For quick planning, our main tool uses about 12 GPM as a conservative starting point for a 3-bathroom home when no measured peak flow is available. It is not a plumbing-code fixture-unit calculation.
Refine it with simultaneous fixtures
Think about what actually runs together during your busiest 10 minutes: showers, faucet use, toilet refill, laundry, dishwasher and tubs.
Use the fixture flow calculator
Why this matters
A filter can have plenty of media life but still create poor shower performance if its service flow or pressure-drop characteristics are too restrictive.
A fixture example for a 3-bathroom home
Using the simple fixture assumptions in our calculator, two showers, two sinks, a toilet refill and a clothes washer fill would total roughly 11.1 GPM. The quick 12 GPM starting target leaves planning room because real fixtures, fill valves, pipe pressure and household habits vary. It is deliberately not presented as a plumbing-code calculation.
Check the number on every whole-house component
Your softener, sediment filter, carbon tank, iron filter and any other point-of-entry equipment all sit in the same flow path. One restrictive component can become the bottleneck. Compare rated or service flow together with pressure drop at that flow; a large media tank is not automatically a high-flow system.
Measured peak flow beats a bathroom shortcut
If you know the actual flow during your busiest period, use it. Otherwise, list the fixtures that realistically overlap and use the fixture flow calculator. Homes with large soaking tubs, body sprays, multiple high-flow showers or unusually high/low water pressure deserve a plumbing-specific check rather than a bathroom-count estimate.
Flow sizing and contaminant capacity are separate decisions. A filter may have years of media life but still be a poor fit if it causes unacceptable pressure loss at your peak demand.