A pump that pushes 50 gpm needs a sand filter bed of at least 2.5 square feet, a cartridge filter of at least 133 square feet, or a DE filter of at least 25 square feet, using the residential ratings on manufacturer data sheets. Under the stricter public pool code the sand bed grows to 3.33 square feet and the cartridge to 167.
How the filter size is calculated
Every filter has a design flow per square foot of filtering surface. Push more water through than that and the filter cleans poorly and loads up fast. So the area you need is the flow divided by the rate:
Filter area (sq ft) = pump flow (gpm) ÷ design rate (gpm per sq ft)
For 50 gpm the residential rates give 50 ÷ 20 = 2.5 sq ft of sand, 50 ÷ 0.375 = 133.3 sq ft of cartridge and 50 ÷ 2 = 25 sq ft of DE grid. Get the flow from the pool pump calculator or from your turnover goal. For a 15,000 gallon pool turning over in 8 hours that is only 31.25 gpm, which needs 1.56 sq ft of sand, but a single-speed pump usually moves more, so size the filter for the pump's full flow.
Design rates: home pools against public pools
| Filter | Home pool rating | Public pool (MAHC 2024) |
|---|---|---|
| High-rate sand | 20 gpm per sq ft | 15 gpm per sq ft, 24 in bed; 12 with a shallower bed |
| Cartridge | 0.375 gpm per sq ft | 0.30 gpm per sq ft design, 0.375 operating limit |
| DE | 2 gpm per sq ft | 2 gpm per sq ft (2.5 with continuous body feed) |
The home figures come from manufacturer data sheets: Hayward's Pro Series table is based on 20 gpm per sq ft, "maximum allowable NSF rating", and Pentair's CCP320 carries 120 gpm on 320 sq ft, which is 0.375. Both are maximums, so pick a filter whose rated flow is at or above your pump's flow.
Sand filter sizes and how much sand
Hayward's Pro Series data sheet lists these high-rate sand filters and the sand each holds:
| Model | Bed area | Design flow | Sand |
|---|---|---|---|
| S180T | 1.75 sq ft | 35 gpm | 150 lb |
| S210T | 2.20 sq ft | 44 gpm | 200 lb |
| S220T | 2.64 sq ft | 52 gpm | 250 lb |
| S244T | 3.14 sq ft | 62 gpm | 300 lb |
| S270T | 3.70 sq ft | 74 gpm | 350 lb |
| S310T2 | 4.91 sq ft | 98 gpm | 500 lb |
| S360T2 | 7.06 sq ft | 141 gpm | 700 lb |
For 50 gpm the smallest that fits is the S220T. The sand works out to about 95 lb per square foot of bed across the eight sizes, so a 24 inch bed, area π ÷ 4 × 2² = 3.14 sq ft, takes about 298 lb, six 50 lb bags. Enter your bed diameter in the calculator and it does this sum; it is an estimate, and the label or manual for your tank has the exact figure. Hayward specifies High Rate Sand No. 20 silica, 0.45 to 0.55 mm.
Cartridge filter sizes
Pentair's Clean and Clear Plus cartridge filters come in four sizes: 240 sq ft (90 gpm), 320 sq ft (120 gpm), 420 sq ft (150 gpm) and 520 sq ft (150 gpm). The flow rating tops out at 150 gpm in the two largest, so extra area buys longer runs between cleaning rather than more flow. At 50 gpm the smallest one covers you.
DE filters
DE grids need the least area, 25 sq ft for 50 gpm, The MAHC allows 2 gpm per sq ft, or 2.5 with a continuous precoat feed. Read the filter's label for its area.
When to clean or replace
Cleaning is triggered by pressure, not the calendar. Hayward says to backwash when the gauge is 8 to 10 psi above the clean reading, and Pentair says to clean cartridges when pressure rises 10 psi above the baseline. No maker's manual we read gives a sand or cartridge replacement age, so follow yours.
Next steps
Check the pump first with the pool pump calculator, see the water in gallons with the pool volume calculator, and find the rest in the equipment category. If a heater sits in the loop, the pool heater calculator sizes it.