Use this as a planning target
Whole-house filters and softeners are flow-limited devices. A system can have enough media capacity yet still feel restrictive if its service flow is below real household demand. This tool uses practical fixture-flow assumptions; measured flow or a plumbing design value is better when available.
Method note. These calculators are for planning. Product selection still needs manufacturer service-flow, pressure-drop, certified performance, plumbing and local-code checks.