Water Softener Size Calculator

A family of 4 with 15 gpg hard water needs about 31,500 grains between regenerations (4 people × 75 gallons × 15 gpg × 7 days). At an efficient 9 lb salt setting, a 32,000-grain softener only delivers about 25,000 grains, so the better pick is a 48,000-grain unit with 1.5 cubic feet of resin.

People in your home

Days between regenerations

Water use per person per day

60 gal75 gal80 gal

75 is the figure softener makers size with. Pick 60 for a low-flow home, 80 to match EPA's average including outdoor use.

Iron (ppm, from your water test)

Water hardness

Iron allowance

4 gpg per ppm5 gpg per ppm

Makers add 4 or 5 gpg of hardness for every 1 ppm of clear-water iron.

Salt setting

48,000-grain softener

1.5 cu ft of resin

Grains needed between regenerations: 31,500
Grains used per day: 4,500
Hardness used (with iron allowance): 15 gpg
Real capacity of this size at 9 lb: 37,500 grains
Lasts about: 8.3 days in your home
Salt per regeneration: 13.5 lb

Show the math

hardness = 15 gpg

grains per day = 4 × 75 × 15 = 4,500

grains needed = 4,500 × 7 = 31,500

capacity per cu ft at 9 lb = 25,000

48,000 (1.5 cu ft) gives 37,500 grains, lasts 8.3 days

All standard sizes for your home at this salt setting
Size	Real capacity	Days it lasts	Status
24,000	18,750	4.2	Too small
32,000	25,000	5.6	Too small
48,000	37,500	8.3	Recommended
64,000	50,000	11.1
80,000	62,500	13.9
96,000	75,000	16.7

48,000-grain softenerBack to calculator

How to use this calculator

Enter how many people live in the home.
Leave water use at 75 gallons unless you know your home uses much less or much more.
Enter your hardness. Use a test strip result or the "total hardness" line in your water utility's annual report, and pick the unit it's given in.
If a water test shows iron, enter it in ppm (the same as mg/L).
Keep 7 days between regenerations unless your dealer or manual says otherwise.
Choose the salt setting you plan to run. The result shows the smallest standard size that covers your home at that setting.

The formula

Softener size comes down to how many grains of hardness your home sends through the resin before each regeneration.

Grains per day = people × gallons per person per day × hardness in gpg

Grains needed = grains per day × days between regenerations

If your water has iron, add 4 gpg to the hardness for every 1 ppm of iron first. Then pick the smallest softener whose real capacity at your salt setting is at least the grains needed.

Example 1: family of 4, 15 gpg, no iron. 4 × 75 × 15 = 4,500 grains a day. Over 7 days that's 31,500 grains. At 9 lb of salt per cubic foot, resin holds about 25,000 grains per cubic foot. A 32,000-grain unit (1 cu ft) gives 25,000, which falls short. A 48,000-grain unit (1.5 cu ft) gives 37,500 and lasts about 8.3 days. It uses 13.5 lb of salt each time it regenerates.

Example 2: couple, 10 gpg, 1 ppm iron. Hardness with iron is 10 + (1 × 4) = 14 gpg. 2 × 75 × 14 = 2,100 grains a day, or 14,700 over 7 days. The smallest standard unit, 24,000 grains with 0.75 cu ft of resin, holds 18,750 at 9 lb. That lasts about 8.9 days and uses 6.8 lb of salt per regeneration.

(Display 6.75 as 6.8 lb in prose; the calculator shows 6.8.)

What this calculator doesn't cover

Peak flow. A big home with 3 or more bathrooms running at once may need a larger valve or tank than the grain math says. Check the valve's rated gallons per minute.
Iron above 2 ppm, manganese, hydrogen sulfide (rotten-egg smell) or tannins. These need their own treatment.
Salt-free conditioners. They don't remove hardness, so they have no grain capacity to size.
Homes that need more than one 96,000-grain tank.
The exact rating of a specific brand. Use the result to shortlist, then check the model's spec sheet.

Real capacity per cubic foot of resin by salt setting

Salt setting (lb per cu ft)	Capacity (grains per cu ft)	Grains per lb of salt
6	20,000	3,333
9	25,000	2,778
12	27,500	2,292
15	30,000	2,000

Based on A.O. Smith's published ratings at 6, 10 and 15 lb and a 2004 Water Conditioning & Purification design guide. Label sizes (32,000 and so on) assume about 32,000 grains per cubic foot at a high salt dose.

Water softener size chart (75 gal per person, 7 days, 9 lb salt setting)

People	5 gpg	10 gpg	15 gpg	20 gpg	25 gpg
1	24,000	24,000	24,000	24,000	24,000
2	24,000	24,000	24,000	32,000	48,000
3	24,000	24,000	32,000	48,000	64,000
4	24,000	32,000	48,000	64,000	80,000
5	24,000	48,000	64,000	80,000	96,000
6	24,000	48,000	64,000	96,000	Over 96,000

Standard softener sizes

Label size (grains)	Resin (cu ft)	Real capacity at 6 lb	at 9 lb	at 15 lb
24,000	0.75	15,000	18,750	22,500
32,000	1.0	20,000	25,000	30,000
48,000	1.5	30,000	37,500	45,000
64,000	2.0	40,000	50,000	60,000
80,000	2.5	50,000	62,500	75,000
96,000	3.0	60,000	75,000	90,000

Tips

City water reports usually list hardness in mg/L. Divide by 17.1 to get gpg, or use the hardness converter.
Buy for the salt setting you'll actually run. At 6 to 9 lb per cubic foot you get 20,000 to 25,000 grains per cubic foot, well below the label.
A metered (demand-initiated) valve regenerates only when the resin is nearly used. NSF/ANSI 44 efficiency ratings don't accept time-clock valves.
A typical 1 cubic foot residential unit is rated for about 9 to 11 gallons per minute. If your home often runs two showers and a washer together, ask about the valve's flow rating.
If you're on a well, get iron and manganese tested before you buy. Those numbers can change the size or the whole system.

FAQ

What size water softener do I need for a family of 4?

It depends on hardness. At 75 gallons per person and 7 days between regenerations, a family of 4 with 10 gpg water needs about 21,000 grains, which a 32,000-grain unit covers at a 9 lb salt setting. At 15 gpg the need rises to 31,500 grains, and a 48,000-grain unit is the right size.

How do I calculate water softener size?

Multiply people by gallons per person per day (75 is the usual figure) and by your hardness in gpg. That's grains per day. Multiply by the days between regenerations, often 7. Then choose the smallest softener whose real capacity at your salt setting is at least that number.

Is a 32,000-grain water softener enough?

At a 9 lb salt setting a 32,000-grain unit delivers about 25,000 grains, or roughly 3,570 grains a day over 7 days. That covers 2 people with water just under 24 gpg, 3 people just under 16 gpg and 4 people just under 12 gpg. Above that, go up a size.

What happens if my water softener is too small?

It runs out of capacity before its next regeneration, so hard water gets through. A metered valve will regenerate more often to keep up, which means more salt and water per week. A timer valve won't adjust, so you'll notice scale and spots.

Can a water softener be too big?

It costs more upfront and, on a timer valve, can regenerate before the resin is used up, which wastes salt. With a metered valve, a slightly larger tank just regenerates less often. Sizing for about a week between regenerations is a common target.

How many grains per gallon is considered hard water?

The Water Quality Association rates 7 to 10.5 gpg as hard and anything above 10.5 gpg as very hard. In mg/L, the USGS calls 121 to 180 hard and over 180 very hard. Under 1 gpg counts as soft water under NSF/ANSI 44.

Does iron in my water change the softener size?

Yes. Softener makers add 4 or 5 gpg to your hardness for every 1 ppm of clear-water iron, so 1 ppm of iron can raise the size you need. Most softeners are rated for no more than 2 ppm of iron. Above that, put an iron filter in front of the softener.