How the calculator works
- Hardness in gpg = mg/L as CaCO₃ ÷ 17.1 (Marlo).
- Compensated hardness = hardness + factor × (iron + manganese). Makers use ×2 (Marlo), ×3 (Hellenbrand), ×4 (Water-Right, CSI) or ×5 (A. O. Smith).
- Capacity needed = people × gallons per person × compensated hardness × days between regenerations.
- Size class: the smallest class with enough resin at your salt setting (table below).
Example (Marlo's): 4 people, 60 gallons, 18 gpg and 1.5 mg/L iron at ×2 → 21 gpg, 15,120 grains for 3 days. Defaults (4 people, 75 gal, 15 gpg, 7 days) → 31,500 grains: 48k class at 6 lb/ft³, 32k at 15 lb/ft³.
Capacity per cubic foot of resin by salt setting
| Salt (lb per ft³) | Hellenbrand ProMate 1 | Marlo | CSI | Hellenbrand grains per lb salt |
|---|---|---|---|---|
| 3.3 | 13,200 | — | — | 4,000 |
| 6 | 22,800 | 20,000 | 21,000 | 3,800 |
| 10 | 28,000 | — | — | 2,800 |
| 15 | 31,500 | 30,000 | — | 2,100 |
Marlo: the reduced (minimum) salt setting 'may not be used if iron is present', because a partly regenerated bed keeps iron that later oxidizes and fouls the resin.
Capacity of each size class at each salt setting
| Class | Resin (ft³) | 3.3 lb/ft³ | 6 lb/ft³ | 10 lb/ft³ | 15 lb/ft³ |
|---|---|---|---|---|---|
| 24k | 0.75 | 9,900 | 17,100 | 21,000 | 23,625 |
| 32k | 1 | 13,200 | 22,800 | 28,000 | 31,500 |
| 40k* | 1.25 | 16,500 | 28,500 | 35,000 | 39,375 |
| 48k | 1.5 | 19,800 | 34,200 | 42,000 | 47,250 |
| 64k | 2 | 26,400 | 45,600 | 56,000 | 63,000 |
| 80k* | 2.5 | 33,000 | 57,000 | 70,000 | 78,750 |
* Not in the Hellenbrand line; sized at the same 32,000 grains per ft³. Capacities are resin volume × the 1.0 ft³ model's per-foot capacity.
Check the service flow, too
Capacity sets how long the softener lasts; service flow is how much water it passes at once. Hellenbrand ProMate 1 specification table. We read the lower row as continuous flow at 10 psi pressure loss and the higher row as peak flow at 15 psi loss.
| Model | Resin (ft³) | Tank | Continuous (gpm, 10 psi loss) | Peak (gpm, 15 psi loss) |
|---|---|---|---|---|
| PM1-024 | 0.75 | 8 × 44 in. | 9.8 | 13.1 |
| PM1-032 | 1 | 9 × 48 in. | 10.1 | 13 |
| PM1-032-10 | 1 | 10 × 44 in. | 11.3 | 14.5 |
| PM1-048 | 1.5 | 10 × 54 in. | 10.5 | 14.1 |
| PM1-064 | 2 | 12 × 52 in. | 14.2 | 18.2 |
When this calculator does not apply
- Iron above 5 mg/L, manganese above 0.5 mg/L, red (oxidized) iron, iron bacteria or rotten-egg odor: treat these ahead of the softener. See the iron filter guide.
- Systems larger than 80,000 grains (farm, commercial, twin-tank).
- Salt-free "conditioners": a different standard, not covered here.
Where sources disagree: Iron factor: Marlo × 2, Hellenbrand × 3, Water-Right and CSI × 4, A. O. Smith × 5 gpg per ppm. None of the four explains its number. The calculator shows the result for × 2 and × 5 next to the factor you pick. Gallons per person per day: Hellenbrand's 2015 PM6 manual says 75 for all uses; its 2019 ProMate 1 manual says 50. Marlo uses 60. We default to 75, the highest published figure, so the softener is not undersized. Capacity at 6 lb salt per ft³: Hellenbrand 22,800, CSI 21,000, Marlo 20,000 grains. At 15 lb: Hellenbrand 31,500, Marlo 30,000. Hellenbrand prints 4,200 grains per lb for its 3.3 lb setting, but its own capacity row (13,200 grains ÷ 3.3 lb) works out to 4,000. The calculator uses the capacity row. Marlo's second worked example multiplies 3 × 60 × 15 and prints 3,240 grains/day; the product is 2,700. We do not use that example. Iron a softener can take: Penn State below 5 mg/L; Nebraska G1714 below 5 mg/L iron and 0.5 mg/L manganese, with makers' maximums of 0.5–5 mg/L; Nebraska G1491 5–10 ppm; NDSU up to 10 ppm iron and manganese.
Related
- Have a full lab report? Well water test results explained.
- No test yet? Well water test kit vs certified lab.
- Softener as one step of several: well water filtration system.
Frequently asked questions
What size water softener do I need for a family of 4?
At 75 gallons each and 15 gpg hardness, four people remove 4,500 grains a day, 31,500 a week: a 32,000-grain class at high salt or 48,000 at the 6 lb low-salt setting. Iron raises it.
How do you calculate water softener size?
People × gallons per person per day × hardness (gpg) = grains per day. Multiply by the days between regenerations. Add 2 to 5 gpg to the hardness for each mg/L of iron, depending on the maker.
How much iron can a water softener remove?
Penn State Extension: dissolved iron below 5 mg/L when pH is above 6.7 and hardness is 3 to 20 gpg. Nebraska Extension adds manganese below 0.5 mg/L. Red, already-oxidized iron fouls the resin.
Sources
- Marlo Inc., Water Chemistry and Treatment (Form 1-246-R1), p.1 and p.4 'Sizing Conventional Water Softeners' and 'Reduced Salt Setting' · checked October 5, 2026
- Hellenbrand, ProMate 1 residential water softener owner's manual (spec sheet 800870 Rev C, 5/13/19), p.3 job specification sheet and specifications table · checked October 5, 2026
- Hellenbrand, PM6 owner's manual (800198), job specification sheet · checked October 5, 2026
- Water-Right, Impression / Impression Plus Series installation instructions and owner's manual (2023), p.8 Programming · checked October 5, 2026
- CSI Water Treatment Systems, High Efficiency Softener installation / operation manual (2025 upload), p.3 specifications and p.11 programming · checked October 5, 2026
- A. O. Smith water softener owner's manual (AO-WH-SOFT-500), pp.4-5 Ferrous Iron and Compensated Hardness · checked October 5, 2026
- Penn State Extension, Water Softening (updated 2025-09-08), Table 1 and text · checked October 5, 2026
- Penn State Extension, Iron and Manganese in Private Water Systems (2025-05-29) · checked October 5, 2026
- Nebraska Extension, Drinking Water: Iron and Manganese (G1714, revised May 2021), pp.3-5 · checked October 5, 2026
- Nebraska Extension, Drinking Water Treatment: Water Softening (Ion Exchange) (G1491, revised September 2014), pp.1-3 · checked October 5, 2026
- NDSU Extension, Water Softening (Ion Exchange) (WQ1031, revised May 2022), p.1 · checked October 5, 2026
- U.S. EPA WaterSense, Guide to Selecting Water Treatment Systems (832-F-24-024, November 2024), p.4 · checked October 5, 2026
Method: The formula, gallons-per-person figures, iron factors and capacity per cubic foot are copied from four softener makers' manuals (Marlo, Hellenbrand, Water-Right, CSI, A. O. Smith) and checked against Penn State, Nebraska and NDSU Extension publications on 2026-10-05. Where makers disagree, every value is listed and the calculator lets you choose. We have not installed or tested any softener.