What size water softener do I need?
Sized on the hardness your resin actually sees — including the load iron and manganese add, which is where most sizing charts go wrong.
The short answer
Multiply the people in your house by 75 gallons a day, multiply by your hardness in grains per gallon, and multiply by seven for a weekly regeneration cycle. Then add roughly 4 grains per gallon for every part per million of iron, because iron consumes capacity too. That total is the capacity you need between regenerations.
Recommended size
48,000grain
- Water used per day
- 300 gal
- Hardness the resin actually sees
- 12.0 gpg
- Grains removed per day
- 3,600
- Capacity needed per 7-day cycle
- 25,200
- Usable capacity of the recommended unitSized at 75% of the rating, because the advertised number assumes the least efficient salt dose.
- 36,000
Sizing assumes weekly regeneration, which is the interval that keeps a resin bed clean without wasting salt. Want to know what it costs to run? Work out the salt.
How the sizing works
A softener is rated by grain capacity — how much hardness it removes before it has to regenerate. Sizing is a matter of working out how many grains your household removes per day, then deciding how often you want the unit to regenerate.
Weekly regeneration is the target. Much more frequent and you are burning salt and working the resin hard for no benefit. Much less frequent and the bed goes stale, channels, and stops making full contact with the water passing through it.
- Daily water use = people x 75 gallons (the usual household average).
- Compensated hardness = hardness + (iron x 4) + (manganese x 4), in gpg.
- Daily grains = daily gallons x compensated hardness.
- Capacity needed = daily grains x 7 days.
Why we size against 75% of the rated capacity
A softener’s advertised capacity is measured at its maximum salt dose, which is also its least efficient. A 32,000 grain unit genuinely delivers 32,000 grains — if you are prepared to spend roughly twice the salt to get the last third of it.
Sizing against about three quarters of the rating is what the industry actually does, and it is why this calculator recommends a larger unit than a manufacturer’s chart sometimes will. The larger unit run efficiently uses less salt than the smaller one run hard, and it lasts longer.
The iron adjustment nobody mentions
Iron and manganese are removed by the same ion exchange process as hardness, so they consume the same finite capacity. The industry rule of thumb is about 4 grains per gallon of extra load for every part per million — meaning 2 ppm of iron adds the equivalent of 8 gpg of hardness.
That is a large adjustment, and leaving it out is the most common reason a softener sized from an online chart regenerates far more often than expected. Above about 3 ppm of iron the calculation stops mattering, because a softener is the wrong machine entirely.
Frequently asked questions
- What size water softener do I need for a family of four?
- On typical 10 gpg water with no iron, a 32,000 grain unit regenerating about weekly. Harder water or any iron pushes that up quickly — at 15 gpg with 1 ppm of iron the same family needs 48,000.
- Is it bad to oversize a water softener?
- Mildly. An oversized unit regenerates too rarely, which lets the resin bed channel and go stale, and it costs more up front. Most control valves have a maximum-days setting to force a cycle, which largely solves it. Undersizing is the worse error.
- How do I convert my hardness from mg/L to grains per gallon?
- Divide by 17.1. So 200 mg/L is about 11.7 grains per gallon. Labs use either unit and rarely say which convention they have followed, so check the header on your report.
- Should I size the softener before or after an acid neutraliser?
- After. A calcite neutraliser raises hardness as it works, by dissolving limestone into the water. Size the softener against a test taken once the neutraliser is running, or you will size against a number that is about to change.