To work out what size water softener you need, multiply the people in your home by about 80 gallons each and by your hardness in grains per gallon. That gives the grains to remove per day. Multiply by 7 for a weekly regeneration and pick the next size up. A family of four on 10 gpg water removes 3,200 grains a day, or 22,400 a week. A 24,000-grain softener covers that on paper, and a 32,000-grain unit covers it at a salt-saving setting.
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Still deciding whether you need one at all? Start with do I need a water softener. The calculator below does the sum for your home. Under it you’ll find a sizing table, three worked examples and the part most sizing guides skip: a softener’s grain rating depends on how much salt it burns.
This calculator needs JavaScript. The formula below lets you do the same sum by hand.
How to size a water softener in three steps
1. Find your hardness in grains per gallon
Your water utility publishes it in its annual water quality report. If the report gives ppm or mg/L, divide by 17.118 to get gpg (the hardness converter does it for you). On a private well you’ll need a test. Not sure whether your number counts as hard? Our guide to what hard water is puts it in context.
If a test found clear-water iron, add 5 gpg for every 1 ppm of iron. That adjustment comes from softener manuals such as Pentair’s, which tells owners to raise the hardness setting by 5 for each ppm of iron.
2. Estimate the grains removed per day
The USGS puts indoor use at 80 to 100 gallons per person per day. Pentair’s manual uses 70. If your bill shows gallons, divide the monthly figure by 30 and then by the number of people; that beats any average.
3. Multiply by the days between regenerations
Seven days is a sensible target (see how often a softener should regenerate). Shorter cycles mean a smaller tank that regenerates often; much longer ones leave resin sitting unused. Then pick the first common size at or above the result: 24,000, 32,000, 40,000, 48,000, 64,000 or 80,000 grains.
What size water softener do I need? Sizing table
Grains needed for 7 days between regenerations at 80 gallons per person per day. The first size is the nominal rating that covers it. The second is the size that still covers it at a medium salt dose (see the next section).
| People | 5 gpg | 10 gpg | 15 gpg | 20 gpg | 25 gpg |
|---|---|---|---|---|---|
| 1 | 2,800 → 24k / 24k | 5,600 → 24k / 24k | 8,400 → 24k / 24k | 11,200 → 24k / 24k | 14,000 → 24k / 24k |
| 2 | 5,600 → 24k / 24k | 11,200 → 24k / 24k | 16,800 → 24k / 24k | 22,400 → 24k / 32k | 28,000 → 32k / 40k |
| 3 | 8,400 → 24k / 24k | 16,800 → 24k / 24k | 25,200 → 32k / 32k | 33,600 → 40k / 48k | 42,000 → 48k / 64k |
| 4 | 11,200 → 24k / 24k | 22,400 → 24k / 32k | 33,600 → 40k / 48k | 44,800 → 48k / 64k | 56,000 → 64k / 80k |
| 5 | 14,000 → 24k / 24k | 28,000 → 32k / 40k | 42,000 → 48k / 64k | 56,000 → 64k / 80k | 70,000 → 80k / over 80k |
| 6 | 16,800 → 24k / 24k | 33,600 → 40k / 48k | 50,400 → 64k / 64k | 67,200 → 80k / over 80k | 84,000 → over 80k |
Why a 32,000-grain softener isn’t always 32,000 grains
A softener’s headline number is its capacity at the highest salt dose. Turn the salt down and capacity drops, but each pound of salt removes far more hardness. Pentair’s performance data sheet for its whole-house softeners shows the pattern clearly:
| Salt per regeneration | Capacity | Share of maximum | Grains per lb of salt |
|---|---|---|---|
| 3.9 lb | 16,660 grains | 54% | 4,272 |
| 7.9 lb | 24,593 grains | 80% | 3,113 |
| 15.8 lb | 30,726 grains | 100% | 1,945 |
At the top dose this unit uses more than twice as much salt per grain as at the lowest one. That’s why the table above has a second column. If you size so that your weekly need fits at a medium setting, you can run the softener efficiently and still get about a week between regenerations. The NSF/ANSI 44 efficiency rating asks for at least 3,350 grains per pound of salt, which on this data means staying below the maximum dose.
Three worked examples
Couple in a condo, 8 gpg. 2 people × 80 gallons × 8 gpg = 1,280 grains a day, or 8,960 a week. The smallest common size, 24,000 grains, would last 18.8 days at full capacity. That’s more than enough. If your valve has a maximum-days setting, a value around 10–14 days stops the resin sitting idle.
Family of three, heavy users, 18 gpg. Their bill shows 9,000 gallons a month, so 9,000 ÷ 30 ÷ 3 = 100 gallons each. 3 × 100 × 18 = 5,400 grains a day, or 37,800 a week. A 40,000-grain unit covers that at full salt and regenerates every 7.4 days. At a medium salt dose a 48,000-grain unit (about 38,400 usable) is the better fit.
Five people on a well, 22 gpg plus 1.5 ppm iron. Compensated hardness is 22 + 1.5 × 5 = 29.5 gpg. 5 × 80 × 29.5 = 11,800 grains a day, or 82,600 a week, which is beyond the largest common single tank. Options: accept regeneration about every 5.4 days on a 64,000-grain unit, or look at a twin-tank system. Iron at this level also needs checking against the softener’s iron limit, which you’ll find in the manual.
Common sizing mistakes
Sizing on the national average. Hardness ranges from soft to very hard between neighbouring cities. Las Vegas water is 16 gpg according to the local utility; plenty of places are under 5. Use your own figure.
Ignoring the salt setting. Buying exactly the nominal size and then setting the salt to “eco” leaves you regenerating every four or five days. Check how much salt a softener uses at each setting before you choose.
Entering the wrong hardness on the valve. A softener set to 25 on 15 gpg water regenerates far too often and wastes salt. Enter the figure from your utility report or a test, plus 5 for each ppm of iron if a test found any. The hardness setting guide has a worked table.
Going huge “to be safe”. An 80,000-grain unit for two people costs more, takes more space and may sit for weeks between regenerations. Bigger is only better up to the point where you get roughly a week per cycle at a reasonable salt dose.
Frequently asked questions
What size water softener do I need for a family of 4?
At 80 gallons per person and a weekly regeneration, a family of four needs 11,200 grains at 5 gpg, 22,400 at 10 gpg, 33,600 at 15 gpg and 44,800 at 20 gpg. That points to 24,000, 32,000, 48,000 and 64,000 grains respectively if you want to run at a medium salt setting.
Is a 32,000-grain water softener enough?
At a medium salt dose (about 25,600 usable grains) it covers a week for two people up to about 22 gpg, and for four people up to about 11 gpg. Above that it will regenerate more often than once a week.
What happens if a water softener is too big?
It still softens water, but it costs more and regenerates rarely. Check your manual for a setting that forces a regeneration after a maximum number of days; it keeps the resin from sitting idle.
What happens if it’s too small?
It regenerates every few days, uses more salt and water per gallon softened, and can run out of capacity before the next cycle if you have guests or run lots of laundry.
How do I find the size of the softener I already have?
Look at the label on the tank or control valve and search for the model number. Many model numbers include the capacity, such as 32 or 48 for 32,000 and 48,000 grains.
