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Pool Filter Sizing

Filter Sizing

Once you have chosen the right type of filter for your pool, it is important to get it sized properly.  It must be sized to match the pump which in turn must match the circulation needs of the pool.  If the filter is undersized, it will fill up quickly and also can be damaged by the force of the pump.  If the filter is undersized, it may not backwash well.

It is important to keep in mind that the manufacturer's flow ratings for filters are often very optimistic.  Our experience suggests that the following guidelines are much more realistic.    

Filter Sizing Chart #1

Optimal Square Footage of Filter for Different Pump Sizes
Pump Size Flow Rate Sand Filter DE Filter Cartridge Filter
3/4 HP 40 2.7 24 100
1.0 HP 50 3.4 36 150-200
1.5 HP 65 4.3 48 300
2.0 HP 87 5.8 60 400
2.5 HP 105 7.0 72 500
3.0 HP 120 8.0 84 600
Maximum Flow Rate Through Filter Media (flow per square foot of filter media)
Filter Type Sand Filter DE Filter Cartridge Filter
Maximum Flow Rate 15 GPM/sq. ft 2.0 GPM/sq. ft 0.25 GPM/sq. ft

Filter Sizing Chart #2

Pool Gallonage Pump Size

Intake Lines (minimum)


 Filter Size


.75 hp 

1 - 1.5" 

Cartridge - 100-200 Sq. Ft. 

Sand - 3.0 Sq. Ft 

DE - 36 Sq Ft 


1.0 hp

1 - 2.0"

20,000 1.5 hp

2 - 1.5" or 1 - 2.0"

Cartridge - 300-400 Sq. Ft. 

Sand - 3.9 Sq. Ft 

DE - 48 Sq Ft 

25,000 1.5 hp

2 - 1.5" or 1 - 2.0"

30,000 2.0 hp

4 - 1.5" or 2 - 2.0"

Cartridge - 400-500 Sq. Ft. 

Sand - 4.9 Sq. Ft 

DE - 60 Sq Ft

35,000 2.0 hp

4 - 1.5" or 2 - 2.0"

40,000 2.5 hp

3 - 2.0"

Cartridge - 500+ Sq. Ft. 

Sand - 6.9 Sq. Ft 

DE - 72 Sq Ft

45,000 2.5 hp

3 - 2.0"

Note that these recommendations are only approximations based on typical conditions.  

On-site analysis must be done to insure that the filter matches up properly with the pump and the pool.  For instance, if the plumbing runs on the equipment are particularly long, then the resistance to flow in the piping will be greater and intake lines may need to be larger than specified in this chart.

Note also that this may differ from the recommendations listed by the manufacturer . 

The literature may indicate that it is OK to match up a 2.5 or 3.0 hp pump to a 60 square foot DE filter, but we never recommend it because we have seen plenty of DE filter grids blown out by these larger sized pumps.  It is always better to oversize the filter.  With an oversized filter, the water flows through the filter media at a slower velocity and that results in finer filtration and longer filter cycles.

Some builders will install a 3.0 hp filter pump on a pool/spa combination in order to give maximum water flow through the spa jets. 

This is nice except for the fact that the pump is so powerful that it has the potential to damage the grids in the filter.  The better way is to install a smaller filtration pump for the pool and then run a separate booster pump.  This keeps you from having to run a large 3.0 hp pump every day on the normal filter cycles.

See also the pump sizing section

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