The swimming pool pump market can look complicated, so we have set out five simple steps to help you choose the right pump for your pool. Get the size right and the pump will run efficiently, protect your filter and last for years.
Many people simply replace an old pump with a like-for-like model. That is fine if the original was correctly specified, but if your previous pump failed early it may have been the wrong size or type in the first place. It is worth checking before you buy a replacement.
Why flow rate matters
There is a common temptation to buy a pump that is more powerful than necessary. We would not recommend it. An oversized pump means higher running costs, it can overpower the filter, and it can actually shorten the pump's life. The most important figure is not the horsepower (HP) or kilowatt (kW) rating, but the flow rate, measured in litres per minute. The same principle applies to every pump we sell, and we explain why in our guide to how to read a pump curve.
The 8-hour turnover rule
As a general rule, all the water in your pool should pass through the filter within an 8-hour period. That single figure drives the whole calculation. To choose your pump you need to know three things:
- How many litres of water are in your pool.
- The flow rate you need to turn that volume over in 8 hours.
- A pump model that delivers that flow rate at the right head.
How to size a pool pump in 5 steps
Step 1: Calculate the volume of your pool
Use the formula Volume (m3) = Length (m) x Width (m) x Depth (m). If the pool is not the same depth throughout, use the average depth, worked out as (shallow depth + deep depth) divided by 2.
These images show a rectangular pool. For a circular pool, calculate the volume using V = π r² h.
Worked example (rectangular pool, same depth): a pool 20m long, 8m wide and 1m deep is V = 20 x 8 x 1 = 160 m3.
Step 2: Convert cubic metres to litres
Step 1 gives you a volume in cubic metres. Convert it to litres, where 1 m3 = 1,000 litres, by multiplying by 1,000.
Worked example: 160 m3 x 1,000 = 160,000 litres.
Step 3: Work out the flow rate to turn the pool over in 8 hours
Divide the volume in litres by 8 to get the litres-per-hour figure.
Worked example: 160,000 litres ÷ 8 hours = 20,000 litres per hour.
Step 4: Convert to litres per minute
Our pumps are rated in litres per minute, so divide the litres-per-hour figure by 60. The result is the flow rate you need, and every swimming pool pump on our site lists this figure on its product page.
Worked example: 20,000 litres per hour ÷ 60 minutes = 333 litres per minute.
Step 5: Choose a pump that matches
Select a pump from our range of swimming pool pumps rated at or just above your required flow rate. In the worked example you would look for a pump delivering around 333 litres per minute, such as a model from the Speroni SWIMM range. The technical data on each product page lists the flow rate, as in the example below, so you can match it to your figure. If you would prefer a different brand, our overview of the Plastica range of swimming pool pumps is a good place to start.
Do not forget head
Flow rate tells you how much water a pump moves, but you also need enough head to overcome the resistance in your system, which is the vertical lift plus the friction losses in the pipework, filter and fittings. A pump that delivers the right flow at zero head may fall short once it is working against a real installation. Our Water Pump Performance Calculator will estimate the friction losses for your pipe run, and the pump curve guide shows how to read flow and head together.
Efficiency and longevity
Choosing the right pool pump is not just about power, it is about efficiency and reliability. A correctly sized pump uses less energy, protects your filter and keeps the water clean and healthy for years. Because pool pumps run for hours every day through the season, the savings from getting the size right add up quickly.
Pool pumps are surface pumps that sit dry beside the pool rather than in the water. If you are weighing up pump types more generally, see surface pump vs submersible pump. And if you are looking to circulate a pond or water feature rather than a pool, see our pond pump guide.
If you would like help choosing, explore our full range of swimming pool pumps, use the Pump Finder, or call the team on 0115 987 0358 for friendly advice.
How to choose a swimming pool pump (video)
Frequently asked questions
How do I work out what size pool pump I need?
Calculate your pool volume in litres (length x width x average depth in metres, then multiply by 1,000), divide by 8 hours, then divide by 60 to get the litres per minute your pump needs to deliver. Match that figure to a pump on its product page.
Is a bigger pool pump better?
No. An oversized pump costs more to run, can overpower the filter and may wear out faster. The right pump is the one sized to turn your pool volume over in about 8 hours, no more powerful than that.
How often should a pool pump turn the water over?
As a general guide, aim to filter the entire pool volume once every 8 hours. That is the turnover period the 5-step calculation above is based on.
What matters more, horsepower or flow rate?
Flow rate. Horsepower and kilowatt ratings tell you how much energy the pump draws, but the litres-per-minute flow rate, read against the head of your system, is what determines whether it will turn your pool over properly.
Do I need to consider head as well as flow rate?
Yes. Head is the resistance the pump works against, including vertical lift and friction losses in the pipework and filter. Choose a pump that delivers your required flow rate at the head of your actual installation, not just its maximum flow figure.
