How to get the numbers you need
Start with a test. Your utility's report may give hardness, and a well needs its own. Check the unit, as reports use grains per gallon, ppm or mg/L, and the last two are the same. Nebraska's table uses 17.1 to convert, so divide ppm by 17.1. Test for iron too, because it uses up softener capacity as hardness does, and a softener has a limit on how much it can handle.
Water use comes next. WQA's current design figure is 75 gallons per person each day. Another of its papers says about 200 gallons a day for four people, so 50 each. A softener only needs to treat what goes through it, and WQA suggests bypassing outside faucets, so low-use households can enter the smaller figure.
Sources pull two ways on size. WQA warns against oversizing, while Lowe's says it is better to overestimate. We take the next common size up and do not push you further. The sizes are from one vendor, Nelsen, and we found no public WQA table, so check your own maker's. A rating means little without its salt dose. Nelsen's 32,000 grain tank is 24,000 at its efficient setting.
A worked example
Here is a family of 4 at 15 grains per gallon with no iron, regenerating every 7 days on full salt. These lines come from the same formula the calculator above runs.
4 people, 15 gpg, 7 days
- 4 × 75 = 300 gal a day
- 300 × 15 = 4500 grains a day
- 4500 × 7 = 31500 grains between regenerations
- The next size up is 32000 grains
With 1 ppm of iron, hardness counts as 19 grains per gallon and the softener goes to 40000 grains. On the efficient salt setting, this family takes 36000 grains. Softening adds sodium too, at about 120 mg/L here, on Nebraska's figure. To see where your water goes, try the water usage calculator.
Softener size by household
These are at 75 gallons per person, full salt, no iron and 7 days between regenerations. Nebraska classes 3.5 to 7 grains per gallon as moderately hard, 7 to 10.5 as hard and above that as very hard.
| People | 5 gpg | 10 gpg | 15 gpg | 20 gpg |
|---|---|---|---|---|
| 1 | 24000 | 24000 | 24000 | 24000 |
| 2 | 24000 | 24000 | 24000 | 24000 |
| 3 | 24000 | 24000 | 24000 | 32000 |
| 4 | 24000 | 24000 | 32000 | 48000 |
| 5 | 24000 | 32000 | 40000 | 64000 |
| 6 | 24000 | 32000 | 48000 | 64000 |
Mistakes when sizing a softener
- Mixing up the units. Entering ppm in a grains field makes the answer 17 times too big.
- Comparing capacities without the salt dose. The same tank can be 32,000 or 24,000 grains on Nelsen's table.
- Ignoring iron. Limits run from 2 to 10 ppm across sources. Above your model's rating, you may need an iron filter.
- Buying the biggest. WQA says to avoid oversizing equipment.
- Softening outdoor water. WQA suggests bypassing outside faucets.
Questions
What size softener for a family of 4?
About 32,000 grains at 15 grains per gallon, on full salt, with a regeneration every 7 days. That is 4 people, 75 gallons each and 4,500 grains a day. Softer water needs less, and iron adds to it, so use your own test.
How do I convert ppm to grains per gallon?
Divide by 17.1, as Nebraska's table does. A reading of 256.5 ppm is 15 grains per gallon. North Dakota State rounds to 17, which makes little difference. Mixing the two up is the costliest mistake here, since it makes the answer 17 times wrong.
What size water softener do I need for 2 people?
About 24,000 grains, which is the smallest size in Nelsen's table. At 15 grains per gallon, 2 people use 15,750 grains in 7 days. Other makers offer smaller tanks, and WQA warns against oversizing, so see if one fits your household.
What size water softener do I need for a family of 6?
About 48,000 grains at 15 grains per gallon on full salt, with a regeneration every 7 days. That is 47,250 grains between regenerations, which sits close to the size, so harder water or more use moves it up a step.
How do I account for iron?
Add about 4 grains per gallon for each ppm of ferrous iron, a rule from Nelsen, as we found none from WQA or an extension service. Lowe's says 3 to 5. Nelsen says most softeners are rated only up to 2 ppm, and limits elsewhere differ.
How often should a softener regenerate?
About every 3 to 7 days, as sources differ. Nelsen says 3 to 5 days is typical, and Lowe's works out its size at 7. Shorter gaps ask for less capacity but use salt more often, and salt setting also changes what a tank can hold.
This softener size is a rule of thumb, using a vendor's iron allowance, so check the manufacturer's capacity table at the salt setting you will use. Test your water first, as hardness and iron vary by supply. WQA warns against oversizing equipment. Softened water gains sodium, which matters if you are on a restricted diet, and high iron may call for a filter.
Sources and review
- Salt-free water softener options, G2275, Nebraska Extension, 2025.
- Water softening (ion exchange), G1491, Nebraska Extension, 2014.
- Water softening (ion exchange), WQ1031, NDSU Extension, 2022.
- Residential water treatment design, Water Quality Association, 2025.
- Getting smart with softeners, Water Quality Association, 2019.
- Softener sizing, Nelsen Corp, no date shown, read November 2026.
- Water softener sizing chart, Lowe's, no date shown, read only as a search excerpt.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Grains per day
Grains = People × Gallons × Hardness used
- Hardness used
- Grains per gallon, plus 4 for each ppm of iron
Worked through: 4 × 75 × 15 = 4500
Capacity
Capacity = Grains per day × Days between regenerations, up to the next size
- Days between regenerations
- 3 to 7, by the sources we read
Worked through: 4500 × 7 = 31500
The constants, and where they come from
PPM_PER_GPG
Published figureParts per million of hardness in one grain per gallon
Applies to: Hardness as calcium carbonate
Source: Salt-free water softener options, G2275, Nebraska Extension, published 2025-12, read 2026-11.
Also from: Water softening (ion exchange), WQ1031, NDSU Extension, published 2022-05, read 2026-11.
Nebraska's table uses 17.1 mg/L per grain per gallon. North Dakota State rounds this to 17, which makes little difference.
IRON_GPG_PER_PPM
Modelled from published dataGrains per gallon added for each ppm of ferrous iron
Applies to: A vendor's rule. No extension service or WQA figure was found
Source: Softener sizing, Nelsen Corp, no date on the page, read 2026-11.
Also from: Water softener sizing chart, Lowe's, no date on the page, read 2026-11.
Nelsen uses 4. Lowe's, read only as an excerpt, says 3 to 5 for every 1 ppm of iron. We use the middle, and show the result so you can see it.
FULL_SALT_SIZES
Published range, narrowedCommon softener capacities at full salt, in grains
- 0
- 24000
- 1
- 32000
- 2
- 40000
- 3
- 48000
- 4
- 64000
- 5
- 80000
- 6
- 110000
Applies to: Nelsen's tanks. Other makers have their own sizes
Source: Softener sizing, Nelsen Corp, no date on the page, read 2026-11.
These are one vendor's tanks. No public WQA sizing table was found, so check your own maker's table before you buy.
EFFICIENT_SALT_SIZES
Published range, narrowedThe same tanks at an efficient salt setting, in grains
- 0
- 18000
- 1
- 24000
- 2
- 30000
- 3
- 36000
- 4
- 48000
- 5
- 60000
- 6
- 84000
Applies to: Nelsen's tanks. Other makers have their own sizes
Source: Softener sizing, Nelsen Corp, no date on the page, read 2026-11.
Also from: Getting smart with softeners, Water Quality Association, published 2019, read 2026-11.
Capacity depends on salt dose. WQA says to set softeners to remove 4,000 grains per pound of salt, and a rating means little without its salt dose.
IRON_RATING_PPM
Published range, narrowedIron above which most softeners are not rated, in ppm
Applies to: Most softeners, and a given model may differ
Source: Softener sizing, Nelsen Corp, no date on the page, read 2026-11.
Also from: Water softening (ion exchange), WQ1031, NDSU Extension, published 2022-05, read 2026-11.
Also from: Water softening (ion exchange), G1491, Nebraska Extension, published 2014-09, read 2026-11.
Limits vary: Nelsen says 2 ppm, Nebraska says above 5 reduces effectiveness, and North Dakota State says up to 10. We warn at the lowest and ask you to check the model.
SODIUM_MG_PER_LITER_PER_GPG
Published figureSodium added to softened water per grain per gallon removed, in mg/L
Applies to: Salt-based ion exchange softeners
Source: Water softening (ion exchange), G1491, Nebraska Extension, published 2014-09, read 2026-11. This source is about 12 years old, and it is the one the figure rests on.
Nebraska says about 8 mg/L for each grain per gallon of hardness. This matters on a sodium-restricted diet, and your doctor can say by how much.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Modelled from published data:
- derived rather than measured, calibrated against the source named beside it.
- Published range, narrowed:
- a source publishes a range and this calculator uses a representative value from it.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the water softener size calculator
- 1
Test your water for hardness and iron
- 2
Enter the people in your home
- 3
Check the units on your hardness figure
- 4
Choose a salt setting
- 5
Check the size against your manufacturer's table
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- A person uses 75 gallons a day, and WQA also gives a lower figure of about 50
- Each ppm of iron counts as 4 grains per gallon, a vendor's rule
- Sizes are one vendor's tanks, and other makers differ
- A softener regenerates every 7 days by default, and sources say 3 to 7
Pro tips
- Test for iron as well as hardness
- Bypass outside faucets, as WQA suggests
- Set softeners to remove 4,000 grains per pound of salt, as WQA says