Head: how high can it push?
Why pump engineers measure pressure in metres — and why a pump lifts petrol exactly as high as water.
⏱ 6 min
- Everyday comparisonStep 1 of 5
Throw a ball straight up
Throw a ball straight up. The faster it leaves your hand, the higher it goes. A tennis ball and a heavier cricket ball, thrown at the same speed, reach the same height.
A pump does the same with liquid. The height it can push the liquid up to is called its .
- Try itStep 2 of 5
Fill the pipe
The pump pushes liquid up a tall pipe that is closed at the top. Speed the pump up and watch how high the liquid climbs — next to a building, for scale.
Then switch the liquid. Watch the height and the pressure gauge separately.
Challenge: Pick the full speed, then switch between water, petrol and brine. Which number changes, the height or the pressure?
- ExplainedStep 3 of 5
Same height, different pressure
The impeller gives every kilogram of liquid the same energy — it depends only on the speed of the vanes. So every liquid rises to the same height.
But a column of heavy brine pushes down harder than a column of light petrol. So the pressure is different: pressure = [[density]] × g × head.
- ExplainedStep 4 of 5
Why engineers love metres
If a pump’s strength were given in bar, you would need a new number for every liquid. In metres of head, one number works for all of them.
For water, a handy rule: 10 metres of head ≈ 1 [[bar]]. A pump with 30 m of head gives cold water about 3 bar.
- The impeller connectionStep 5 of 5
Reading a pump label
A label that says H = 32 m means the pump can lift water 32 m high when nothing is flowing. When water does flow, the pipes eat part of that head as friction, so it delivers a bit less.
Each storey of a building is about 3 m. So that pump could (just about) reach the top of a 10-storey building.
📌 Remember this
Head is how high a pump can push a liquid, in metres. It is the same for every liquid; the pressure it makes depends on the liquid’s density. For water, 10 m ≈ 1 bar.