For a typical two-bathroom home, a whole-house filter in the 9–11 GPM service-flow range is often a practical starting point. SpringWell CF1 is rated 9 GPM for 1–3 bathrooms; RKIN CS8 is 9 GPM service / 12 GPM peak and CS9 is 11 / 17 GPM.
Estimate simultaneous use
Think in realistic combinations: one shower plus laundry, or two showers plus a faucet. The filter should handle that peak without becoming the main restriction in the plumbing.
Manufacturer examples
SpringWell sizes CF1 for 1–3 bathrooms at 9 GPM service flow. RKIN sizes CS8 for 1–3 bathrooms at 9 GPM service and CS9 for 2–5 bathrooms at 11 GPM service.
When to move up
Choose the larger flow tier when the home has high-flow fixtures, multiple simultaneous showers, a large soaking tub or consistently high municipal pressure and demand.
Do not size from tank width alone
Compare service flow, connection size and pressure behavior. A physically larger tank is useful only if the valve and media bed support the flow you need.