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Pool Filter Size Calculator — Sand, Cartridge & DE Sizing
Find the right pool filter size for your pool based on volume and turnover rate. Compares sand, cartridge, and DE filter sizing side by side. No sign-ups. Instant results.
💡 Quick answer — pool filter sizing basics
Required flow rate: Pool gallons ÷ (turnover hours × 60) = GPM
Standard turnover: 8 hours for residential pools | 6 hours or less for commercial
Sand filter: ~15 GPM per sq ft |
Cartridge filter: ~0.375 GPM per sq ft |
DE filter: ~2 GPM per sq ft
Bigger filter area = lower flow rate per sq ft = better filtration and longer time between cleanings.
Use the calculator below for your exact pool size.
💡 Filter size is based on flow rate, not just pool volume — a 20,000 gallon pool at a 6-hour turnover needs a larger filter than the same pool at a 12-hour turnover.
Buy what your calculator recommended:
Example calculation
A 20,000 gallon pool with a standard 8-hour turnover:
| Step | Value |
|---|---|
| Pool volume | 20,000 gallons |
| Turnover rate | 8 hours |
| Required flow rate | 41.7 GPM (2,500 GPH) |
| Minimum sand filter | 2.8 sq ft (≈ 24" tank) |
| Minimum cartridge filter | 111 sq ft |
| Minimum DE filter | 20.8 sq ft |
Formula: 20,000 ÷ (8 × 60) = 41.7 GPM. Sand: 41.7 ÷ 15 = 2.8 sq ft. Cartridge: 41.7 ÷ 0.375 = 111 sq ft. DE: 41.7 ÷ 2 = 20.8 sq ft. Add 25% for the recommended (not bare-minimum) size — see the calculator above.
Sand vs cartridge vs DE filter size — full comparison
The three filter types need dramatically different surface areas for the same flow rate, because each filters at a different rate per square foot. Smaller isn't better or worse — it reflects a tradeoff between filtration fineness, maintenance style, and water use.
| Filter type | Design rate | Filters down to | Cleaning method |
|---|---|---|---|
| Sand | ~15 GPM/sq ft | 20–40 microns | Backwash (2-3 min) |
| Cartridge | ~0.375 GPM/sq ft | 10–20 microns | Remove & hose off |
| DE (diatomaceous earth) | ~2 GPM/sq ft | 2–5 microns | Backwash + recharge with DE powder |
🏖️ Sand filters
Cheapest to buy and maintain, smallest footprint. Least fine filtration of the three. Backwashing uses 200-400+ gallons of water each cleaning. Good default choice for most residential pools.
🧻 Cartridge filters
No backwash water waste — just rinse the cartridge with a hose. Finer filtration than sand. Needs the largest surface area, so cartridge filter housings are physically bigger than sand tanks for the same flow rate.
💎 DE filters
Finest filtration available — can trap particles as small as 2-5 microns, even some algae and Cryptosporidium cysts. Higher maintenance: requires recharging with DE powder after every backwash. Best for pool owners prioritizing water clarity above all else.
⚖️ Bigger isn't always better
Oversizing your filter (not your pump) is generally beneficial — it lowers the flow rate per square foot, improving filtration and extending time between cleanings. But a filter sized far beyond your pump's flow rate may not get enough velocity to backwash effectively.
Pool turnover rate calculator — how it works
Turnover rate is the time it takes for a volume of water equal to your entire pool to pass through the filter once. It's the foundation every filter sizing calculation is built on — pool volume alone doesn't tell you what size filter you need; volume combined with your target turnover time does.
| Turnover time | Typical use case | Flow rate for a 20,000 gal pool |
|---|---|---|
| 6 hours | Heavy bather load, commercial-style circulation | 55.6 GPM |
| 8 hours | Standard residential recommendation | 41.7 GPM |
| 10 hours | Lighter use, energy-conscious | 33.3 GPM |
| 12 hours | Minimal use, variable-speed pump running longer at lower speed | 27.8 GPM |
Formula: pool gallons ÷ (turnover hours × 60) = GPM. Many pool owners with variable-speed pumps choose a longer turnover time (10-12 hours) run continuously rather than a shorter turnover run for fewer hours — this often uses less total energy while still achieving full daily circulation.
Sand filter tank size chart
Sand filters are sold by tank diameter, not square footage. Use this reference to translate your calculated filter area into the tank size you'll see listed on product pages.
| Tank diameter | Filter area | Typical pool size |
|---|---|---|
| 19" | ~2.0 sq ft | Above-ground pools, small pools |
| 24" | ~3.1 sq ft | Up to ~20,000 gallons |
| 30" | ~4.9 sq ft | 20,000–30,000 gallons |
| 36" | ~7.1 sq ft | 30,000+ gallons, larger inground pools |
Never go smaller than a 24" tank for an inground pool, even if your calculated minimum is lower — smaller tanks are typically only rated for above-ground use. When in doubt, size up to the next tank diameter.
Signs your pool filter is the wrong size
🔴 Too small
Cloudy water despite balanced chemistry, rapid pressure gauge climb, needing to backwash or clean more than once a week, and a pump that seems to be working harder than expected.
🟡 Too big for your pump
Pressure gauge barely moves between cleanings even after weeks of use, and backwashing doesn't seem to fully clear the sight glass — a sign there isn't enough flow to properly lift and clean the media.
✅ Right-sized
Pressure climbs gradually and predictably, cleaning is needed every 2-4 weeks under normal use, and water stays clear between chemical treatments.
🔧 Still cloudy after resizing?
If water stays hazy even with a correctly sized filter, the cause is usually chemistry, not equipment. Check our pool clarifier calculator or shock calculator for next steps.
Pool filter size calculator FAQs
What size pool filter do I need?
Filter size depends on your required flow rate, which is based on pool volume and desired turnover time — not pool volume alone. For an 8-hour turnover, a 20,000 gallon pool needs roughly 42 GPM of flow. That translates to about a 3 sq ft sand filter, 110 sq ft cartridge filter, or 21 sq ft DE filter at conservative design rates. Use the calculator above for your exact numbers.
How do I calculate pool filter GPM?
GPM (gallons per minute) = pool volume in gallons ÷ (turnover hours × 60). For a 15,000 gallon pool with an 8-hour turnover: 15,000 ÷ (8 × 60) = 31.25 GPM. This is the minimum flow rate your pump and filter need to fully circulate your pool water within that time window.
What is a good pool turnover rate?
8 hours is the standard turnover rate for residential pools, meaning the entire pool volume passes through the filter once every 8 hours. Some pool owners use 6 hours for heavier bather loads or 10–12 hours for lighter-use pools to save on pump energy. Public and commercial pools are typically required to turn over in 6 hours or less.
Sand vs cartridge vs DE — which filter size is right for me?
Sand filters need the smallest footprint but filter the least fine (20–40 micron), are cleaned by backwashing, and are the cheapest to buy and maintain. Cartridge filters need the largest surface area but filter finer (10–20 micron), have no backwash water waste, and are cleaned by manually rinsing the cartridge. DE filters need a mid-size footprint, filter the finest (2–5 micron), and are cleaned by backwashing plus recharging with new DE powder. Choose based on filtration quality needs, maintenance preference, and water conservation priorities, not just size.
Can a pool filter be too big?
Practically, no — oversizing a filter (not your pump) is safe and generally beneficial. A larger filter area lowers the flow rate per square foot of media, which improves filtration quality, extends time between cleanings, and reduces backpressure. The only caveat is that a sand or DE filter sized dramatically larger than your pump's flow rate may not backwash effectively, since backwashing needs adequate flow to lift and clean the media.
What happens if my pool filter is too small?
An undersized filter forces water through the media too fast for effective filtration, leading to cloudy water, rapid pressure buildup, more frequent cleaning or backwashing, and reduced filter lifespan. It also makes your pump work harder against higher resistance, increasing energy costs and wear on pump seals.
What size sand filter tank do I need?
Sand filter tanks are sized by diameter, not just square footage. A 3 sq ft filter area corresponds to roughly a 24-inch tank, commonly used for pools up to 20,000 gallons. A 4.9 sq ft filter area corresponds to roughly a 30-inch tank, suitable for larger inground pools. Use the calculator above to get your exact recommended tank diameter.
How often should I clean or backwash my filter?
Clean or backwash your filter when pressure rises 8–10 PSI above your clean, freshly-backwashed baseline reading — not on a fixed calendar schedule. Sand filters are backwashed for about 2–3 minutes until the sight glass runs clear. Cartridge filters are removed and hosed off. DE filters are backwashed like sand filters, then recharged with fresh DE powder through the skimmer.
Does my pump size need to match my filter size?
Your filter's maximum rated flow should be at least equal to — ideally 20% or more above — your pump's actual output flow rate. A filter smaller than your pump's flow rate will be overwhelmed and filter poorly. A filter larger than your pump's flow rate is safe; the pump simply won't push it to its maximum rated capacity.