Water Softener Size Calculator

Use the calculator above to get the grains your household needs between regenerations. It works in grains: how many grains of hardness your household removes each day, and how many build up before the unit regenerates. The answer is only as good as the numbers you put in, so test your water first and use your own household’s water use where you know it.

The formula, step by step

1. Adjust hardness for iron. Adjusted hardness = hardness (grains per gallon) + 5 × iron (ppm). The 5 is a common installer rule of thumb of about 4 to 5 grains per ppm. Leave iron at 0 if you have none.
2. Grains per day. People × gallons per person per day × adjusted hardness.
3. Grains between regenerations. Grains per day × days between regenerations. This is the first figure the calculator shows.
4. Optional reserve. Grains between regenerations × 1.25 for a 25% reserve. This is the second figure. Vendors disagree on the reserve: some add 25%, others divide by 0.65, which adds about 54%. The calculator shows the 25% version.

The calculator writes the same formula out in plain text under the inputs, so you can check it by hand.

A worked example

Four people, 75 gallons per person per day, 15 grains per gallon, no iron, 7 days between regenerations:

• 4 × 75 = 300 gallons a day
• 300 × 15 = 4,500 grains a day
• 4,500 × 7 = 31,500 grains between regenerations
• 31,500 × 1.25 = 39,375 grains with the 25% reserve

Compare the 31,500 or the 39,375 figure with the unit’s rated grain capacity at the salt setting you plan to run. A unit rated 32,000 grains covers the first figure but not the second, so the reserve choice can change which unit you need. A 48,000-grain Fleck 5600SXT lists about 36,000 grains at 9 lb of salt and about 45,000 at 15 lb, so it covers the first figure at either setting and the second at the higher one.

Penn State Extension uses the reverse calculation. It divides a unit’s capacity by the grains used per day to get the interval. Its example, a 20,000-grain unit with 4 people, 75 gallons and 10 grains per gallon, uses 3,000 grains a day and regenerates about every 6 to 7 days. Both methods use the same inputs.

Sample results

These rows use the calculator’s defaults of 75 gallons per person and 7 days. The last column adds the optional 25% reserve, rounded to the nearest grain.

People	Hardness (gpg)	Grains per day	Grains between regenerations	With 25% reserve
2	10	1,500	10,500	13,125
3	10	2,250	15,750	19,688
4	15	4,500	31,500	39,375
4	20	6,000	42,000	52,500
5	20	7,500	52,500	65,625
6	25	11,250	78,750	98,438

These rows use no iron. With 2 ppm of iron, the calculator adds 10 grains to a hardness of 10 (10 + 5 × 2 = 20), so four people give 6,000 grains a day, the same as the 20-grain row above.

Notice how sharply the answer moves at higher hardness. Four people at 20 grains per gallon need 42,000 grains between regenerations, a third more than four people at 15.

Inputs that matter most

Gallons per person per day. Penn State uses 75 gallons as its example, and vendors give 75 to 80. The calculator starts at 75. Penn State’s water-use figures show the spread: drinking and cooking about 1 gallon a person a day, a bath 25 to 60 gallons, and a washing machine load 20 to 33 gallons.

Hardness. Use a measured number. The hardness converter turns ppm into grains per gallon, using 17.1 ppm per grain.

Days between regenerations. A shorter interval needs fewer grains held between regenerations, but it means more regenerations. Each regeneration uses about 50 gallons of water, according to Nebraska and Penn State Extension, plus salt. A longer interval needs more grains of capacity. The calculator offers 3, 4, 5, 7 and 10 days, with 7 as the default.

Iron. Iron adds to the sizing load, and the 5-per-ppm rule raises the grain number quickly. Iron above the unit’s rated limit is a filtering problem, not a sizing fix. The extension guides give different limits, so use the number the maker rates. Our guide to iron in well water explains the levels.

Before you buy

Rated grain capacity depends on the salt setting. US Water Systems lists a 1.0 cubic-foot Matrixx at 15,000 grains on 3 pounds of salt, 24,000 on 6 pounds and 32,000 on 15 pounds. Compare the calculator’s result with the rating at the salt dose you intend to run, not the largest number on the box.

SpringWell lists its SS1 at 32,000 grains and its SS4 at 48,000 grains without a salt setting, so ask the seller which salt dose the rating assumes before you compare.

Check flow too. A softener with a 10 GPM rating will not serve a household that needs more than that at peak. Our flow rate calculator gives a starting figure, and the best water softener guide lists the sizes and flow ratings of the picks. For one to three people, see the small water softener guide, and for every model side by side the comparison page.

Finally, potassium chloride salt costs more and uses more salt per regeneration, so the running cost of the unit can differ from the sizing calculation alone.

Common mistakes

• Keeping the default 75 gallons without thinking. It is a starting point, not your use.
• Typing ppm into the grains box. The calculator works in grains per gallon. Convert first.
• Forgetting iron. Even a little iron changes the answer under this method.
• Comparing with the wrong rating. Match the grain figure to the rating at the salt setting you will run.
• Treating the result as a promise. Real capacity depends on the salt setting, water quality and the valve.

Use the calculator to see the shape of the answer, then check it against a lab hardness test and the brand’s own grain rating before you pay for a unit.