What size pool filter do I need?
Filter size follows the flow through the filter, not the gallons in the pool. Divide the highest flow your pump sends through it by the rate the media is allowed to run at, and the answer is square feet of media. The residential pool energy standard, ANSI/APSP/ICC-15, and California's energy code, §150.0(p), set those rates at 0.375 GPM per sq ft for cartridge, 15 for sand and 2 for DE, applied to "the maximum flow rate through the filter". At 50 GPM that is 134 sq ft of cartridge, 3.4 sq ft of sand bed (a 27-inch tank) or 25 sq ft of DE grid.
The flow to use is the highest one, which is rarely the overnight filtration speed. If the same pump runs a spa, a pressure cleaner or a waterfall through the filter, or you vacuum at full speed, that flow passes through the media too. The pump sizing calculator works out the flow you need; the chart below turns it into a filter.
Pool filter size by flow
pooltoolbox.com · minimum sizes at the ANSI/APSP/ICC-15 and California §150.0(p) rates
Pool filter size chart
| Highest flow | Cartridge (0.375) | Sand bed (15) | Sand tank | DE (2) |
|---|---|---|---|---|
| 30 GPM | 80 sq ft | 2.0 sq ft | 21" | 15 sq ft |
| 40 GPM | 107 sq ft | 2.7 sq ft | 24" | 20 sq ft |
| 50 GPM | 134 sq ft | 3.4 sq ft | 27" | 25 sq ft |
| 60 GPM | 160 sq ft | 4.0 sq ft | 31" | 30 sq ft |
| 75 GPM | 200 sq ft | 5.0 sq ft | 36" | 38 sq ft |
| 90 GPM | 240 sq ft | 6.0 sq ft | 36" | 45 sq ft |
| 120 GPM | 320 sq ft | 8.0 sq ft | 2 tanks | 60 sq ft |
| 150 GPM | 400 sq ft | 10.0 sq ft | 2 tanks | 75 sq ft |
Minimum media area at the residential code rate in GPM per sq ft (metric: 0.92, 36.7 and 4.9 m/h), rounded up, computed by our filter engine. Sand tank: the smallest in Hayward's ProSeries table whose published bed area covers it. Use the highest flow that passes through the filter. Residential cartridges are rated to 150 GPM at most.
How many GPM can my filter handle?
There are two answers, and the box prints the bigger one. Manufacturers rate residential filters at the most the media will pass: up to 1 GPM per sq ft for cartridge, 20 for sand and 2.5 for DE in current manuals. The code rate is lower because it is the public-pool rate in the NSF/ANSI 50 equipment standard, and ANSI/APSP/ICC-15 requires home filters to be "at least the size specified in NSF/ANSI 50 for public pool intended applications". Both are real ratings. They answer different questions: the box says what the filter survives, the code says what it is sized for.
| Filter | Area | Box rating | Per sq ft | At the code rate |
|---|---|---|---|---|
| Pentair Clean & Clear 100 | 100 sq ft | 100 GPM | 1 | 37.5 GPM |
| Pentair Clean & Clear 200 | 200 sq ft | 150 GPM | 0.75 | 75 GPM |
| Hayward SwimClear C3030 | 325 sq ft | 122 GPM | 0.38 | 121.9 GPM |
| Hayward SwimClear C4030 | 425 sq ft | 150 GPM | 0.35 | 150 GPM |
| Pentair Triton II TR60 | 3.14 sq ft | 63 GPM | 20.1 | 47.1 GPM |
| Pentair Triton II TR100 | 4.91 sq ft | 74 GPM | 15.1 | 73.7 GPM |
| Hayward Pro-Grid DE6020 | 60 sq ft | 120 GPM | 2 | 120 GPM |
| Pentair FNS Plus 60 | 60 sq ft | 150 GPM | 2.5 | 120 GPM |
Box ratings from each owner's manual: Pentair Clean & Clear, Hayward SwimClear, Pentair Triton II, Hayward Pro-Grid, Pentair FNS Plus. Last column: area × the code rate, or the box rating where that is lower. Computed by our engine and pinned to the manuals in its tests.
Pentair's Clean & Clear manual lists its 100 sq ft filter at 6,000 gallons an hour for residential use and 2,280 for commercial: 100 GPM and 38 GPM. The second figure is the code rate. A 100 sq ft cartridge on a pump pushing 100 GPM is within its label and 2.7 times the flow the code sizes it for. Hayward's multi-cartridge SwimClear line is rated at the code rate to begin with, which is why its numbers look small beside a single-cartridge box.
The gap is widest for cartridges (2.7×), smaller for sand (1.33×) and DE (1.25×). Some makers already rate at the code figure: Pentair's larger Triton tanks are rated at 15 GPM per sq ft, Hayward's Pro-Grid DE at 2. Running a filter at its box rating is legal and it will filter. It will also need cleaning far more often, for reasons the head-loss section below explains.
What size sand filter do I need?
Sand tanks: flow, backwash and sand
| Tank | Bed area | Code flow | Box flow | Backwash needs | Sand |
|---|---|---|---|---|---|
| 14" | 1.1 sq ft | 16 GPM | 22 GPM | 17 GPM+ | 50 lb |
| 16" | 1.4 sq ft | 21 GPM | 28 GPM | 21 GPM+ | 100 lb |
| 18" | 1.8 sq ft | 27 GPM | 36 GPM | 27 GPM+ | 150 lb |
| 21" | 2.2 sq ft | 33 GPM | 44 GPM | 33 GPM+ | 200 lb |
| 22" | 2.6 sq ft | 39 GPM | 52 GPM | 39 GPM+ | 250 lb |
| 24" | 3.1 sq ft | 46 GPM | 62 GPM | 47 GPM+ | 300 lb |
| 27" | 3.7 sq ft | 55 GPM | 74 GPM | 56 GPM+ | 350 lb |
| 31" | 4.9 sq ft | 73 GPM | 98 GPM | 74 GPM+ | 500 lb |
| 36" | 7.1 sq ft | 106 GPM | 142 GPM | 107 GPM+ | 700 lb |
Tank sizes, bed areas and sand charges from Hayward's ProSeries manual, Table 2. Code flow at 15 GPM per sq ft, box flow at 20, both rounded down; backwash at 15 GPM per sq ft of bed (Sta-Rite), rounded up. Other makers' tanks of the same size differ slightly.
Pick the tank whose code flow covers your highest flow. At 50 GPM that is the 27-inch; a box rating of 20 GPM per sq ft would allow the 22-inch. One tank tops out at the 36-inch's 106 GPM; beyond that, two tanks or a cartridge. The same code also wants a backwash valve of at least 2 inches or the return pipe's size, whichever is larger, so take the 2-inch valve version where a tank comes both ways.
How much sand a tank takes depends on the tank, not its size label. Hayward's 24-inch takes 300 lb, Sta-Rite's 250 lb, and Pentair's 24-inch Triton TR60 325 lb of sand, or 50 lb of pea gravel under 275 lb of sand. Pentair's manual says why: sand density varies, so the number on the chart is a reference and the gap between the sand and the top diffuser, the freeboard, is what has to be right. Fill to the freeboard in your manual.
Can a sand filter be too big?
Yes, and the limit is the backwash. A sand filter cleans itself by running water up through the bed hard enough to lift and tumble it. Sta-Rite's engineering manual puts the bed's rise at about 6 inches at 15 GPM per sq ft, and says high-rate sand filters backwash at the same rate they filter. A 27-inch tank needs 56 GPM to backwash; a 36-inch needs 107. Below that the water channels through the dirt instead of carrying it out, and the pressure after a backwash creeps up week by week.
This is the trap with a variable-speed pump. It filters happily at 40 GPM, and a backwash at the same speed lifts only 2.7 sq ft of bed, about a 22-inch tank. So backwash at the speed that gives the tank's backwash flow, and if the pump cannot reach it, a larger tank is the wrong buy. A backwash runs 2 to 5 minutes, depending on the manual, plus a short rinse; at 55.5 GPM for 3 minutes that is 167 gallons to waste, which the cost to fill calculator can price as a top-off.
A high-rate sand filter traps dirt through the depth of the bed, not just on top. To release it, the backwash has to fluidize the bed: push water up fast enough that the grains separate and rub against each other. Public-pool designers use at least 15 GPM per sq ft, more in warm water, which is thinner and lifts sand less. Filtering slower than the bed's rating is fine; backwashing slower is the problem.
What size cartridge filter for my pool?
The usual rule, 100 sq ft per 10,000 gallons, has no flow in it, so it can only be right by coincidence. Here is what the code size comes to at the largest filtration flow the same standard allows, one turnover in six hours or 36 GPM, whichever is greater.
| Pool | Code filtration flow | Cartridge | Sand tank | DE | "100 per 10,000" rule |
|---|---|---|---|---|---|
| 10,000 gal | 36 GPM | 96 sq ft | 22" | 18 sq ft | 100 sq ft |
| 15,000 gal | 41.7 GPM | 112 sq ft | 24" | 21 sq ft | 150 sq ft |
| 20,000 gal | 55.6 GPM | 149 sq ft | 31" | 28 sq ft | 200 sq ft |
| 25,000 gal | 69.4 GPM | 186 sq ft | 31" | 35 sq ft | 250 sq ft |
| 30,000 gal | 83.3 GPM | 223 sq ft | 36" | 42 sq ft | 300 sq ft |
| 40,000 gal | 111.1 GPM | 297 sq ft | 2 tanks | 56 sq ft | 400 sq ft |
Code filtration flow: gallons ÷ 360 minutes or 36 GPM, the ANSI/APSP/ICC-15 maximum for filtration. Areas at the code rates, rounded up; computed by our engine. A faster pump, or a spa or feature on the same filter, needs the larger flow's size.
The rule overshoots for big pools filtered slowly and undershoots the moment the pump runs faster. A 20,000-gallon pool filtered at its code flow of 55.6 GPM needs 149 sq ft of cartridge; the same pool on a single-speed pump moving 80 GPM needs 214 sq ft. If you are choosing between two cartridge sizes, take the larger: there is no backwash to limit it and no energy penalty, and it runs longer between cleanings. The ceiling is the ports: residential cartridges are rated to 150 GPM however large the element.
What size DE filter, and how much DE?
DE filters are rated at 2 GPM per sq ft of grid by Hayward and 2.5 by Pentair; the code holds them to 2, and Sta-Rite's engineering manual recommends 1.5 for a longer run between backwashes. At 60 GPM that is 30 sq ft at the code rate, 40 at Sta-Rite's, against the 24 a 2.5 rating would allow. Hayward and Pentair sell grid sets of 24, 36, 48, 60 and 72 sq ft, so round up to the next one.
After each backwash the grids need a fresh coat. Pentair's FNS Plus manual asks 0.1 lb of DE per sq ft and Hayward's Pro-Grid 0.125: a 48 sq ft filter takes 4.8 to 6 lb, added through the skimmer with the pump running. The calculator shows the range for your size; your own manual's figure wins.
Does a bigger filter lower filter pressure?
Less than you would think, when it is clean. Sta-Rite's head-loss chart lists cartridge filters from 25 to 135 sq ft, each at 0.75 GPM per sq ft. Moved to the same 50 GPM by the square law, they all land within 1.5 ft of each other, though the largest has 5.4 times the media.
| Cartridge | Published | At 50 GPM | Flow per sq ft at 50 GPM |
|---|---|---|---|
| 25 sq ft | 1.1 ft at 18.75 GPM | 7.8 ft | 2 |
| 35 sq ft | 2 ft at 26.25 GPM | 7.3 ft | 1.43 |
| 50 sq ft | 4.3 ft at 37.5 GPM | 7.6 ft | 1 |
| 70 sq ft | 7.5 ft at 52.5 GPM | 6.8 ft | 0.71 |
| 75 sq ft | 8 ft at 56.25 GPM | 6.3 ft | 0.67 |
| 100 sq ft | 17.5 ft at 75 GPM | 7.8 ft | 0.5 |
| 135 sq ft | 28 ft at 101.25 GPM | 6.8 ft | 0.37 |
Published column: Sta-Rite Engineering and Design Manual, p. 17, clean filters. 50 GPM column: scaled by flow squared, the same rule our total dynamic head calculator uses for equipment. An older manual; your filter's own curve overrides it.
The clean loss sits in the tank, ports and valve, so it follows the flow, not the media area. What the extra area changes is how fast the pressure climbs as dirt builds up, and that is where the box rating costs you.
Cartridge and DE filters build a layer of trapped dirt, a cake, on the media. In the textbook model of a filter cake at a steady flow, the pressure across it rises with the square of the flow per square foot. Double the area at the same flow and the pressure climbs 4 times more slowly; run a cartridge at its 1 GPM box rating instead of the 0.375 code rate and the model has it reaching the cleaning point 7 times as fast. Pool dirt is lumpier than the textbook and flow falls as pressure rises, so treat those as the direction and rough size of the effect, not a promise. Sand traps dirt through its depth and does not follow this model.
The calculator's "Head loss from the manual" section takes one point off your filter's curve and the cleaning pressure rise from the manual (7 to 12 psi in the ones we checked), and carries both into the total dynamic head calculator.
Is my filter too small for my pump?
The signs, roughly in order of how early they show:
- Pressure is back at the cleaning point within days of a cleaning or backwash, in ordinary weather.
- A cartridge needs hosing every week or two; a sand filter wants backwashing every few days.
- Water clears slowly after a storm or an algae treatment, because the filter is plugging as fast as it catches.
Each has other causes, from a chlorine problem to a failed cartridge, so check with numbers: enter the filter's area and your highest flow in the calculator. Above 100% of the code rate, the fix is less flow or more filter. On a variable-speed pump, less flow is free: filter at a lower speed and keep high speed for short jobs. A bigger pump makes it worse, which is the pump side of the same question in the pump sizing calculator. If the pressure is high from the moment the filter is clean, the restriction is somewhere else; total dynamic head covers reading the gauge. Which filter type to choose is on the gear page, and what a filter can and cannot clear is in the cloudy water guide.