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What Size Cartridge Filter for My Pool?
Quick answer: For a standard 8-hour turnover, you need roughly 1 m² of cartridge filter area for every 7,200 litres of pool water. A 50,000-litre pool needs a cartridge filter rated around 7 m² (about 75 sq ft) of filtration area. Undersizing the filter is the single most common cause of short cartridge life and cloudy water — bigger is close to free in running cost, and too small forces the filter to work harder and clog faster.
Work out the filter size for your pool
Cartridge filters are sized by surface area, not flow rate alone, because the area is what determines how much debris the cartridge can hold before pressure builds up and flow drops off. The standard planning method is turnover-based:
- Work out your pool volume in litres (length × width × average depth × 1,000 for a rectangular pool, in metres).
- Divide by 480 (8 hours × 60 minutes) to get the flow rate in litres per minute needed for an 8-hour turnover.
- Divide that flow rate by roughly 15 — the standard cartridge filtration rate in litres per minute per square metre — to get the minimum filter area you need.
Planning figures for common pool sizes (8-hour turnover):
| Pool volume | Required flow | Minimum cartridge area |
|---|---|---|
| 20,000 L | ~42 L/min | ~2.8 m² (~30 sq ft) |
| 30,000 L | ~63 L/min | ~4.2 m² (~45 sq ft) |
| 40,000 L | ~83 L/min | ~5.6 m² (~60 sq ft) |
| 50,000 L | ~104 L/min | ~6.9 m² (~75 sq ft) |
| 70,000 L | ~146 L/min | ~9.7 m² (~105 sq ft) |
| 90,000 L | ~188 L/min | ~12.5 m² (~135 sq ft) |
These are planning figures, not a replacement for the manufacturer's rated flow and area on the specific cartridge filter you're looking at — always check the datasheet, since rated area varies between single and multi-cartridge units of similar physical size.
Why bigger is usually the right call
Unlike a pump, an oversized cartridge filter doesn't cost you more to run — it just has more surface area to catch debris before it needs cleaning, and lower flow resistance per square metre means less strain on the pump. The main reason not to oversize dramatically is upfront cost and physical space, not running cost. If you're between two sizes, sizing up is the safer choice.
Cartridge vs sand filtration
Cartridge filters trap finer particles than a sand filter and don't need backwashing, which saves water — you clean the cartridge by hosing it off instead. The trade-off is a cartridge needs physical removal and cleaning every few weeks in swim season, and eventually needs replacing (typically every 2–5 years depending on use and water balance), whereas a sand filter's media lasts years longer between changes but uses more water on backwash cycles.
When to clean or replace your cartridge
Clean the cartridge when your filter pressure gauge reads 8–10 psi above the clean, just-cleaned baseline reading — that pressure rise means debris is restricting flow, not that the filter needs replacing yet. Rinse thoroughly with a hose, working into the pleats, and consider a degreasing filter cleaner solution every few cleans to clear oils and sunscreen residue that a hose alone won't shift. Replace the cartridge itself when it no longer restores full flow after a proper clean, when pleats are visibly torn or collapsed, or once it's had roughly its expected service life for your bather load and season length.
FAQ
What happens if my cartridge filter is too small? It clogs faster, needs cleaning more often, and struggles to keep up with debris load and turnover — often showing up as cloudier water despite a filter that's technically working.
Can I fit a bigger cartridge filter than my pump needs? Yes, and it's generally a good idea. A larger filter area relative to your pump's flow rate means lower pressure and longer intervals between cleans, with no downside beyond upfront cost.
How often should I clean a cartridge filter? As a guide, every 4–6 weeks in peak swim season and less often in cooler months — but let the pressure gauge, not the calendar, be the real trigger.
How long does a pool cartridge filter last? Typically 2–5 years with regular cleaning, though heavy bather load, poor water balance, or infrequent cleaning can shorten that considerably.