What Size Water Pump Do I Need?

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Pumps aren't sized in watts or horsepower first. They're sized by how much water they need to move and how high they need to push it, including the pipe's friction. Get those two numbers and the maker's chart does the rest.

1. The two numbers

• Flow: how much water per minute, in litres per minute (L/min).
• Total head: how hard the pump has to push, in metres. It's the height you lift the water, plus the friction in the pipe, plus any pressure you need at the outlet.

You then choose a pump whose performance curve gives at least your flow at your total head.

2. Step 1: the flow

Work it out one of two ways.

• You know the flow you want. A garden hose often runs somewhere around 10 to 20 L/min.
• You know the litres you need each day. For example, a stock trough or a tank top-up. Divide by the minutes of pumping. For a solar pump, the pumping hours are roughly the peak sun hours you get.

Flow (L/min) = litres per day ÷ (pumping hours × 60).

3. Step 2: the height to lift

Measure from the lowest water level (not the top of the tank, and not the water level in a wet season) up to the highest outlet.

4. Step 3: pipe friction

Water rubbing along the inside of the pipe costs head. Friction goes up with:

• longer pipe;
• smaller pipe;
• more flow;
• bends, valves, tees and filters. The calculator adds 10% for these as a starting point.

Use the pipe's inside diameter. It's smaller than the size printed on the pipe, so check the pipe maker's chart.

The calculator uses the Hazen-Williams formula with C = 140, a typical figure for smooth plastic pipe such as poly or PVC. Older, rough or kinked pipe has more friction.

5. Step 4: pressure at the outlet

Filling an open tank or trough needs no extra pressure (0 kPa). Sprinklers, taps and houses do need pressure, so use the fitting maker's figure. Every 9.81 kPa adds about 1 metre of head.

6. Worked example: filling a stock trough

A trough needs about 2,000 litres a day, pumped over 4 hours. The water lifts 12 metres from the lowest dam level to the trough, through 150 metres of poly pipe with a 20 mm inside diameter. It runs into an open trough (0 kPa). The calculator's default pump efficiency is 40%.

So you're looking for a pump that gives at least 8.3 L/min at about 14.5 m of head.

Now try a hose-sized flow

Push the flow up to 15 L/min through the same 150 m of pipe, and the picture changes.

The bigger pipe saves about 5 metres of head and around 30 W. On a long run, pipe size matters as much as the pump.

7. Reading the pump maker's chart

• Find your total head on the chart.
• Read across to the flow the pump gives at that head.
• If the pump gives at least your flow there, it's a candidate.
• A pump that only just reaches your head will give a trickle. Leave some room.

8. Things that catch people out

• Suction lift. A pump on the surface can only suck water up a limited height, often only a few metres in practice. Deeper water needs a submersible or bore pump. Check the maker's suction lift figure.
• Running dry. Many pumps are damaged if they run without water. Check the maker's limits.
• Water quality. Dirty dam water and sand can wear a pump. Check what the pump is rated for.
• Startup power. The wattage here is a rough guide to the power drawn while running. A mains pump's startup current can be much higher, so check the maker's figures when sizing a generator, inverter or solar system.

9. Electrical safety

Any work on 240V AC wiring for a pump, including new circuits and outdoor outlets, must be done by a licensed electrician.

10. Common mistakes

• Measuring the lift from the top of the tank instead of the lowest water level.
• Using the printed pipe size instead of the inside diameter.
• Ignoring friction on long runs.
• Choosing by watts or horsepower instead of the flow-and-head chart.
• Expecting a surface pump to suck from deep water.

11. Quick summary

• Flow (L/min) and total head (m) are the two numbers that matter.
• Total head = lift + pipe friction + outlet pressure.
• Measure the lift from the lowest water level.
• On long runs, a bigger pipe can save a lot of head.
• Choose from the maker's performance curve, and check suction lift and dry-running limits.

Try it with your own figures: Pump Size Calculator.

General information only. Check the pump maker's charts and instructions.