Water softener grain capacity calculator

Grain capacity is the number of grains of hardness a softener removes before it has to regenerate. The arithmetic is short. What makes most sizing numbers untrustworthy is the assumptions buried behind them, so here every assumption is a field you can change, labelled as an assumption, with the running total shown at each step.

Your water and your household

Hardness (gpg) From your utility's report or a tap test. Verified utility figures or convert from mg/L. People in the household Everyone whose water passes through the softener.

Assumptions every one of these is a default we chose, not a measured or published value — change them

Gallons per person per day Assumption. A placeholder, not a published figure. Your water bill divided by days and people is a far better number, and outdoor irrigation is usually not softened at all. Days between regenerations Assumption. How long you want one cycle to last. Longer means a larger unit and less salt handling; shorter means a smaller unit cycling more often. Reserve allowance (%) Assumption. Spare capacity so a heavy day does not run the resin out before the cycle ends. Set it to 0 to see the bare arithmetic. Usable share of nominal rating (%) Assumption. A unit's advertised grain rating is measured at a salt dose per its own spec sheet, and running it at a lower dose yields less. At 100% this tool takes the rating at face value. Lower it only to a figure from the unit's own documentation.

The arithmetic, step by step

Nothing here is proprietary. Four multiplications and a division:

Step	Formula	What it means
1. Daily water use	`people × gallons per person per day`	Total gallons through the softener each day
2. Daily hardness load	`daily gallons × hardness in gpg`	Grains of hardness the resin takes out per day
3. Grains per cycle	`daily grains × days between regenerations`	The load one cycle has to carry
4. With reserve	`grains per cycle × (1 + reserve ÷ 100)`	Usable capacity required
5. Nominal rating to look for	`required usable ÷ (usable share ÷ 100)`	The number printed on the unit

A grain is a unit of mass, 1/7000 of a pound, and one grain per gallon equals 17.1 mg/L. So "grain capacity" is literally the mass of hardness minerals a resin bed can hold between regenerations.

What this calculator deliberately does not do

It does not recommend a product. No softener has been tested by the author, and nothing here is a review.
It does not estimate salt use or running cost. That depends on the regeneration dose in a specific unit's documentation, which is a per-model figure, not arithmetic.
It does not size for flow rate. Capacity and service flow are two different constraints, and a unit with enough grains can still be undersized for simultaneous showers. The unit's spec sheet gives its service flow.
It does not know whether you should soften at all. At the soft end of the verified utility list — Portland's Bull Run supply at about 9 mg/L, Boston's MWRA at about 16 — there is very little hardness to remove.
It does not use your utility's figure as if it were your tap. A system range is not an address. That caveat sits on every utility page here for the same reason.

Starting from a utility figure

If you came here from a utility page, the hardness field is already filled with that system's published figure, converted to gpg at 17.1 mg/L per grain. The figures behind those pages were each read from the utility's own report, with the report year shown. Indianapolis is the clearest case for changing the default rather than trusting it: Citizens Energy Group publishes a system-wide range of 148 to 460 mg/L, which is 8.7 to 27 gpg, and sizing at 8.7 gpg against sizing at 27 gpg produces a threefold difference in required capacity.

So put your own tap test in if you have one, and read your report's hardness line if you do not. On a private well, no utility figure applies to you at all.