What Size Water Softener Do I Need?

The grain math in three steps

1. 1

   Cause 1: Daily water use

   Hellenbrand's manual uses 75 gallons per person per day if all the water in the house is softened, 50 if toilets are left on hard water, and 30 if only the hot water is softened. Penn State Extension uses the same 75-gallon figure in its example. If you have your water bill, use your real number instead.

2. 2

   Cause 2: Grains per day

   Multiply daily gallons by hardness in gpg. If your test is in mg/L, divide by 17.1 first, the conversion Morton gives. On a well with iron, add your manufacturer's iron allowance to the hardness before you multiply. Whirlpool, for one, says to add 5 for each 1 ppm of iron.

3. 3

   Cause 3: Capacity

   Multiply grains per day by the days you want between regenerations. HomeGuide suggests 3 to 7 days. Much longer than a week usually means the softener is bigger than the house needs.

   HomeGuide's example: 400 gallons a day at 20 gpg is 8,000 grains a day, and at 4 days between regenerations that calls for 32,000 grains.

Quick sizing chart

Using 75 gallons per person per day and 7 days between regenerations:

Round up to the next size you can buy. Then read the next section, because the label number isn't always what you get.

Rated capacity vs real capacity

A softener's capacity changes with how much salt it uses per regeneration. Use more salt and you get more capacity, but less of each pound goes to work. Some models are named or labeled by the capacity they reach at a high salt dose.

Hellenbrand's H100 manual shows how big the gap is. At its high-efficiency salt setting, the H100-48 delivers 34,400 grains using 9 pounds of salt per regeneration. The H100-24 gives 17,200 grains and the H100-96 gives 68,810. The H100-48 only reaches 48,460 grains at its high salting setting of 22.5 pounds. If you want to run efficiently, check the capacity at the low salt setting and size up one step if needed.

Efficiency matters to some sewer districts too. Minnesota's pollution agency cites a Madison Metropolitan Sewerage District program in Wisconsin that calls for replacement softeners with at least 4,000 grains per pound of salt efficiency.

What about a 30,000-grain or 80,000-grain softener?

Going by the chart, a 30,000-grain unit fits a couple or a small family on moderately hard water. From the chart above, two people on 20 gpg water need about 21,000 grains for a weekly cycle.

An 80,000-grain unit is for large households or very hard water, by the same math. Six people on 20 gpg water need about 63,000 grains a week, and some wells run harder than that. Penn State calls anything over 10.5 gpg very hard.

Bigger isn't automatically better. An oversized softener costs more up front and, on a timer, regenerates on schedule whether it needs to or not. Undersized is worse, though: it regenerates every day or two, burns salt, and lets hard water slip through when the household runs a lot of water at once.

Check flow rate as well

Grain capacity tells you how long a softener lasts between cycles. Flow rate tells you whether it can keep up when two showers and the washing machine run at once. Check the service flow rate on the spec sheet against your peak demand and your pipe size, especially in a bigger house. A softener with too little flow causes a pressure drop at the taps.

Next steps

1. Test your raw water hardness, and iron if you're on a well. Our hardness setting page explains testing and conversion.
2. Run the math above with your household's numbers.
3. Compare models at their efficient salt setting, not just the label rating.
4. Price it out with our water softener cost guide, and see water softeners for well water if you're on a private well.