What Size Pool Pump Do I Need?

The heart of any swimming pool is the circulation pump, which allows the water to be filtered, heated, vacuumed and chemically treated. Without a pump the pool will quickly become useless.Therefore, when first installing or replacing your pump it is extremely important that it is the correct size for your pool. It is essential that the pump is neither too big nor too small, as there can be problems associated with both, such as insufficient filtration or the creation of too much pressure on the pipe work or filter installation. The ideal situation is to have a pump that filters all of your pool water in a 6 to 8 hour period, depending on the bather load.

When you are simply replacing a worn out pump, many people will automatically put in a pump of the same size as they are taking out. However, this supposes that the original was the correct size in the first place. As such it is therefore worth going through the correct sizing procedure anyway.

There are two main factors that need to be measured, "flow rate" and "head".

The amount of water moved in any given period of time is known as the flow rate. This is usually worked out in gallons per minute. To work out your required flow rate you first need to work out your pool volume. For these formulae, take the measurements in feet. To work out the average depth, add the deepest depth to the shallowest depth and divide by two.

Pool Water Volumes.

Rectangular Pool : Length x Width x Average Depth x 6.25 = Gallons

Round Pool : Radius x Radius x Average Depth x 20 = Gallons

Shaped or Oval Pools : Avg Width x Avg Length x Avg Depth x 6.25 = Gallons

Once you know the volume, decide how quickly you want to turnover the pool water. Ideally this will be about every 6 hours, but can vary according to bather load. If we assume turnover required is six hours then the flow rate in hours is the volume divided by 6. This figure must then be divided by 60 to get gallons per minute.

Once we have established the required flow rate, we need to determine the head loss, which is a measure of the resistance that the water is subjected to as it is circulated around the pool.

This is very difficult to do, and is only theoretically possible on new builds. There are very many variables and too many unknowns. To be completely accurate you need to know the exact length of all pipe work, exact location of joints, what they are made of, who by, etc,etc.

In reality, it is only possible to make a good estimate. For most domestic pools, it is accepted that the average figure for "head" is 10 metres or 33 feet. This will usually be reasonably accurate for most pools where the pump is situated no more than 30 feet from the pool, and on the same level. Take this 10m figure as a starting point and adjust upwards or downwards if there are any unusual features, such as the pump on a lower level.

Once we have calculated the required flow rate, and estimated the head, then we need to look at the pump curve charts provided by the pump supplier.

All pump curve charts show the flow along the horizontal axis, measured against the head along the vertical axis. At the point where your two readings cross, go to the next pump curve up. That is then the pump you require.


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Francis Lowe is a leading expert in swimming pool maintenance with a huge wealth of experience developed from 40 years in the industry.
To see a full range of pool equipment, including swimming pool pumps and pool pumps please see the website of specialist supplier poolwarehouseuk.com