Hardness conversion
gpg = mg/L as CaCO₃ ÷ 17.1. USGS field documentation uses 17.1 mg/L per grain per gallon.
Hardness class
USGS: 0–60 mg/L soft; 61–120 moderately hard; 121–180 hard; above 180 very hard.
Daily softening load
daily grains = hardness (gpg) × people × softened gallons/person/day. The main planner defaults to 75 gallons/person/day because Penn State Extension uses that value in a sizing example. Users can change the value in both the main planner and the dedicated calculator; measured softened-water use is preferable when available.
Planning capacity band
We multiply daily load by seven days and add 20% reserve, then round up to a common nominal grain band. This is only a shopping/sizing band. Final selection must use the specific product's usable capacity at the intended salt dose.
Flow estimate
If you do not enter measured peak flow, the main planner uses a conservative bathroom-count heuristic. The dedicated fixture-flow calculator is more explicit. Neither is a plumbing-code fixture-unit calculation.
Iron, manganese and pH flags
We flag iron above 0.3 mg/L, manganese above 0.05 mg/L and pH outside 6.5–8.5 because these are EPA secondary (aesthetic/nuisance) guidance values. We do not automatically convert iron or manganese into “extra hardness.”
Health-contaminant handling
The planner does not prescribe treatment for lead, arsenic, PFAS, nitrate, bacteria or other health concerns. It directs users to lab/CCR evidence and contaminant-specific certified treatment.
Sizing confidence grade
The main planner grades only the quality of the sizing inputs, not water safety. A laboratory or utility hardness value carries more weight than an estimate; entered/measured peak flow carries more weight than a bathroom-count estimate; and measured softened household use carries more weight than a per-person default. For private wells, known iron and manganese results improve the sizing-data grade.
Product Fit Checker
The Product Fit Checker performs two independent comparisons: candidate service flow versus the household flow target, and candidate usable capacity versus the design grain requirement. It does not infer missing product data and does not convert a peak-flow number into a service-flow rating. Certification, plumbing, backwash/drain requirements and contaminant selection remain separate checks.
Why nominal capacity is not treated as usable capacity
The planner stores nominal capacity only as a shopping label. Sizing decisions should use the capacity documented for the operating point or salt dose you plan to use. The Softener Efficiency calculator therefore takes usable capacity and salt dose as separate inputs and reports the arithmetic ratio in grains per pound.