Softener sizing is arithmetic, and the inputs are the household's water use and the water's hardness.
The calculation
daily grain load = people × gallons per person per day × hardness in gpg
Gallons per person per day is conventionally taken as 75, which is a reasonable planning figure for a household without unusual use.
Hardness in grains per gallon (gpg) comes from a water test. If the result is in parts per million or milligrams per litre, divide by 17.1 to get grains.
Four people, 75 gallons each, water at 15 gpg: 4 × 75 × 15 = 4,500 grains per day.
The iron adjustment
Iron fouls softener resin and consumes capacity, so it is added to the hardness figure before the calculation.
The usual convention adds 4 to 5 gpg for each part per million of dissolved iron. Water at 15 gpg hardness with 2 ppm iron is treated as roughly 24 gpg.
Manganese is handled similarly, at a higher factor.
Iron above a few parts per million is generally better removed by a dedicated iron filter ahead of the softener rather than loaded onto it.
Choosing the capacity
Softeners are rated by grain capacity — 32,000, 48,000, 64,000 and so on — which is the total hardness a full resin bed can remove between regenerations.
The aim is to regenerate every three to seven days.
More often than that wastes salt and water and wears the valve. Much less often lets the resin bed sit loaded, which reduces efficiency and can allow bacterial growth.
At 4,500 grains a day, a unit delivering around 25,000 grains of working capacity per regeneration runs about five and a half days. That is a comfortable target.
Rated capacity is not working capacity
A 32,000 grain unit does not deliver 32,000 grains per regeneration in normal service.
Manufacturers publish capacity at several salt doses. Running at the maximum dose gives the headline number at poor salt efficiency; running at a lower dose gives less capacity per regeneration and considerably more grains removed per pound of salt.
Most units are set for efficient operation, which means the working capacity is meaningfully below the rating on the box. Size against the working figure.
What oversizing costs
A very large unit regenerating every two weeks uses more salt per regeneration and leaves the bed standing loaded for long periods.
It also costs more to buy, and the flow rate through an oversized bed at low demand can channel rather than contact the resin evenly.
Bigger is not better here.
What undersizing costs
Regenerating daily, or running out of capacity before regeneration, which delivers hard water at the tap — usually noticed at the end of a heavy day.
Flow rate, separately
Capacity is about how much hardness; flow rate is about how fast.
A unit has a maximum service flow before the water passes through too quickly to be softened properly. A household running two showers and a washing machine at once needs a unit rated for that flow, regardless of its grain capacity.
Both figures are on the specification, and a unit adequate on one and short on the other will disappoint.
Demand-initiated regeneration
A meter-controlled softener regenerates based on water actually used. A timer-based one regenerates on schedule whether or not the resin is exhausted.
Demand initiation saves salt and water, and it handles a week away or a week of guests correctly. Any unit bought now should have it, and an old timer unit is worth replacing on this ground alone.