gravity varies slightly, but this effect is negligible for most engineering applications.