Softeners are sold by grain capacity: how many grains of hardness the resin can remove before it has to regenerate. Too small and it regenerates constantly, wasting salt and water. Too big and the resin sits unused. Here’s the math, with every assumption shown. Want to skip ahead? Use the sizing calculator.
Step 1: Get your hardness number
You need hardness in grains per gallon (gpg). Reports often list it in mg/L or ppm “as calcium carbonate”; divide by 17.1 to get gpg (1 grain = 64.8 mg and 1 US gallon = 3.785 L, so 64.8 ÷ 3.785 ≈ 17.1). City water customers can find it in their utility’s Consumer Confidence Report; well owners should use a lab test. See How to Find Your Water Hardness.
If your water has iron, add about 4 gpg for every 1 ppm (mg/L) of iron. That’s a rule of thumb (sources use 3 to 5), and it’s our assumption. A few ppm of iron or more usually calls for a dedicated iron filter, so get advice from a water-treatment professional.
Step 2: Grains per day
Grains per day = people × gallons per person per day × compensated hardness (gpg)
We default to 75 gallons per person per day, a common rule of thumb. Your water bill gives a better number: divide monthly gallons by 30 and by the number of people.
| Household | Hardness | Gallons/day | Grains/day |
|---|---|---|---|
| 2 people | 10 gpg | 150 | 1,500 |
| 4 people | 15 gpg | 300 | 4,500 |
| 4 people | 15 gpg + 1 ppm iron (19 gpg) | 300 | 5,700 |
| 6 people | 25 gpg | 450 | 11,250 |
Step 3: Pick a capacity
Maximum grain ratings assume a heavy salt dose. Most owners set a softener for salt efficiency, which gets less capacity per regeneration but more grains per pound of salt. We assume you can use about 67% of the maximum rating at an efficient setting, and we aim for about 7 days between regenerations. Both are our assumptions.
Rated capacity needed ≈ grains per day × 7 ÷ 0.67
For the 4-person, 15 gpg home: 4,500 × 7 ÷ 0.67 ≈ 47,000 grains, so a 48,000-grain class softener. At 67% usable, it regenerates about every 7 days.
For a reference point, NSF/ANSI 44’s “efficiency rated” claim requires at least 3,350 grains of hardness removed per pound of salt and no more than 5 gallons of regeneration water per 1,000 grains removed, according to NSF’s listing notes (checked 2026-10-19).
Step 4: Check flow rate
Capacity isn’t the only spec. A softener also has to keep up with your peak flow in gallons per minute (gpm), when showers, a washer, and a faucet all run at once. Makers list a flow rating next to each size. Larger homes with more bathrooms need higher flow ratings, often the next size up.
Real sizes and prices from an official store
Here’s how one retailer, US Water Systems, labels its metered softeners on its official product pages. Prices are list prices shown on 2026-10-19 and change often. Grain numbers are the maker’s maximum ratings.
| Model (official page) | Max capacity | Flow | Price (checked 2026-10-19) |
|---|---|---|---|
| Fleck 5600 SXT Metered | 26,000 grains | 7 gpm | $799.00 |
| Fleck 5600 SXT Metered | 35,000 grains | 10 gpm | $876.00 |
| Fleck 5600 SXT Metered | 53,000 grains | 15 gpm | $1,010.00 |
| Fleck 5600 SXT Metered | 70,000 grains | 20 gpm | $1,045.00 |
| Flexx-HD Economy Metered | 35,000 / 53,000 / 70,000 grains | 10 / 15 / 20 gpm | $1,179.95 / $1,414.95 / $1,575.00 |
| Matrixx Smart Metered | listed by flow | 10 / 15 / 20 gpm | $1,466.95 / $1,635.00 / $1,775.00 |
For our 4-person, 15 gpg example (needs ≈ 47,000 rated grains), the 53,000-grain class would give about 35,500 usable grains at 67%, or roughly 7.9 days between regenerations.
Certification check: On 2026-10-19 we searched NSF’s NSF/ANSI 44 softener listings for “US Water” and NSF returned no matching products. The product pages describe NSF-certified components (such as resin or tank materials), which isn’t the same as a system certified to NSF/ANSI 44. Read more in our certifications guide.
Quick checklist
- Hardness in gpg, plus iron if you’re on a well.
- Real water use from your bill, if you can.
- Capacity that gives roughly a week between regenerations at an efficient salt setting.
- A flow rating that covers your peak use.
- Certification claims verified in NSF’s listings, not just on the box.
- Install per the manual and local plumbing code, including a proper drain air gap.