Field guide
Plumber Water Softener Pages: The Hardness Number Decides the Size
The number that decides everything
A water softener is not sized by the size of the house. It is sized by one number: hardness, measured in grains per gallon. The US Geological Survey classifies water as soft at 0 to 3.5 grains per gallon, moderately hard at 3.6 to 7.0, hard at 7.1 to 10.5, and very hard above 10.5. One grain per gallon equals 17.1 milligrams per liter, so a lab report in mg/L converts by dividing by 17.1. Typical US municipal water runs 7 to 10 grains per gallon; well water often runs 15 to 25 or more. Below 3.5, a softener is solving a problem you do not have. Above 10.5, the scale on the fixtures is not cosmetic. It is the water telling you the number.
The page should insist on a test before a quote. A softener sized off a guess either regenerates constantly and wastes salt, or runs out of capacity mid-week and lets hard water through for days. Get the number from the water report, a test strip, or a lab. If the water is from a well, test for iron too: for every 1 part per million of iron, add 4 grains per gallon to the hardness figure before calculating, because iron consumes softening capacity the same way hardness does.
The sizing math, worked once
The formula is short enough to print on the page. People times 75 gallons a day times hardness in grains per gallon gives the grains removed per day. Multiply by 7 for a weekly target, then round up to the next standard size: 24,000, 32,000, 40,000, 48,000, 64,000, or 80,000 grains. A family of four at 10 grains per gallon removes 3,000 grains a day, 21,000 a week, and lands on a 24,000-grain unit. The same family at 20 grains per gallon needs 42,000 a week and lands on a 48,000-grain unit. The goal is a softener that regenerates about once a week: more often wastes water, a cycle can use up to 50 gallons, and wears the control valve faster, while less often risks running out of capacity mid-week.
Oversizing is not free either. A tank that sits idle too long can let bacteria and iron foul the resin, and the bigger tank costs more for no benefit. The page should say that plainly, because the upsell to the biggest tank is the oldest move in water treatment sales, and the homeowner who can do the math is the homeowner who buys the right size. Add a line about flow: a 1.5 cubic foot tank is the starting point for homes with four tub bathrooms, because filling tubs can draw about 12.5 gallons a minute at once.
What a softener does not fix
A salt-based ion-exchange softener is the only type that actually removes hardness. It swaps calcium and magnesium for sodium on a resin bed, then flushes the resin with salt brine when it is exhausted. Salt-free conditioners, magnetic devices, and scale inhibitors do not remove hardness, and a page that says so is doing the reader a favor. A softener also does not filter. It does not remove sediment, bacteria, or most chemicals, and it needs a sediment filter ahead of it and, on a well, an iron filter or acid neutralizer where the water calls for one. Below pH 6.5, an acid neutralizer comes first, and the calcite it dissolves adds hardness, so the softener gets bigger.
The treatment order is worth printing: pressure tank, sediment filter, iron filter or acid neutralizer if needed, then the softener, then any UV or drinking water filter. A page that lays out the order is a page that gets bookmarked, and a page that gets bookmarked is a page that brings the call when the homeowner is ready to buy.
Related reading
If this topic is useful, these earlier guides go deeper on connected questions: Plumber Sewer Replacement Pages: Trenchless Is Two Different Jobs, Plumber Tankless Pages: Sell the Descale, Not Just the Install, and Plumber Drain Camera Pages: Sell the Recording, Not the Look.
Frequently asked questions
How hard is my water, and how do I find out?
Hardness is measured in grains per gallon: soft is 0 to 3.5, hard is 7.1 to 10.5, very hard is over 10.5. Get the number from your water report, a test strip, or a lab test.
How do I size a water softener?
Multiply people by 75 gallons a day by hardness in grains per gallon, then by 7 days, and round up to the next standard size: 24,000, 32,000, 48,000, or 64,000 grains.
Does iron in well water change the size?
Yes. Add 4 grains per gallon for every 1 ppm of iron before calculating. Iron consumes softening capacity, so unadjusted math undersizes the unit.
Do salt-free softeners remove hardness?
No. Only salt-based ion-exchange softeners remove hardness. Salt-free conditioners and magnetic devices do not remove calcium or magnesium.