Divide your pool volume by the turnover you want in minutes to get the flow: gallons ÷ (hours × 60) = GPM. Then divide that flow by the media's design rate — 15 GPM per sq ft for sand, 2 for DE, 1 for cartridge. A 20,000-gallon pool on an 8-hour turnover needs 41.7 GPM, which is a 24-inch sand tank, about 21 sq ft of DE, or 42 sq ft of cartridge as a bare minimum — and with a filter, buying larger than the minimum is always the right call.
Filter Size Starts With Flow, Not Gallons
A filter is rated by how much water it can clean per minute without the water moving through the media too fast to be filtered. So the question "what size filter for a 20,000-gallon pool" has to be turned into a flow question first: how many gallons a minute do you need to move to turn the water over in the time you want?
The standard target is one full turnover in 8 hours. That is a convention rather than a law — many owners run 6 hours in high season and 10–12 in the shoulder months, and a variable-speed pump changes the arithmetic by running longer at a much lower flow. Pick the shortest turnover you actually intend to use, because that is the flow the filter has to handle.
GPM = gallons ÷ (turnover hours × 60)
Then filter area = GPM ÷ design rate, where the design rate is 15 GPM per sq ft for sand, 2.0 for DE and 1.0 for cartridge (NSF/ANSI 50 and manufacturer design guides).
Those design rates are the whole reason the three media give such different-looking numbers for the same pool. Sand passes fifteen times more water per square foot than cartridge does, so a sand filter with 3 sq ft of media and a cartridge filter with 45 sq ft can be correctly sized for the same pool. Comparing filters on square footage across media types is meaningless.
What to Buy by Pool Size
The table converts the minimum into what is on the shelf: the smallest standard sand tank diameter that meets the requirement, and the next stock size up for DE and cartridge.
| Pool volume | Flow (8 hrs) | Sand tank | DE | Cartridge |
|---|---|---|---|---|
| 10,000 gal | 20.8 GPM | 16" | 24 sq ft (min 10.4) | 50 sq ft (min 20.8) |
| 15,000 gal | 31.3 GPM | 21" | 24 sq ft (min 15.6) | 50 sq ft (min 31.3) |
| 20,000 gal | 41.7 GPM | 24" | 24 sq ft (min 20.8) | 50 sq ft (min 41.7) |
| 25,000 gal | 52.1 GPM | 27" | 36 sq ft (min 26.0) | 75 sq ft (min 52.1) |
| 30,000 gal | 62.5 GPM | 30" | 36 sq ft (min 31.3) | 75 sq ft (min 62.5) |
| 40,000 gal | 83.3 GPM | 36" | 48 sq ft (min 41.7) | 100 sq ft (min 83.3) |
Read those as floors with one step of headroom, not as targets. Unlike a pump, a filter cannot really be too big: extra area means longer runs between cleanings, more dirt held before the pressure climbs, and less resistance for the pump to fight. The owners who are happy with their cartridge filters are usually running two or three times the minimum, and the ones cleaning cartridges every month bought exactly the minimum.
For a turnover other than 8 hours, or a volume that isn't on this list, the pool filter size calculator runs the same rule on your own figures. If you are unsure of your real volume, start with the pool volume calculator rather than the number on the build contract.
Why the Three Media Give Different Numbers
The design rate is a trade between throughput and fineness. Sand lets water through fastest and catches the least; DE catches the most and needs the most area per gallon. That single trade explains the whole table.
| Media | Design rate | Filters down to | Sizing consequence |
|---|---|---|---|
| Sand | 15 GPM/sq ft | ~20–40 microns | Sold by tank diameter; a few sq ft covers a big pool |
| DE | 2.0 GPM/sq ft | ~2–5 microns | Middling area, finest water, media recharged after backwash |
| Cartridge | 1.0 GPM/sq ft | ~10–20 microns | Largest area on paper; no backwash, so no water wasted |
The 1.0 GPM per sq ft used here for cartridge is the longevity figure rather than the absolute maximum. Manufacturers will quote higher, and the filter will pass the water, but the element loads up faster and does not last as long. Sizing at 1.0 is what makes the difference between cleaning a cartridge twice a season and cleaning it monthly.
Choosing between the three is a separate question from sizing them — running cost, water waste, maintenance effort and how clear you want the water all come into it. Our sand vs cartridge vs DE comparison lays those side by side. Once you own the filter, how much media it takes is fixed by the unit, and the sand and DE quantity calculator gives the charge for your tank.
Matching the Filter to the Pump and the Pipe
A filter never works alone. The pump has to push water through it, and the plumbing sets a ceiling on how much water can move at all. Get the three out of step and the symptoms look like a filter fault when they are a system fault.
Two rules keep them in step:
- The filter's maximum flow rating must be at least the pump's flow at its operating point. A filter fed more than it is rated for passes dirt straight through, and on a sand bed it can channel the media.
- The pipe caps everything. At sane water velocity, 1.5-inch plumbing carries about 43 GPM and 2-inch about 73 GPM. A pump that wants more than the pipe can deliver just makes noise and cavitation.
The order that works when you are specifying a new system: real volume first, then the turnover you want, then the filter sized generously, then the pump sized against the system you actually built. Every filter adds resistance, and that resistance grows as the media loads — so an 8-hour turnover on a clean filter quietly becomes a ten-hour one before the next cleaning without anything looking wrong.
Variable-speed pumps change the calculation
A variable-speed pump run long and slow moves the same volume at a much lower GPM, which is easier on the filter and far cheaper in electricity. It does not let you buy a smaller filter — size for the highest speed you will use, typically the cleaning or spa mode — but it does mean an oversized filter pays back twice, in longer cleaning intervals and in lower head at low speed.
If there is a heater in the loop, it has a minimum flow of its own and will trip out below it, so check it against the same number. Sizing for the heater is covered in what size pool heater do I need.
Is the Filter You Already Have Too Small?
Most people arrive at this question because something is wrong rather than because they are shopping. The label on the tank gives you both numbers you need: the filter area in square feet and the maximum flow in GPM. Compare the area against the table above for your volume, and the flow rating against what your pump actually delivers.
The signs of a genuinely undersized filter are consistent:
- Pressure climbs back to the cleaning threshold within days rather than weeks.
- The water never quite clears in summer, even with chemistry in range.
- The interval between backwashes or cartridge cleanings keeps shrinking as the season goes on.
A filter that fills up in a week is not a filter that needs cleaning more often — it is a filter with too little area for the pool it serves.
Rule out the cheaper explanations first, though:
- Chemistry, not hardware. Cloudy water is far more often chemistry — see how to fix cloudy pool water.
- Sand older than five to seven years. It channels and stops filtering properly.
- A worn or dirty element. A cartridge past a couple of seasons loses pleat integrity, and a filter that was never cleaned properly behaves exactly like one that is too small.
Where People Go Wrong
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Buying exactly the minimum
The calculated area is the smallest filter that can legally pass the flow, not the one to own. One size up is cheap at purchase and pays back every week in cleaning intervals — this is the single most common regret with cartridge systems.
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Comparing square footage across media
A 3 sq ft sand filter and a 45 sq ft cartridge can serve the same pool. Square feet only means something next to the design rate for that media, so compare sand with sand and cartridge with cartridge.
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Sizing off the builder's gallon figure
Contract volumes are frequently rounded or optimistic, and everything downstream inherits the error. Measure the pool and work it out yourself — the volume calculator handles rectangles, ovals and freeform shapes.
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Upgrading the pump and leaving the filter
A bigger pump on the old filter pushes water through the media faster than it can be cleaned, so the water gets worse rather than better. If the pump goes up, check the filter's maximum flow rating before anything else.