Water Softener Size Calculator: Salt and Regeneration

Daily compensated load is $L = Q(H + k_{Fe}Fe)$, using US gallons and grains/US gal. GE's household sizing guide supplies the default iron allowance, 5 gpg per mg/L Fe. The coefficient is an adjustable sizing convention, not iron stoichiometry.

Aldex C-800, Rev. ADX13643-1123, PDF p. 1, gives two operating points: 20,000 grains/ft³ at 6 lb NaCl/ft³ and 30,000 at 15 lb/ft³. A custom rating must supply both values. No intermediate-dose curve is assumed. The mass efficiency step converts CaCO₃-equivalent grains to pounds before dividing by pounds of NaCl, giving a dimensionless mass ratio.

With reserve fraction $r$, required resin is $V_L=LT/[c(1-r)]$. Peak-flow screening uses Aldex's upper normal rate, 5 US gpm/ft³ (PDF p. 1). New-bed sizing rounds $\max(V_L,V_{flow})$ upward in 0.25 ft³ planning increments; this is not a product catalogue. Installed-bed mode uses the entered volume.

Operating capacity is $C=Vc$, usable capacity $C_u=C(1-r)$, and actual interval $D=\min(C_u/L,D_{override})$. Useful service volume is $QD$. Salt per cycle is $Vs$; regeneration water is the selected bed-volume ratio times resin volume. Annual totals multiply each full cycle by $365/D$.

Hardness conversion uses the physical grain and US liquid gallon, retaining full precision (about 17.118061 mg/L per gpg). Vermont Department of Health gives the sodium approximation $\Delta Na=0.46H_{removed}$ in mg/L. This covers Ca/Mg hardness exchange only, not compensated iron or background sodium.

The illustration caps elapsed service time at the scheduled regeneration, then advances a sharp front by $f=\mathrm{clamp}(tL/C,0,1)$ through an effective operating-capacity bed. Reserve and any calendar override stop service before the entire budget is used. This is a capacity-budget illustration, not a transport or breakthrough model.

Custom NaCl ratings are checked against the ideal exchange ceiling. DuPont's Ion Exchange Memento Manual, Rev 0 (March 2024), pp. 7–8, gives rounded equivalent weights of 50.0 for CaCO₃ and 58.5 for NaCl. One equivalent of exchanged hardness requires one equivalent of regenerant sodium, so the ideal mass ratio is 50/58.5 = 0.8547 (about 5,983 grains per lb NaCl). A custom operating capacity above that ratio is rejected. This is only a necessary physical bound; real regeneration consumes more salt. Peak service flow must also be no smaller than daily average demand after unit conversion.

• Steady daily softened demand and feed hardness; full removal of Ca/Mg hardness, with no blending or bypass within the stated demand.
• Sodium chloride regeneration with the selected paired operating capacity and dose; fresh, correctly operated resin. No certification is implied by the estimate.
• Default demand is an illustrative four-person household at 60 US gal/person/day, not a measured consumption standard. Default reserve is 10%, following the convention in Chem-Aqua TB4-019; use the reserve strategy required by the actual controller. Do not apply both this reserve and an additional identical controller reserve.
• Default 7-day target and 14-day calendar override are editable planning choices, not sanitation limits or universal manufacturer recommendations.
• Default total regeneration water is an illustrative 10 bed volumes (74.8 US gal/ft³). Aldex lists 20 US gal/ft³ slow rinse and 30 US gal/ft³ fast rinse, plus a backwash of at least 10 minutes; those rinse values alone do not establish total full-cycle water. Enter a measured or equipment-rated total including all steps and brine makeup.
• Every regeneration uses a full programmed salt and water cycle, even under calendar-limited operation. No proportional brining, variable-fill control, holidays, startup losses or deterioration model.
• Annual values use 365 days of continuous operation; fractional regenerations express a long-run average.

Conventional downflow, sodium-cycle strong-acid cation softening for household and light-commercial planning. Follow the supplier's feed-quality, chlorine, iron, brine concentration, resin depth and flow limits; the Aldex flow figure is only a screening criterion. It does not establish pressure loss, leakage or valve suitability. An iron allowance cannot predict fouling or prove removal of manganese, oxidized iron or organic complexes.

Use measured inlet hardness and softened demand. No hardness-breakthrough guarantee is made. The chart and front use an effective operating budget, not the resin's total exchange capacity or a resolved mass-transfer zone. The sodium estimate is added sodium from hardness only; total sodium requires inlet/finished-water testing and consideration of other ions.

Not a design for demineralization, potassium-chloride regeneration, medical water, high-purity process water, pressure vessels or uninterrupted soft-water service. Check the selected equipment's published rating, backwash supply and drain requirements. Installation work belongs to a qualified person.

Softening does not disinfect water, and it adds sodium: on a sodium-restricted diet, ask your doctor about softened drinking water.