Water softener size calculator
Softeners are sold by a grain number that assumes the heaviest salt dose, then left on a lighter one. Size from the house's own water use, hardness and iron, choose the interval and the salt setting, and the resin volume falls out, along with how often it will really regenerate and how much salt it eats.
Pre-filled with a four-person house on 15 gpg water. Put your own test result in.
From a lab test or the utility's water quality report.
38,400
grains between regenerations
1.5 ft³ of resin, run at 10 lb/ft³
- Hardness
- 15.0 gpg (very hard)
- Compensated hardness, iron included
- 15.0 gpg
- Household use
- 320 gal/day
- Hardness removed per day
- 4,800 grains
- One day of reserve
- 4,800 grains
- Resin needed at this salt setting
- 1.37 ft³
A 1.5 ft³ unit (sold as a 48,000-grain softener) removes 42,090 grains per cycle at the medium salting setting. On your water it regenerates about every 7.8 days, using 15.0 lb of salt a time — about 58 lb a month.
| Resin | Sold as | Grains per cycle | Days on your water |
|---|---|---|---|
| 0.75 ft³ | 24,000 grain | 21,045 | 3.4 |
| 1 ft³ | 32,000 grain | 28,060 | 4.8 |
| 1.5 ft³ | 48,000 grain | 42,090 | 7.8 |
| 2 ft³ | 64,000 grain | 56,120 | 10.7 |
What a softener size actually is
A softener is sized in grains of hardness it can remove before it has to regenerate, not in gallons. The sum is three numbers multiplied together: the gallons the house softens a day, the hardness of that water in grains per gallon (with iron added on top as "compensated hardness"), and the days you want between regenerations. That gives the grains the resin must hold. The resin volume that holds them depends on how much salt each regeneration is allowed to use, which is the part most sizing charts skip.
This page sizes the unit. What it costs to buy and fit is a separate question with its own variables, the loop and the drain mostly, covered in water softener installation cost.
How to use the calculator
- 1
Get a real hardness number
Use a lab test or the utility's water quality report. Reports usually give hardness in mg/L (or ppm) as calcium carbonate; switch the unit and the tool divides by 17.12, the NIST factor for one grain per U.S. gallon (17.118 06 mg/L). Well water needs an iron result too, and the iron must be the dissolved, ferrous kind.
- 2
Set the gallons per person
USGS puts indoor home use at about 80–100 gallons per person per day, so the tool starts at 80. Hellenbrand's own sizing worksheet uses 50 for everything softened, 40 if the toilets stay on hard water and 30 if only the hot side is softened. Better than either is the house's own meter: a winter bill divided by the days and the people. If that number looks high, run the meter test on the leak cost calculator before you size anything around it.
- 3
Choose the interval and the salt setting
Longer between regenerations means more resin. A lower salt setting means more resin for the same interval but less salt per grain removed. Pick the setting you will actually leave the valve on, not the one printed on the box.
- 4
Read the size, then check the days
The copper ring shows the smallest resin volume that covers the load. The table under it shows how many days each size really lasts on your water, which is the number to compare when two quotes name different tanks.
Why the salt setting decides the size
The grain number on a softener is its capacity at the heaviest salt dose, and almost nobody runs it there. Hellenbrand's E3 table shows the same cubic foot of resin removing 32,310 grains on 15 lb of salt but 13,972 on 3.3 lb. A.O. Smith's AOW-S400 is sold as a 36,000-grain unit; its manual lists that at 15 lb and 13,400 grains at its 3 lb high efficiency setting, which is the factory default.
| Setting | Salt per ft³ | Grains per ft³ | Grains per lb |
|---|---|---|---|
| High efficiency | 3.3 lb | 13,972 | 4,234 |
| Low salting | 6 lb | 22,930 | 3,822 |
| Medium salting | 10 lb | 28,060 | 2,806 |
| High salting | 15 lb | 32,310 | 2,154 |
Capacity falls more slowly than the salt. Chem-Aqua's technical bulletin 4-006 puts it plainly: a 50% cut in salt costs only 27% of the capacity, and doses above 15 lb per cubic foot do not significantly add any. That is the case for a bigger tank on a lighter setting. NSF/ANSI 44 sets the bar for an "efficiency-rated" softener at 3,350 grains removed per pound of salt, on a demand-initiated unit.
Size on the setting, then label it
Worked example: four people on a well
The lab report reads 342 mg/L hardness and 1 ppm ferrous iron. Divided by 17.12, the hardness is 20.0 gpg. The softener's manual adds 5 gpg per ppm of iron, so the compensated hardness is 25.0 gpg. Four people at 80 gallons is 320 gallons a day, so the resin removes 7,993 grains a day. A week plus a day of reserve is 63,946 grains.
| Interval and setting | Grains needed | Resin | Real interval | Salt a month |
|---|---|---|---|---|
| 7 days, medium salting | 63,946 | over 2 ft³ | – | – |
| 7 days, high salting | 63,946 | 2 ft³ (64,000) | 7.1 days | 127 lb |
| 4 days, medium salting | 39,966 | 1.5 ft³ (48,000) | 4.3 days | 105 lb |
At the medium setting a week does not fit in any single residential tank on that table. Two honest fixes come out of the same arithmetic. Run the 2 ft³ unit at high salting and it lasts about a week, using 30 lb a regeneration. Or accept a shorter interval: the 1.5 ft³ unit at medium salting regenerates every four days or so and burns less salt over a month, because capacity per pound is better at the lighter dose.
Iron, flow and the checks that overrule the sum
Only dissolved, ferrous iron is a softener's job, and it costs capacity. How much is a manufacturer's call: A.O. Smith tells owners to add 5 gpg (86 mg/L) for every 1 ppm of ferrous iron, Hellenbrand's worksheet adds 3 gpg. Use the figure in the manual of the unit you are fitting; the calculator offers both.
| Iron in the test | What a softener does with it |
|---|---|
| Ferrous (clear-water, dissolved) | Removes it, at the cost of capacity. Count it in compensated hardness. |
| Ferric (particles) | Fouls the resin bed. Filter it out ahead of the softener. |
| Organic or bacterial | Needs separate treatment. |
| Colloidal (suspended) | Cannot remove it. |
The limits are printed too. Hellenbrand designs its E3 units for up to 100 gpg of hardness and 2 ppm of ferrous iron; A.O. Smith lists a maximum compensated hardness of 90 gpg on the AOW-S400 and 10 ppm of ferrous iron. Above the figure in your unit's manual, the answer is iron treatment ahead of the softener, not a bigger softener.
Grains are only half of sizing. The tank also has to pass the house's peak flow without starving the top floor: the AOW-S400 is rated 7.5 gpm at a 15 psi drop, the Hellenbrand E3-032 13 gpm at the same drop. Check the peak demand from the fixture unit calculator against the service flow, and carry that pressure drop into the water pressure calculator before you promise a shower nobody will complain about. A house that softens only the hot side sizes the softener from the water heater's demand, not the whole house's.
Regeneration is water too
Code notes
IPC 2021 608.4, in the section on protecting the potable supply, names softeners among the appliances and devices that handle or treat potable water: on paper the unit is supply equipment, not a drain appliance. The manufacturers ask for the regeneration discharge to reach a floor drain or standpipe through an air gap or siphon break; how the discharge is classed and which detail is accepted is decided by the authority having jurisdiction (AHJ), along with the adopted code edition and any local amendment. The cross-connection logic is the same one behind backflow preventer requirements.