How the math works
Softenry mathGrains removed a week = people × gallons a day × (hardness + 4 × iron ppm) × 7. Salt a week = grains ÷ grains per pound of salt.
SourceSalt efficiency falls as the salt setting rises. Water Conditioning & Purification's NSF/ANSI 44 article notes California requires efficiency-rated softeners to reach at least 4,000 grains per pound, and gives an example falling to about 1,867 at a heavy setting.
Questions
How much salt does a softener use a month?
For three people on 15 gpg water at an efficient setting, about 6 lb a week, so one 40 lb bag every six or seven weeks; at a heavy setting about twice that.
How can I cut salt use?
Set the hardness to your test result, use a metered valve, and use the lowest salt setting that keeps the water soft.