Calculators
Sizing calculators for home water treatment: grain capacity, flow rate, micron rating and more, with the formula shown.
Inputs you already have, one formula, one answer. Every calculator shows its working and names the source the formula came from, so you can check it rather than trust it.
Water softener size calculator: grain capacity from people, hardness and iron
Grains = people x 75 gal/day x (gpg + 5 per mg/L iron) x days between regenerations. 4 people at 10 gpg for 7 days is 21,000 grains. Calculator and the size table.
Whole-house filter flow rate calculator: GPM from fixtures and bathrooms
Peak gpm = showers running at once x 2.5 + one faucet at 2.2, or WQA fixture units x 0.4 + 2. A 2-bath home pulls about 7 gpm; a 20 x 4.5 in housing is rated to 15.