Softener Sizing

Notes

• Size by grains per day, not by house size. A softener does not care how many bathrooms there are — it cares how many grains of hardness pass through it. 4 people at 75 gallons each through 15 gpg water is 4,500 grains a day, and everything else follows from that one number.
• The number on the box is the maximum, not the working capacity. A 48k unit only reaches 48,000 grains at the highest salt dose, where salt efficiency is at its worst. Run at a sensible dose it delivers about 32,160 grains per cycle — which is the figure used above, and the one your salt bill will reflect.
• Regenerate roughly weekly. Stretching the cycle to save salt lets the resin bed sit loaded and channel, so hard water finds a path straight through a softener that looks like it is working. This size regenerates about every 7.1 days.
• Add iron to hardness before sizing. Every 1 ppm of dissolved iron behaves like roughly 4 gpg of hardness. A well at 15 gpg with 3 ppm iron is really sizing at 27 gpg, and skipping that step is the most common reason a correctly sized softener still delivers hard water. At this hardness the margin for that mistake is already thin.
• Get a real water test first. Municipal water reports publish hardness for free; well water needs a lab, and the lab will also give you the iron and pH figures that decide whether a softener alone is even the right equipment.