The filtration rate is the rate (in metres per hour) at which the pool water moves down through the filter during normal operation. It is not to be confused with the flow rate, which is the rate (in cubic metres per hour) at which water is moving through the circulation system.

The filtration rate is calculated by dividing the flow rate by the surface area of the filter. For example:

**Circulation Rate:** 100 cubic metres per hour

**Filter Surface Area:** 5.71 metres squared

**Filtration Rate:** 125m3 / 5.71m2 = 21.89m3/m2/hr

You may already have a flow rate meter fitted onto the circulation pipework in your plant room to tell you what the flow rate is. If you do not have one fitted, it is highly recommended that you get one fitted as soon as possible as it will be difficult to calculate the turnover time and filtration rate without having the key piece of data that a flow rate meter provides.

If you are trying to calculate your filtration rate, but don’t know the surface area of the filter, you can easily calculate what the filter surface area is by using the formula: r2 x π. This formula means the radius squared multiplied by pi. See below for an example using a filter that is 2.7 metres wide:

**Width of filter: **2.7 metres (this is also the diameter)

**Radius: **2.7m / 2 = 1.35m

**Radius Squared: **1.35m x 1.35m = 1.82 m2

**Surface Area: **1.82m x 3.14 (pi) = 5.72 m 2

There are three categories of filtration rate and they are set out below:

**Low Rate:** up to 10 m3/m2/hr

- very good filtration, but requires a very large surface area

**Medium Rate:** 10 - 25 m3/m2/hr

- recommended for public pools

**High Rate:** +25 m3/m2/hr

- recommended for small domestic pools only because this rate is too fast to deal with pollution in public pools

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