The short answer
A family of 4 with 15 grains per gallon of hardness needs a 24,000 grain water softener: 300 gallons a day carries 4,500 grains, or 18,000 grains over 4 days between regenerations. Add 4 grains per gallon for each ppm of iron.
Softener size by household and hardness
Softener size (grains, at the highest salt rating) by people and hardness in grains per gallon, no iron, 4 days between regenerations and 75 gallons a person.
| People | 5 gpg | 10 gpg | 15 gpg | 20 gpg | 25 gpg | 30 gpg |
|---|---|---|---|---|---|---|
| 1 | 24k | 24k | 24k | 24k | 24k | 24k |
| 2 | 24k | 24k | 24k | 24k | 24k | 24k |
| 3 | 24k | 24k | 24k | 24k | 32k | 40k |
| 4 | 24k | 24k | 24k | 32k | 40k | 48k |
| 5 | 24k | 24k | 32k | 40k | 64k | 64k |
| 6 | 24k | 24k | 40k | 48k | 64k | 80k |
How it works
The formula
A softener's resin holds a fixed number of grains of hardness before it has to regenerate with salt. So the size is the hardness in the water you use between regenerations.
grains a day = people × 75 gal × hardness
capacity = grains a day × days between regenerations
Assumptions
- The method, the 75 gallons a person, the iron allowance and the tank capacities are from Nelsen Corporation's softener sizing guide. Change the gallons if your water bill shows your real use.
- Sizes, with grains removed at full salt and at the lower, efficient salt setting: 24k (18k), 32k (24k), 40k (30k), 48k (36k), 64k (48k), 80k (60k), 110k (84k). We pick by the lower figure.
- Water used outdoors usually bypasses the softener; if yours doesn't, count it in the gallons.
Worked example: 4 people, 15 gpg, 1 ppm iron
- 15 + 4 × 1 = 19 grains per gallon
- 4 × 75 = 300 gal a day × 19 = 5,700 grains a day
- 5,700 × 4 days = 22,800 grains → 32,000 grain softener (24,000 at the lower salt setting)
Too big or too small
- Too small: hard water by the end of the cycle. It regenerates every day or two, using more salt and water, or lets hard water through.
- Too big: fouled resin. Weeks between regenerations let iron and sediment build up in the resin.
- Test, don't guess. Hardness and iron vary well to well; a lab or kit test is cheap.
Questions people ask
What size water softener do I need for a family of 4?
At 15 grains per gallon, 4 people use about 300 gallons a day, which carries 4,500 grains of hardness. Over 4 days that's 18,000 grains, so a 24,000 grain softener. With 1 ppm of iron as well, it's 22,800 grains and a 32,000 grain unit.
How do I convert ppm hardness to grains per gallon?
Divide by 17.1. A water report showing 250 ppm (or mg/L) as calcium carbonate is 14.6 grains per gallon.
Why does iron change the size?
Ferrous iron uses up the resin like hardness does. Count each ppm of iron as 4 grains per gallon of hardness. Most softeners are rated for up to 2 ppm of iron; above that, put an iron filter ahead of the softener.
Is a bigger softener better?
Up to a point. A larger one regenerates less often and can run at a lower, more efficient salt setting. Too large and it regenerates so rarely that the resin can foul, and some controllers regenerate on a schedule anyway, wasting salt. Size for about 3 to 5 days between regenerations.
What do the grain ratings mean?
The number on the box is what the resin removes at the highest salt setting. At a lower salt setting it removes less but uses salt more efficiently: a 32,000 grain unit removes about 24,000. We pick the size by the lower figure, so it keeps up while saving salt.