Pool Filter Size Calculator
Sizes your filter to your pump's ACTUAL flow — the operating-point flow from the pump-sizing calculator, not the nameplate — and shows you why bigger is genuinely better here, opposite of the pump page. Sand, cartridge, or DE — three correct areas for the same pool, the universal industry misconception corrected.
Hook
Pool filters are sold by square footage — and the number lies across media.
A sand filter rated at 4 square feet and a cartridge rated at 150 do the same job for the same pool. The area number means nothing on its own, because each kind of filter is designed to run at a wildly different speed. What actually matters is the filtration rate— how fast water moves through each square foot — and getting that right (usually by sizing generously) is the difference between crystal water and a filter you're cleaning every week.
Promise
This calculator sizes your filter to your pump's actual flow — the operating-point flow from the pump-sizing calculator, not the nameplate — and shows you the minimum area for sand, cartridge, and DE, plus the recommended 67 % generous size-upfor cleaner water and longer runs between cleanings. Unlike a pump, where bigger is usually a mistake, a bigger filter is genuinely better here, and we'll show you exactly why. Every term derived on the page.
Here's the deal: a filter's job is to pass your pool's water through media slowly enough to catch the dirt. Push water through too fast — too few square feet for your flow — and the dirt blows through; the filter clogs in days and never gets the water truly clear. Give it more area and the water slows down, the media catches finer particles, and you clean it far less often. With filters, unlike pumps, generosity pays.
What you'll give us
The design flow (from turnover, or pasted from the pump-sizing calculator's operating-point output), the filter media, and optionally your existing filter area to check whether it's adequate. The volume calculator deep-links here with ?gal= prefilled; the pump-sizing calculator can deep-link with ?gpm=.
The calculator
Pick the flow source, the media, and (optionally) your existing area. The result panel shows the minimum and the generous-recommended areas for your media, the cross-media comparison (the rating game), the undersized verdict for an existing filter, and the honest media trade-off summary.
Bigger is better here — the opposite of the pump page
If you've used our pump-sizing calculator, you learned that a bigger pump is usually a mistake — it costs more, runs louder, and the pipes won't let it deliver. (The pump-cost calculator puts a dollar on that: roughly $9 more every month for the next-size-up pump.) Filters are the opposite, and it's worth understanding why, because the instinct to “not oversize” is exactly wrong here.
A pump pushes against resistance that climbs with size; a filter just provides area for water to pass through slowly. More area means the water moves slower through the media, which means finer particles get caught and the filter holds more dirt before it needs cleaning. With a pump, size to your need. With a filter, size up — about 67 % over the bare minimumis the engine's honest recommendation, derived from running the filter at 60 % of its media's max design rate.
Slower is cleaner: the filtration rate
The single number that decides how well your filter works isn't its size — it's its rate: how many gallons per minute pass through each square foot. Push water through fast and it doesn't have time to drop its dirt; the fine particles ride straight through and your water stays faintly cloudy no matter how long you run the pump.
Slow it down — more area for the same flow — and the media catches finer particles, and it can hold more dirt before the pressure climbs and you have to clean it. A sand filter run at half its maximum rate doesn't just filter a bit better; it filters finerand runs far longer between cleanings. 6 sqft of sand at 65 GPM (10.83 GPM/sqft) beats 4 sqft (16.25, near max) on both clarity and cycle length — and that's the same flow through the same media. The rate is the lever.
The rating game: why area is meaningless across media
Here's the thing the square-footage number hides. A filter's area only means something next to its media's design rate — and those rates are wildly different. Sand runs fast, so it needs little area; cartridge runs slow, so it needs a lot.
For the same pool and the same 65 GPM flow, you might need under four square feet of sand, or over one hundred and seventy of cartridge— and both are correctly sized. So when you see “150 sq ft cartridge” next to “4.9 sq ft sand,” you're not comparing big to small. You're comparing two filters that do the same job, sized for their media. Judge a filter by its rate for your flow, never by its area alone — that's the universal industry misconception this page corrects.
Sand, cartridge, or DE? — even-handed
Three media, three sets of trade-offs, no universal winner. Sand is the cheapest and simplest, backwashes in minutes, and filters to about twenty to forty microns — fine for most pools. Cartridge filters finer (ten to twenty microns), needs no backwashing so it saves water, but you hose the elements by hand and replace them every few years.
DE is the finest (three to five microns — polished, sparkling water) but it's the most maintenance and the messiest to clean. Pick for your priorities — simplicity, water savings, or the clearest possible water — then size that media generously. The choice is yours; the sizing math is the same.
Where the numbers come from
The cluster's fourth hydraulics module, purely additive. Imports the operating-point flow physics from the pump-side, the filter ΔP constants from the flagship — and adds only the filtration-rate model.
Step 1 · design flow = pump operating-point flow
The filter must handle the actual flow your pump delivers — the operating point on your plumbing, NOT the box rating. We consume
requiredFlowGpmfrom the page-14 turnover engine OR you paste the GPM directly from the pump-sizing calculator. Sizing to the nameplate HP guesses wrong by a meaningful margin.Step 2 · filtration rate = flow ÷ area (EXACT)
rate (GPM/sqft) = flow (GPM) / area (sqft). The core metric — the rate of how fast water passes through each square foot of media. Below the media max: cleaner water + longer cycles. Above: filtration fails. The math is exact; the max-rate constants below are typical-design figures.Step 3 · the media max rates (typical design figures)
Sand (high-rate): 18 GPM/sqft max — fast, short bed depth, coarsest filtration.
Cartridge: 0.375 GPM/sqft max — slow, pleated paper, mid-fine.
DE: 2 GPM/sqft max — finer than cartridge, DE-coated grids.
These are the typical design figures; specific products vary. The 48× spread between sand and cartridge is the reason the rating game exists.
Step 4 · recommend generously (67 % over minimum)
We recommend running at 60 % of the media's max rate, which sizes the area to 1.67× the minimum — about 67 % over the floor. That headroom IS the bigger-is-better wedge: finer particle capture + longer cycles between cleanings. The minimum is a floor, not the recommendation. The 67 % figure is derived from the engine's
RECOMMENDED_RATE_FRACTION_OF_MAXconstant, not a hand-typed range — single source of truth.Step 5 · sanity check (E1 / E2)
Operating point 65 GPM, sand media.
- minArea = 65 / 18 = 3.61 sqft (the floor — running at max rate, no headroom).
- recArea = minArea / 0.6 = 6.02 sqft (the generous recommendation).
- Across media at the same 65 GPM: 3.61 sqft sand / 32.50 DE / 173.33 cartridge — all correctly sized.
Step 6 · the filter-ΔP reconciliation with the pump page
Filter ΔP figures here come from the SAME constants the pump-sizing calculator shipped — Sand 10 ft at 50 GPM, Cartridge 8 ft, DE 6 ft — imported from
lib/hydraulics/headloss.ts, never redeclared. The reconciliation assertion proves the two pages cannot disagree about the same filter's resistance.
Eight worked examples
All values from the asserted lib/hydraulics/filter.ts engine. Design flow = 65 GPM (the operating point from the standard pump-sizing case).
E1 — Minimum filter area by media (the core case)
Operating flow 65 GPM (standard pool + 1.5HP pump from the pump-sizing calculator). → minimum area: 3.61 sqft sand, 32.50 sqft DE, 173.33 sqft cartridge.
Takeaway: all three are correctly sized for the same pool — the area numbers look wildly different only because each media runs at a different rate.
E2 — Size up for cleaner water (the bigger-is-better wedge)
Sand at 65 GPM: minimum 3.61 sqft at 16.25 GPM/sqft (at max) vs the generous 6.02 sqft at 10.83 GPM/sqft (60 % of max). → the 6.0 sqft filter runs in the headroom zone — finer particle capture + longer between cleanings.
Takeaway: unlike a pump, sizing the filter generously is the right move — slower water means cleaner water and less maintenance. Size up.
E3 — The rating game (the integrity wedge)
Same 65 GPM across media: 3.61 sand / 32.50 DE / 173.33 cartridge. A “173 sqft cartridge” and a “3.6 sqft sand filter” are the same-capacity filter for this pool. The area number is meaningless without the media's rate.
Takeaway: never compare filters by square footage across types. A big cartridge number and a small sand number can be identical jobs. Judge by rate for your flow.
E4 — The undersized filter (the matching warning)
A 2.5 sqft sand filter on a 65 GPM flow → 26.00 GPM/sqft, well above the 18 max (1.44× the max).
Dirt blows through (cloudy water that won't clear) AND the filter clogs in days. Size up to at least 3.61 sqft (the floor), better 6.02.
Takeaway:a filter too small for your pump's flow fails twice — it filters badly and needs constant cleaning. Match it to the operating flow.
E5 — Matching the filter to the real pump flow (the cross-page tie)
Pump-sizing says the operating point is 65 GPM(not the 1.5HP “nameplate” some assume means more flow). Size the filter to 65, not to a guessed higher number — using the actual delivered flow avoids both over- and under-buying.
Takeaway: size to what your pump actually delivers (the operating point), which the pump calculator gives you — not to the horsepower on the box.
E6 — Cartridge: the water-saving choice (media trade-off)
65 GPM, cartridge (~173.33 sqft minimum, recommend ~288.89 sqft): no backwash → saves the water sand/DE send down the drain each clean.
Takeaway: cartridge filters cost you labor and area but save the water backwashing wastes — a real consideration where water is dear.
E7 — DE: the clearest water (media trade-off)
65 GPM, DE (~32.50 sqft minimum, recommend ~54.17 sqft): filters to 3–5 microns — the sparkling end of the range. The trade: finest filtration, but the most maintenance and the messiest (the DE powder) and backwash water use.
Takeaway:if you want the clearest possible water and don't mind the upkeep, DE is the finest filter — at the cost of the most hands-on maintenance.
E8 — Metric / larger pool
A larger pool with an operating flow ~90 GPM → minimum areas scale linearly: ~5.0 sand, ~45.0 DE, ~240 cartridge (sqft).
Takeaway: the rate math is identical at any flow — more flow needs more area, in the same media-relative proportions.
Reference tables
T1 · Minimum filter area by flow × media
EXACT rate math · minimum area = flow ÷ media max-rate. The minimum (filter at media max — no headroom); the generous recommended area is ~1.67× this (60 % of max rate).
| Flow (GPM) | Sand (sqft) | DE (sqft) | Cartridge (sqft) |
|---|---|---|---|
| 40 | 2.22 | 20.0 | 107 |
| 50 | 2.78 | 25.0 | 133 |
| 60 | 3.33 | 30.0 | 160 |
| 70 | 3.89 | 35.0 | 187 |
| 80 | 4.44 | 40.0 | 213 |
| 90 | 5.00 | 45.0 | 240 |
| 100 | 5.56 | 50.0 | 267 |
T2 · Filtration rate & fineness by media
TYPICAL design figures — max rates vary by specific product. The rate math is EXACT; these are the design ceilings each media is built around.
| Media | Max rate (GPM/sqft) | Recommended rate (60 %) | Fineness (μm) |
|---|---|---|---|
| Sand | 18 | 10.80 | 20–40 |
| Cartridge | 0.375 | 0.22 | 10–20 |
| DE | 2 | 1.20 | 3–5 |
T3 · Media comparison — even-handed trade-offs
Even-handed · no “best” — pick for your priorities, then size that media generously. Typical recommended area is for the 65 GPM standard pool.
| Media | Cost | Maintenance | Backwash / water | Typical recommended sqft @ 65 GPM |
|---|---|---|---|---|
| Sand | Low (~$300–$600) | Low — backwash 5 min | Yes (uses water) | 6.0 |
| Cartridge | Mid (~$400–$900) | Mid — hose elements, replace every few years | No (saves water) | 288.9 |
| DE | Mid-High (~$700–$1,500) | Highest — recharge DE after each backwash | Yes (uses water) | 54.2 |
Tables released CC BY 4.0. T1 area math is exact (flow ÷ media max). T2 max rates are typical design figures (vary by product). T3 cost/maintenance values are typical residential ranges.
Methodology & sources
Filtration rate = flow ÷ area — filtration physics. The design flow is the pump's OPERATING-POINT flow from the SHIPPED pump-sizing calculator, not the pump's nameplate HP — what your filter actually has to handle. Required-flow-from-turnover comes from the page-14 run-time calculator's requiredFlowGpm. One source for each.
Media max-rate constants (typical design figures).Sand at 18 GPM/sqft, DE at 2.0, cartridge at 0.375 — the values around which each media is designed. The rate math (flow ÷ area = GPM/sqft) is EXACT physics; the max-rate constants vary by specific product and shoulder the “typical” honesty label. The 48× spread between sand and cartridge is THE reason filter areas can't be compared across media.
The 67 % generous size-up. We recommend running at 60 % of the media max, which sizes the area to 1.67× the minimum — a 67 % size-up. This figure is derived from the engine's RECOMMENDED_RATE_FRACTION_OF_MAX constant, NOT a hand-typed range: same single-source discipline as every other physical constant in this site. The minimum is a floor (filter at max rate, no headroom); the recommended generous area is the headline.
The pump-filter ΔP reconciliation. The filter ΔP figures here (Sand 10 ft / 50 GPM, Cartridge 8, DE 6) are IMPORTED from lib/hydraulics/headloss.ts — the same constants the pump-sizing calculator shipped — never redeclared. The reconciliation assertion proves the two pages CANNOT disagree about the same filter's resistance. Inventing different ΔP figures here would silently make the pump page and the filter page describe the same filter differently — exactly the integrity failure the assertion pattern exists to prevent.
Bigger is better here — the pump-filter mirror.Where the pump-sizing page's identity is BIGGER-IS-USUALLY-WORSE, this page's is BIGGER-IS-GOOD. Two pages, opposite truths, both correct — different physics (pump pushes against rising resistance; filter just provides area for slower flow). We do NOT import the pump page's oversizing-penalty logic; the contrast IS the page's identity.
Even-handed media + filtration ≠ sanitation. Sand / cartridge / DE each have real trade-offs (fineness, maintenance, water use, cost); no “best” — pick for your priorities, then size that media generously. And the boundary inherited from the run-time page: filtration catches particles; chlorine sanitizes. A bigger filter doesn't replace chemistry (see chlorine, pH, LSI).
Cluster closer. The pump-running-cost calculator closes the cluster — it consumes the operating-point power and reuses lib/thermal/fuelcost.ts's electricity rate, so the pump-cost and cost-to-heat pages cannot disagree about a kWh price. The engine investment compounds across the four clusters.
Reference tables T1/T2/T3 released under CC BY 4.0. T1 area math is EXACT; T2 max rates are typical design figures; T3 cost/maintenance values are typical residential ranges.
Frequently asked questions
- What size filter do I need for my pool?
Size it to your pump's operating-point flow (not its nameplate HP), then pick the smallest area that delivers your media's recommended rate. For 65 GPM on the standard pool: ~3.6 sqft sand minimum, ~6 sqft sand recommended generous; ~32.5 sqft DE; ~173 sqft cartridge. All three are correctly sized for the same pool — the area numbers differ only because each media's design rate is different.
- Can a pool filter be too big?
No — bigger is genuinely better here, the opposite of pump sizing. More filter area means the water moves slower through the media, which means finer particles get caught and the filter holds more dirt before it needs cleaning. The recommended generous size-up over the bare minimum is about 67 %(running at 60 % of the media's max rate). With pumps, size to your need; with filters, size up.
- Why is a cartridge filter so much bigger than a sand filter?
Because each media has a wildly different design rate. Sand runs at up to 18 GPM/sqft, cartridge at only 0.375 — a 48× difference. So for the same flow, cartridge needs about 48× the area sand does. A 173 sqft cartridge and a 3.6 sqft sand filter can be the same job for the same pool. Area is meaningless across media; only the rate matters.
- Sand, cartridge, or DE — which is best?
No universal best — each has real trade-offs. Sand: cheapest, simplest, backwashes in minutes, filters to 20-40 microns. Cartridge: finer (10-20 μm), saves water (no backwash), but more hand-cleaning. DE: finest (3-5 μm, polished sparkling water) but the most maintenance and the messiest. Pick for your priorities — simplicity, water savings, or finest possible — then size that media generously. The sizing math is the same.
- What happens if my filter is too small?
It fails twice. With too few square feet for your flow, the rate exceeds the media's max design rate, and the dirt blows through (cloudy water that won't clear) AND the filter clogs in days (constant backwashing/cleaning). The double penalty of too-small. Size up to at least the minimum, ideally the generous recommendation (~67 % bigger than the minimum).
- What size filter for my pump?
Size to the pump's OPERATING-POINT flow(the actual delivered flow on your plumbing, from the pump-sizing calculator), NOT the nameplate HP. The nameplate is wrong by a meaningful margin because pumps don't deliver a fixed flow — they deliver wherever their curve crosses your plumbing's resistance. A 1.5 HP pump might deliver 65 GPM on standard plumbing, not the 80+ the box implies; size the filter to 65.
- What is filtration rate / GPM per square foot?
It's the real metric: how many gallons per minute pass through each square foot of filter media. The lower the rate, the cleaner the filtration (finer particles get caught) and the longer between cleanings. Each media has a max design rate (sand 18, DE 2.0, cartridge 0.375 GPM/sqft); we recommend running at about 60 % of that max for headroom.
- Does a bigger filter mean cleaning it less often?
Yes — meaningfully. More area means slower rate, and slower rate means each square foot of media accumulates dirt more slowly while filtering finer particles. A 6 sqft sand filter at 65 GPM (10.83 GPM/sqft, 60 % of max) cycles materially longer between cleanings than a 4 sqft (16.25 GPM/sqft, 90 % of max). Cleaning frequency drops; clarity improves.That's the bigger-is-better wedge.
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