Water Pump Calculator
Home & ConstructionPump HP from flow and total head, with friction worked out by Hazen-Williams.
Runs entirely in your browser โ nothing is uploaded
Total head is three things added together, and sizing on the first alone is the intuitive mistake: the vertical lift, the friction in the pipe, and any pressure needed at the outlet. On a long or narrow run the friction can exceed the lift โ which is why a pump that looks generously sized still fails to deliver.
What people do next
Features
- Friction loss by Hazen-Williams, for the actual pipe size and material.
- Fittings counted as an equivalent length of straight pipe.
- Head broken down so you can see what is costing you power.
- Shows what the next pipe size up would save.
- Flags excessive pipe velocity and impossible suction lifts.
- Runs entirely in your browser โ nothing is uploaded.
How to use the Water Pump Calculator
- 1Enter the flow you need and the vertical lift.
- 2Give the pipe run, size and material.
- 3Set the residual pressure wanted at the outlet.
- 4Read the total head and the motor size to buy.
Frequently asked questions
How do I calculate the HP of a pump?
Power = ฯ ร g ร Q ร H รท efficiency, where Q is flow in cubic metres per second and H is total head in metres. Divide by 746 for horsepower. A domestic duty of 60 litres per minute against 43 m of total head at 50% efficiency comes to about 1.1 HP, so you buy a 1.5 HP pump โ motors only come in fixed sizes.
What is total head?
Everything the pump works against, added together: the vertical lift from water level to discharge, the friction losses in the pipe and fittings, and any residual pressure you want at the outlet. Sizing on the vertical lift alone is the common error, and it leaves a pump that runs continuously but never quite fills the tank.
Why does pipe size matter so much?
Because friction loss falls with close to the fifth power of diameter. Going from a 20 mm to a 25 mm pipe roughly halves the friction; going to 32 mm cuts it to a fifth. On a long run that difference can be several metres of head, which is often a whole motor size. One pipe size up is almost always cheaper than one motor size up, and the tool shows you the comparison.
How high can a pump suck water?
About 10 m in theory, since that is what atmospheric pressure supports, and roughly 7 m in practice once pipe friction and vapour pressure are allowed for. Beyond that a surface pump cavitates and stops drawing. The answer is a submersible pump, or moving the pump down closer to the water rather than buying a bigger one.
What flow rate do I need?
Work back from the job. A domestic tap runs about 10 litres per minute, a shower 8 to 12. To fill a 1,000 litre overhead tank in twenty minutes you need 50 lpm. Sizing for far more flow than you use means a pump that short-cycles, which wears the motor faster than running does.