Pool Filter Size Calculator (sq ft or m²)

Q: What size pool filter do I need for a 50,000 litre pool?

On an eight-hour turnover a 50,000 L pool needs about 104 L/min of flow (50,000 ÷ 480). Divide that by the media loading rate and the minimum areas are roughly 0.13 m² of sand, 1.28 m² of DE, or 2.56 m² of cartridge — 1.4, 13.8 and 27.5 sq ft. In practice a cartridge is bought several times larger than its minimum, because the sustained loading a cartridge is happy at is far below its peak design rate. Enter 50,000 with the unit chip set to L and the calculator above prints all three.

Q: How do I convert a filter size from square feet to square metres?

Multiply square feet by 0.09290304 — the exact factor published in NIST’s SI guide, Appendix B.8 . A 100 sq ft cartridge element is 9.29 m²; a 150 is 13.94 m². Flow converts with the gallon factor from the same table: 1 US gallon = 3.785412 L, so 1 GPM = 3.785412 L/min. The calculator prints both units on every line, so you should not have to do either conversion by hand.

Q: Is there an Australian standard turnover rate I should size to?

Not one national number we can hand you. Requirements for public pools and spas in Australia are made at state and territory level — NSW Health, for example, publishes its own Public Swimming Pool and Spa Pool Advisory Document covering pools in that state. Find the document that governs the venue you are working on and size to what it says; a calculator cannot substitute for it. Private backyard pools generally sit outside those codes altogether, which is where the eight-hour turnover survives as a design convention rather than a rule.

Size a sand, cartridge, or DE pool filter in square feet or square metres: required flow from turnover (or your pump), then minimum filter area per media. No login.

Required flow rate

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Enter your pool's flow (from turnover, or your pump's GPM) to see the filter size.

Computed pool volume From dimensions (turnover mode).: —
Flow source The math, with your numbers.: —

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How this is calculated

Filter sizing starts with flow, not the filter. The required flow is the gallons the system must move per minute to turn the pool over in your chosen time: required GPM = volume ÷ (turnover hours × 60). (In By pump GPM mode you skip this and use your pump's max flow directly.) The minimum filter area for each media is then area = required GPM ÷ the media's design rate.

required GPM = volumeGal / (turnoverHours × 60)     (or: your pump's GPM)
min area     = required GPM / design rate (GPM per sq ft)

metric form: required L/min = litres / (turnoverHours × 60)
             min area (m²)  = required L/min / rate (L/min per m²)

design rates:  sand 20 · DE 2 · cartridge 1 GPM per sq ft
               sand 815 · DE 81.5 · cartridge 40.7 L/min per m²
recommended  = min area × 1.25 (optional headroom)

units: 1 US gal = 3.785412 L · 1 sq ft = 0.09290304 m² (NIST SP 811 B.8)
       1 imperial gal = 4.54609 L — NOT the same gallon

dimensions mode: volumeGal = pool-volume engine (shape × avg depth × 7.48 gal/ft³)
                 avgDepth = (shallow + deep) / 2 (deep blank ⇒ shallow)

We publish the design rate we used rather than hiding it — sand 20, DE 2, cartridge 1 GPM/sq ft — so you can translate GPM into the square feet you actually shop for. These rates are a band, not a single magic number: cartridge especially is sold much larger than the 1 GPM/sq ft peak figure because sustained daily loading is closer to 0.3–0.35 GPM/sq ft. The pump-vs-filter check compares your pump's GPM against a filter's max throughput (sand 22, DE 1.75, cartridge 0.35 GPM/sq ft) — a pump must never exceed the filter's rated flow.

In From dimensions mode the gallons come from the same engine the Pool Volume Calculator uses (7.48 gal/ft³, primary-source validated shape constants), not a rule-of-thumb multiplier. Suggested stock sizes are illustrative shopping hints, not a product catalog. This is an advisory planning figure — verify against the filter's rated capacity and your pump's flow curve before buying.

Square footage is also what the service side of the filter runs on. Once a DE filter is sized, the grid area decides the recharge, and the DE powder recharge calculator converts that area into the scoops a backwash actually needs. If you clean filters for customers rather than just spec them, the filter cleaning pricing calculator turns cartridge count, teardown time and media cost into a price for the visit. And when the sizing exercise ends with the verdict that the existing tank is simply too small, what the swap quotes at with hardware, labor and disposal on separate lines is the next question to answer.

Working in litres and m²? Enter litres, L/min and metres — every result prints its metric twin, so you never convert by hand. The engine computes in US gallons and converts at the edge using the exact NIST factors (1 US gallon = 3.785412 L, 1 sq ft = 0.09290304 m²). One trap worth naming: an imperial gallon is 4.54609 L, about 20% larger than the US gallon most pool charts use — if a chart just says "gallons", check which gallon before you trust the square footage.

Sources

How To Size a Pool Filter. InYo Pools. Retrieved Jun 20, 2026.
Pool Pump Size Calculator. Pool Research. Retrieved Jun 20, 2026.
Filter Sizing for Swimming Pools (Ultimate Tech Manual). PoolSpaNews. Retrieved Jun 20, 2026.
NIST Guide to the SI, Appendix B.8 — gallon and square-foot conversion factors. NIST. Retrieved Aug 23, 2026.
Public Swimming Pool and Spa Pool Advisory Document (example of a state-level public-pool code). NSW Health. Retrieved Aug 23, 2026.

Filter sizing starts with flow, not with the filter. Work out the flow your pool needs in order to turn its whole volume over in the time you want — volume divided by (turnover hours × 60) — then divide that flow by the media’s design loading rate to get the minimum filter area. The calculator above runs both steps and prints every answer twice: square feet and square metres, GPM and litres per minute.

The same arithmetic, in litres and square metres

Nearly every published filter loading rate is written in US units, because the trade literature that fixed them is American. That does not make the rates American — they convert cleanly. The Ultimate Tech Manual from Pool & Spa News gives the three figures most of the trade works from: high-rate sand filters water at 20 GPM per square foot, DE models at 2 GPM/sq ft, and cartridge units at 1 GPM/sq ft. Converted with the exact factors in NIST’s SI guide, Appendix B.8 (1 US gallon = 3.785412 L, 1 sq ft = 0.09290304 m²), the same rates read:

Media	Design rate (US)	Design rate (metric)
High-rate sand	20 GPM/sq ft	815 L/min per m²
DE	2 GPM/sq ft	81.5 L/min per m²
Cartridge (peak)	1 GPM/sq ft	40.7 L/min per m²
Cartridge (sustained)	0.3–0.35 GPM/sq ft	12–14 L/min per m²

A metric calculation therefore never has to touch gallons at all. A 50,000 L pool on an eight-hour turnover needs 50,000 ÷ (8 × 60) = 104 L/min, and 104 L/min ÷ 815 L/min per m² = 0.13 m² of sand. The tool reaches the same answer through its gallons core and shows you both figures side by side, so nothing is lost in translation.

Pool filter size chart — eight-hour turnover

Pool volume	Required flow	Sand (min)	DE (min)	Cartridge (min)
20,000 L · 5,280 gal	41.7 L/min · 11.0 GPM	0.05 m² · 0.6 sq ft	0.51 m² · 5.5 sq ft	1.02 m² · 11.0 sq ft
30,000 L · 7,930 gal	62.5 L/min · 16.5 GPM	0.08 m² · 0.8 sq ft	0.77 m² · 8.3 sq ft	1.53 m² · 16.5 sq ft
40,000 L · 10,570 gal	83.3 L/min · 22.0 GPM	0.10 m² · 1.1 sq ft	1.02 m² · 11.0 sq ft	2.05 m² · 22.0 sq ft
50,000 L · 13,210 gal	104.2 L/min · 27.5 GPM	0.13 m² · 1.4 sq ft	1.28 m² · 13.8 sq ft	2.56 m² · 27.5 sq ft
60,000 L · 15,850 gal	125.0 L/min · 33.0 GPM	0.15 m² · 1.7 sq ft	1.53 m² · 16.5 sq ft	3.07 m² · 33.0 sq ft
75,000 L · 19,810 gal	156.2 L/min · 41.3 GPM	0.19 m² · 2.1 sq ft	1.92 m² · 20.6 sq ft	3.83 m² · 41.3 sq ft

Those are minimum areas at the design rates above, not shopping sizes. Round up to the next stock size, and treat the cartridge column as a floor rather than a target — see the sustained-loading trap below. Sand is efficient enough per unit of area that its column is usually cleared by any standard tank, so the binding constraint there is the tank’s own maximum flow rating rather than its bed area — that is the check the tool runs in By pump GPM mode. If you do not know your volume, the pool volume calculator works in litres or gallons from your pool’s shape and depths, and the pool surface area calculator covers the resurfacing and cover side of the same measurement.

Turnover time is a convention, not a universal rule

The eight-hour turnover the chart uses is a design habit, and it is worth being straight about where it comes from. InYo Pools puts it as a suggestion rather than a requirement — “we suggest having a pump and filter that can cycle through your pool’s volume within 8 hours” — and that is exactly the right weight to give it for a private backyard pool.

Public and commercial pools are a different matter: they are governed by the health code of the jurisdiction they sit in, and that code, not a calculator, sets the turnover you have to meet. In Australia those requirements are made at state and territory level rather than nationally — NSW Health, for instance, publishes its own Public Swimming Pool and Spa Pool Advisory Document for public pools and spas in that state. There is no single national figure we can honestly quote you, so we will not invent one. Find the document that covers the venue you are working on and size to what it says. It is a better job to do before the season than during it — in Australia and New Zealand that means late winter, since the opening window runs through September and October, the mirror image of the northern-hemisphere calendar most pool content assumes. See sizing a filter ahead of the AU/NZ opening season for the rest of that pre-season checklist.

Three traps that only show up in metric

Not every gallon is the same gallon. A US gallon is 3.785412 L; an imperial gallon is 4.54609 L, about 20% larger. Both factors come from the same NIST conversion table. A chart that says “gallons” without saying which one is a chart you cannot safely convert, and the 20% gap is more than enough to push you a filter size in the wrong direction. Everything on this page, and everything the calculator computes, is US gallons.

Cartridge filters are bought far larger than the design rate suggests. The 1 GPM/sq ft figure is a peak rate. Pool Research puts a cartridge’s practical per-square-foot loading at 0.3–0.35 GPM — 12 to 14 L/min per m² — and at that rate a 50,000 L pool wants roughly 7.3–8.5 m² (79–92 sq ft) of element, which is why 100 sq ft (9.3 m²) cartridges are ordinary on residential pools. Size up if in doubt; the penalty for a slightly large cartridge is money, and the penalty for a small one is cleaning it every other week.

Filter area is not sand quantity. A sand filter carries two separate specs: the bed area that sets its flow rating, and the sand charge the tank holds, quoted in kilograms in metric markets and pounds in the US. Sizing is the first; the charge comes off the tank’s own label and does not substitute for it. If a search sent you here looking for how much sand a filter takes, that number belongs to the specific tank, not to your pool’s volume.

Where this sits beside the rest of the pool set

Flow is the shared variable across the whole equipment pad, so the sizing tools chain together. The pool pump size calculator picks the pump for the flow you just computed, and the pool pump run time calculator turns that flow back into daily hours once the pump is on the wall, and the pool pump energy cost calculator prices what the pump costs to run at that flow against published US, UK, and Australian electricity rates. If the pump is variable-speed, the variable-speed pump savings calculator prices the difference in running cost — the slower, longer turnover a VS pump makes practical is easier on the filter as well as the power bill. On a salt pool the salt cell sizing calculator sizes the chlorinator to the same volume, and the pool water chemistry hub covers the balancing side that filtration alone will not fix.

Filtration is advisory design guidance here, not a regulatory rate or an engineering specification. Verify any size against the filter’s own rated capacity, your pump’s flow curve, and the code that governs the pool before you buy anything.

Frequently asked questions

What size pool filter do I need for a 50,000 litre pool?

On an eight-hour turnover a 50,000 L pool needs about 104 L/min of flow (50,000 ÷ 480). Divide that by the media loading rate and the minimum areas are roughly 0.13 m² of sand, 1.28 m² of DE, or 2.56 m² of cartridge — 1.4, 13.8 and 27.5 sq ft. In practice a cartridge is bought several times larger than its minimum, because the sustained loading a cartridge is happy at is far below its peak design rate. Enter 50,000 with the unit chip set to L and the calculator above prints all three.

How do I convert a filter size from square feet to square metres?

Multiply square feet by 0.09290304 — the exact factor published in NIST’s SI guide, Appendix B.8. A 100 sq ft cartridge element is 9.29 m²; a 150 is 13.94 m². Flow converts with the gallon factor from the same table: 1 US gallon = 3.785412 L, so 1 GPM = 3.785412 L/min. The calculator prints both units on every line, so you should not have to do either conversion by hand.

Is there an Australian standard turnover rate I should size to?

Not one national number we can hand you. Requirements for public pools and spas in Australia are made at state and territory level — NSW Health, for example, publishes its own Public Swimming Pool and Spa Pool Advisory Document covering pools in that state. Find the document that governs the venue you are working on and size to what it says; a calculator cannot substitute for it. Private backyard pools generally sit outside those codes altogether, which is where the eight-hour turnover survives as a design convention rather than a rule.

Why is a cartridge filter so much bigger than a sand filter for the same pool?

Because the media works differently. High-rate sand is rated at about 20 GPM per square foot (≈815 L/min per m²) while cartridge is rated at roughly 1 GPM per square foot (≈40.7 L/min per m²) — a twentyfold difference in the area needed for the same flow. Cartridge is then sized larger again: many technicians target a sustained loading nearer 0.3–0.35 GPM per square foot (≈12–14 L/min per m²) so the element is not cleaned every fortnight, which is why 100–150 sq ft (9.3–13.9 m²) elements are common on ordinary residential pools.

Should I size the filter to the pump, or the pump to the filter?

Size both to the flow the pool needs, then check they agree. Work out required flow from volume and turnover first, pick a filter whose area covers it, and confirm the filter’s maximum rated throughput is at or above the pump’s flow — a pump must never over-drive a filter. Switch the tool to By pump GPM with a single media selected and it runs that check for you. Sizing the pump itself is the job of the pool pump size calculator.

Sources

How To Size a Pool Filter. InYo Pools. Retrieved Aug 23, 2026.
Pool Pump Size Calculator. Pool Research. Retrieved Aug 23, 2026.
Filter Sizing for Swimming Pools (Ultimate Tech Manual). PoolSpaNews. Retrieved Aug 23, 2026.
NIST Guide to the SI, Appendix B.8 — gallon and square-foot conversion factors. NIST. Retrieved Aug 23, 2026.
Public Swimming Pool and Spa Pool Advisory Document. NSW Health. Retrieved Aug 23, 2026.

How we choose and check our formulas