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Softener sizing

The grain capacity your water needs

Household size and hardness set the baseline. Iron and manganese then consume capacity on top of it. Leave them out and a correctly bought softener regenerates every other day.

At 75 gallons per person per day.
Reported in mg/L? Divide by 17.1 first.
Leave at zero if a filter removes it upstream.
Same again: zero if it is filtered out first.

The arithmetic

Nothing here is complicated. What separates a right answer from a wrong one is whether iron and manganese are counted at all.

  1. 1Compensated hardness. Measured hardness, plus 4 grains per gallon for every 1 ppm of iron, plus 4 for every 1 ppm of manganese.
  2. 2Daily water use. People multiplied by75 gallons.
  3. 3Daily grain load. Daily use multiplied by compensated hardness.
  4. 4Capacity. Daily load multiplied by7 days, then rounded up to a standard cabinet size.

Why iron belongs in the calculation

Softener resin picks up positively charged dissolved metals. Calcium and magnesium are the target, and iron and manganese are taken alongside them, occupying sites that would otherwise be available for hardness.

So a softener on iron-bearing water reaches the end of its capacity sooner than the hardness figure predicts. Sized without compensation it regenerates more often than intended, uses more salt, and wears its valve faster. There is a second cost too: iron does not release cleanly during regeneration, and what stays behind oxidises on the resin and permanently reduces capacity.

Which is why, above roughly 1 ppm, the better answer is removing iron upstream rather than sizing around it. Compensation is for the iron a softener is reasonably expected to carry, not a licence to carry any amount.

These two factors are the weakest-sourced numbers here. No extension service, EPA publication or NSF standard publishes a compensation factor; the figures in circulation come from softener manufacturers' programming manuals, which give iron somewhere between 4 and 5 and manganese between 2 and 4. Counting iron imperfectly still beats not counting it, and both figures are set where being wrong costs least.How every number here is graded.

Questions

Why is my answer bigger than the one I got elsewhere?
Because iron and manganese are counted. Both consume exchange capacity alongside hardness, and a calculation that asks only for hardness leaves them out. On water with 15 grains per gallon and 0.8 ppm of iron, leaving the iron out understates the requirement by more than 20%.
Should I just buy the next size up to be safe?
Oversizing is the lesser error, but it is not free. A softener that regenerates rarely leaves resin sitting in stale water between cycles, and very large units cost more in salt per regeneration. Sizing for roughly a weekly cycle is the usual target for a reason.
My lab reported hardness in mg/L. What do I enter?
Divide by 17.1 first. One grain per gallon is 17.1 mg/L, so 180 mg/L is about 10.5 grains per gallon. Entering mg/L into a grains field overstates the requirement by a factor of seventeen, and it is the most common mistake in softener sizing.
How solid are the iron and manganese factors themselves?
They are the weakest numbers on this site. No extension service, EPA publication or NSF standard publishes a compensation factor. The figures in circulation come from softener manufacturers' own programming manuals, which disagree: iron between 4 and 5 grains per gallon per ppm, manganese between 2 and 4. Counting iron imperfectly still beats the usual alternative of not counting it at all, and each figure here is set where being wrong costs least.
Does the iron figure belong here at all?
Only if the softener is going to see the iron. Where a filter upstream removes it first, the resin never encounters it and the compensation should be left out. Whether it reaches the resin depends on what sits upstream, so size from the whole test rather than from the hardness line alone.

Before you buy a softener

  • Hard water from a well

    Dissolved calcium and magnesium. It scales water heaters, coats fixtures, stops soap working, and shortens the life of every appliance that heats water.

  • Water softeners for well water

    Trades the calcium and magnesium that cause scale for sodium, on a resin bed that regenerates itself with brine. It is the right answer for hard water and the wrong answer for an iron problem, though it is sold for both.

  • Iron in well water

    The most common well water complaint in the country. Above 0.3 ppm it stains everything water touches, and the form it arrives in decides whether a filter or a softener is the right answer.

  • Manganese in well water

    Iron's quieter companion. It stains black or grey rather than orange, shows up at a twentieth of iron's concentration, and needs a distinctly higher pH to remove.

  • Water softener vs iron filter: which your well needs

    They are not alternatives. An iron filter oxidises and removes iron; a softener exchanges calcium and magnesium. Water with both problems needs both, in that order, and water with only one needs only one.