The short answer: selection is "which technology fixes what the test found" — that's the field guide from step 2. Sizing is "how big does it have to be to actually work in this house" — and that's where most bad installs come from. A system sized to fit the utility room instead of the family's water use either regenerates constantly, runs out of capacity mid-shower, or can't backwash because the well can't push enough water. Four numbers decide it: grain capacity, service flow, backwash flow, and the tank and valve that deliver them.
This is step 3 of the Roadmap. It assumes you've done step 1, the test and know step 2, the equipment.
Selection: the technology picks itself if you tested
Walk the test results through the field guide:
- Hardness → softener (strong-acid-cation resin, sodium form).
- Chlorine or chloramine → carbon or catalytic carbon, ahead of the softener.
- Iron, manganese, sulfur → an oxidizing stage (air injection, Katalox, greensand, Birm, or Pyrolox for heavy loads), ahead of the softener.
- Low pH → calcite or corosex, early.
- High TDS, nitrate, arsenic, fluoride → reverse osmosis at the kitchen sink (or targeted resin/media).
- Bacteria → UV, last.
- Sediment → a sediment stage, first.
Then chain them in the order the treatment-train rules require. If you find yourself choosing equipment before mapping the test to the technology, you've skipped a step, and the house will tell you about it in six months.
Sizing a softener: grain capacity
A softener's job is measured in grains of hardness it can remove between regenerations. Three inputs:
Hardness, in grains per gallon. From your test. If there's dissolved iron in the water, the standard practice is to compensate for it — iron uses up softening capacity fast — so the effective hardness number goes up. The manufacturer's sizing table will give the compensation rule for their resin.
Daily water use. People in the home times an assumed gallons per person per day (sizing tables use a household average). Big families, teenagers and jetted tubs push it up.
Regeneration frequency. Hardness × daily gallons = grains per day. You want a capacity that regenerates on a sensible interval — often every few days to a week — not every night (wasting salt and water, wearing the resin) and not every three weeks (resin sits fouled, capacity drops). Metered valves regenerate on gallons used, which is what you want; timer valves regenerate on a schedule regardless.
Two systems with the same "grain rating" on the box can perform very differently depending on how much salt they're programmed to use per regeneration — high-salt settings squeeze more grains out of the resin but cost more salt per grain. Efficiency settings matter; learn the valve's programming, not just the tank size.
Service flow: can it keep up?
Service flow is the gallons per minute the system can deliver treated while the house is using water. Size it to the home's peak demand — two showers and a washing machine at once — not to the average. Undersize it and the family gets pressure drop and hard water sneaking through during peak use because the water moved too fast through the bed to be treated. The tank diameter, media depth and valve all set the service-flow ceiling.
Backwash flow: the one people forget
Backwashing media needs water flowing up through the bed fast enough to lift and clean it. Dense media — Pyrolox and Filox especially — need a lot of backwash flow; lighter media like Filter-Ag, Katalox Light and carbon need much less. On a well, the pump and pressure tank have to deliver that backwash flow or the media never gets cleaned and the system slowly fails.
Before you spec an iron filter on a well: measure what the well can actually deliver in gallons per minute at the treatment location. If it can't push the backwash a dense media needs, choose a lighter media or a smaller tank diameter. This single check prevents the most common well-water callback there is.
Tank and valve sizing
The tank diameter and height set the bed volume (capacity) and the flow ceilings. The valve — the control head on top — sets how the system regenerates, backwashes and meters, and each valve has its own flow limits. A big tank on a small valve chokes; a big valve on a small tank wastes money. Match them to each other and to the service and backwash numbers, using the manufacturer's tables. Where you're chaining stages, each tank gets sized for its job — the iron filter for backwash and the well's flow, the softener for grains and service flow.
Point-of-use RO sizing
Reverse osmosis at the sink is sized differently — by daily gallons of drinking and cooking water and by the membrane's production at the home's pressure and temperature. Feed it soft water (from the softener upstream) to protect the membrane, and if the family drinks a lot of water or wants the fridge and ice maker on it, size the storage tank and membrane up.
The rules of thumb, and where they stop
Sizing tables and rules of thumb get you to a solid answer for most homes. Where you stop guessing and get specific help: very high iron, mixed iron and manganese with low pH, unusual flow demands, commercial applications, and anything with arsenic, nitrate or bacteria. Your manufacturer's tech support exists for this — use it, and that's part of why step 8, choosing the right manufacturer, matters.
What you'll learn in the program
Step 3 inside Water CheatCode covers the sizing worksheet — from test result and household to grain capacity, regeneration interval, service flow, backwash flow and the tank/valve pair — plus the iron-compensation rule, valve programming and efficiency settings, and the well-flow check that stops iron-filter callbacks. Free to join.
Previous: Step 2 — the equipment field guide. Next: Step 4 — installation fundamentals.
FAQ
How do you size a water softener?
Take hardness in grains per gallon from the test (compensated upward if there's dissolved iron), multiply by the household's daily gallons to get grains per day, and choose a capacity that regenerates on a sensible interval — every few days to a week — on a metered valve. Then confirm the tank and valve can deliver the home's peak service flow.
What is service flow rate in water treatment?
The gallons per minute the system can deliver treated while the house is drawing water. Size it to peak demand (multiple fixtures at once), not the average. An undersized system drops pressure and lets hard water slip through during heavy use.
Why does backwash flow matter for an iron filter?
Backwashing lifts and cleans the media bed, and dense iron media like Pyrolox need a lot of upward flow to do it. On a well, if the pump can't deliver that flow at the treatment location, the media never gets clean and the system fails. Measure the well's real GPM before choosing the media and tank size.
What's the difference between a metered and a timer softener valve?
A metered valve counts gallons used and regenerates when capacity is reached, so it adapts to actual use. A timer valve regenerates on a fixed schedule regardless of use — wasting salt and water in light-use periods and risking hard water in heavy ones. Metered is the standard choice.
When should I call the manufacturer for sizing help?
For very high iron, iron and manganese with low pH, unusual flow demands, commercial jobs, and anything involving arsenic, nitrate or bacteria. Rules of thumb cover most homes; the hard cases are exactly what a good manufacturer's tech support is for.