Sizing · shell

What size water softener do I need

Capacity follows from three things: how hard your water is, how much of it you use, and how often you are willing to regenerate. Every assumption in that arithmetic is shown here rather than buried inside it.

Unpublished draft · not indexed

This page is a shell. Its structure is settled and its figures are not, so nothing on it should be relied on or quoted yet. It is excluded from the sitemap and marked noindex until every figure below carries a source and a date.

Every XXX on this page is a placeholder, not a figure. It marks a cell whose table would not read without something in it. No number on this page has been invented to fill a gap, and none ever will be. A figure here is either measured and dated, or it is visibly absent.

Outstanding: 2 figures · 2 shown as XXX

resinCapacityGrainsPerCubicFootXXX : Exchange capacity per cubic foot of resin at a stated salt dose. Capacity is not one number: it moves with salt dosing, and quoting the high figure oversizes every result. A manufacturer table is now recorded in manufacturer-specs.ts showing a 1.5 ft³ tank at 26,781 gr @ 6 lb against 47,604 gr @ 18 lb. The sizing model needs a DOSE to compute at before this becomes a figure; that decision is targetSaltDose and has not been made. Likely source: Resin manufacturer specification sheets.
targetRegenerationIntervalDaysXXX : The regeneration interval the model targets, and why. Too long breeds bacteria in the bed; too short wastes salt and water. Likely source: Manufacturer guidance, cited.

The model, and what it assumes

Daily hardness load is household size times daily use times hardness. Capacity is that load times the regeneration interval, divided by the resin’s exchange capacity. Each input is a published figure, and each is contestable, so each is named.

Input	Value	Why it is contestable
Gallons per person per day	82	Varies by region and household habits
Resin capacity per cubic foot	XXX	Moves with salt dose; the high figure oversizes every result
Target regeneration interval	XXX	Too long breeds bacteria in the bed; too short wastes salt and water
Salt efficiency	4,463 grains per pound of salt	Turns a sizing result into an annual running cost

You need your hardness before any of this means anything. Get the measured figure for your ZIP code, then come back.