Method version 1.3 · Sources checked September 21, 2026
How the sizing estimate works
The result is a calculated working-capacity requirement for a stated planning scenario. It is not a product rating, a certified system design or a controller setting. Source links appear beside the facts they support. A source for one step does not validate the entire system.
1. Normalize hardness and estimate use
GPG = hardness in mg/L as CaCO₃ ÷ 17.1. For dilute household water, ppm is treated as equivalent to mg/L. Pentair explains the conversion.
Default daily use = people × 75 US gallons. HomeWater101’s sizing guide uses this baseline and a seven-day treatment load. This is a published planning convention, not measured household consumption. Replace it with actual softened-water use where available, excluding bypassed outlets.
2. Allow for dissolved iron
Effective hardness = hardness in GPG + (5 × dissolved iron in ppm). Morton specifies this adjustment for its softeners treating ferrous iron. Applying it here is a screening convention, not evidence that another model can treat iron. Unknown iron is not a verified zero. Ferric iron needs a separate treatment assessment.
3. Show treatment load and reserve separately
Daily load = daily gallons × effective hardness. Seven-day load = daily load × 7. The displayed scenario adds 1 day of demand: working-capacity requirement = daily load × 8.
Pentair’s Fleck 5800 SXT manual, page 6, explains that delayed regeneration needs reserve to cover use until regeneration starts. Page 11 describes fixed, percentage and variable reserves. It does not prescribe a universal one-day allowance.
The extra day is therefore an explicit planning scenario for daily delayed regeneration, not a manufacturer mandate or a guaranteed safety margin. Actual reserve depends on demand and controller operation. Immediate-regeneration systems may differ. A supplier must confirm the appropriate reserve and maximum interval.
4. Use actual capacity at a stated salt dose
An advertised grain label does not establish capacity at every salt setting. The Whirlpool WHES40/WHES44 manual, document 7392319 revision G, page 3, gives these model-specific ratings:
| Model | Capacity (grains) | Salt dose (lb) |
|---|---|---|
| WHES40 | 17,300 | 3.4 |
| WHES40 | 31,600 | 9.0 |
| WHES40 | 40,000 | 15.5 |
| WHES44 | 18,700 | 3.7 |
| WHES44 | 34,800 | 9.7 |
| WHES44 | 44,000 | 16.7 |
The manual limits its efficiency rating to the minimum salt dose and rated service flow, and describes variable salt dosing. These figures cannot be transferred to another model or treated as user-selectable controller settings.
The calculator no longer translates the requirement into a generic nominal size or predicts a regeneration interval from an assumed capacity. Ask the supplier for the selected model’s operating capacity, salt dose, service flow and controller behavior.
5. Worked example
Four people at 10 GPG with no dissolved iron: 300 gallons/day × 10 grains/gallon = 3,000 grains/day. Seven days requires 21,000 grains. The one-day reserve scenario adds 3,000, producing 24,000 grains. This is calculated arithmetic, not a measured product rating. It does not mean every softener advertised as “24,000 grains” is suitable.
6. Screen products separately
Published specifications and missing data remain visible on each product card. A product below the scenario load is excluded from this scenario, not declared unsuitable for every possible regeneration schedule. Unverified working capacity never becomes a confirmed match. Bathroom count does not prove service-flow suitability. Commissions and prices do not influence the calculation or ordering.
Water source and manganese
Water source changes warnings and product screening, not the capacity arithmetic. Bathroom count screens published product ranges only. Manganese is not included in the numerical capacity calculation and requires a separate product and treatment assessment. No manganese multiplier is assumed. Read the iron and manganese guide.
Well water and limitations
The calculator cannot identify contaminants or establish drinking-water safety. EPA recommends annual private-well testing for coliform bacteria, nitrates, TDS and pH, plus iron and manganese testing when considering softening. Seek certified laboratory testing and local advice.
Salt consumption and actual regeneration intervals are omitted because they require the selected equipment and operating settings. Interface input limits are software limits, not treatment-performance ratings.