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What Well Water Filtration System Do I Need?

Tell the tool how many bathrooms and people the well serves and which problems your water test or your eyes have found. It returns the service flow every whole-house stage needs to be rated for, the stages in the order they should be plumbed, and the verified systems we cover that match.

What is wrong with the water? (check all that apply)

From the pump label or the well log. A treatment system cannot deliver more than the pump does.

Required service flow

10GPM

Every whole-house stage you buy should be rated for at least this service flow.

Why this number
  1. Showers at once: the smaller of 2 bathrooms and 4 people ÷ 2, rounded up = 2 (our assumption). × 2.5 GPM each (federal maximum showerhead flow, 10 CFR 430.32) = 5 GPM.
  2. One kitchen faucet: + 2.2 GPM (federal maximum faucet flow).
  3. One more fixture (toilet refill, washer or hose): + 2.2 GPM (our assumption).
  4. Total: 9.4 GPM, rounded up to a whole number, never below 7 GPM = 10 GPM.

How the flow rate is calculated

Whole-house treatment equipment is rated by service flow in gallons per minute. Undersize it and pressure drops whenever two showers run; oversize an oxidizing filter and the media may not backwash properly. The tool estimates peak demand from fixtures, not from square footage, because fixtures are what draw water.

Fixture rates are the federal maximums in 10 CFR 430.32: showerheads may deliver no more than 2.5 gallons per minute and faucets no more than 2.2. WaterSense-labeled fixtures are lower (2.0 and 1.5 GPM), so sizing on the federal maximums is conservative. Simultaneity is our assumption: the number of showers running at once is the smaller of your bathroom count and half your household rounded up, plus one kitchen faucet, plus one more fixture (a toilet refilling, a washer or an outside tap) at 2.2 GPM. The result is rounded up to a whole number and never below 7 GPM, the smallest size that makes sense for a whole-house system.

If you enter your pump output, the tool caps the requirement there. A treatment system rated higher than the pump cannot deliver more water than the pump does; a well contractor, not a filter, is the answer when the pump cannot keep up.

Why the stages are in that order

Each stage protects the next one, which is why every state extension guide and every manufacturer manual puts them in the same sequence. A sediment filter goes first so grit never reaches a control valve. An oxidizing filter for iron, manganese and hydrogen sulfide comes next, because those foul softener resin and coat a UV sleeve. A softener follows, sized on compensated hardness. UV disinfection is always the last whole-house stage, since it needs clear, iron-free water to reach the dose an NSF/ANSI 55 Class A listing requires. Nitrate and PFAS are treated at the kitchen tap with reverse osmosis, because whole-house treatment for drinking-water contaminants is rarely worth the cost.

How the product matches are chosen

The tool looks only at products in our verified data that are not dealer-only, keeps those whose published service flow meets your requirement, and picks the ones that cover the most of the problems you checked, preferring ranked systems and then lower price. Treatment claims are the manufacturers' own; a product listed for iron is rated for a maximum iron level, so compare that number to your lab result on its product page before buying. When no single system matches, the problem pages describe the treatment types and the installer directory lists companies that design custom trains.

Before you size anything

Test the water. EPA recommends private wells be tested every year for total coliform bacteria, nitrate, total dissolved solids and pH, and the only way to choose between an iron filter and a softener, or to know whether you need UV at all, is a number from a certified lab. The testing guide covers what to order and how to read the report; the cost guide covers what the resulting system will run.

Frequently asked questions

How many GPM does a well water filtration system need?

Enough to keep up with the fixtures that run at the same time. Our tool adds the showers likely to run at once (never more than your bathrooms or half your household) at the federal maximum of 2.5 gallons per minute each, plus one faucet at 2.2 GPM and one more fixture, and rounds up. A one-bathroom, two-person house lands at about 7 GPM; three bathrooms and six people at about 12 GPM. Your well pump sets the ceiling.

What if my well pump produces less than the tool recommends?

Then the pump, not the filter, limits your water, and buying a larger system gains nothing. Size the treatment stages to the pump output, keep pressure drop low with full-port valves and the right pipe size, and talk to a well contractor if peak demand regularly exceeds what the pump and pressure tank can deliver.

Why does the order of the treatment stages matter?

Each stage protects the next. Sediment filtration keeps grit out of every valve downstream. Iron, manganese and hydrogen sulfide must be oxidized and filtered out before a softener, or they foul the resin, and before UV, or they coat the quartz sleeve. UV goes last because it needs clear, low-iron water to work. Nitrate and PFAS are treated at the kitchen tap because that is the water you drink.

Can one system treat everything?

Combination systems bundle an oxidizing iron filter with a softener or UV in one package, and the tool prefers them when they cover more of what you checked. No whole-house unit treats nitrate or PFAS economically; those are point-of-use problems. If nothing matches, the problem pages list the treatment types and a local installer can design a custom train from your lab report.

I do not know what is wrong with my water. What should I do first?

Get a certified lab test. EPA and CDC recommend testing private wells every year for total coliform bacteria, nitrate, total dissolved solids and pH, and adding iron, manganese, hardness and hydrogen sulfide whenever you see stains, scale or smell odor. Treatment chosen without a test is a guess, and the wrong system can make the water worse.

Sources

Figures and quotations above come from these pages as read on 2026-10-05. These are reference links, not affiliate links.

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