Quick take
Homes with large rain heads, body sprays or multiple showers running together should size from measured or manufacturer-rated fixture flow, not bathroom count. That often pushes the home toward 12–15+ GPM service-flow systems.
Why bathroom count can underestimate demand
One oversized shower system can draw more water than several conventional low-flow fixtures combined.
Choose service-flow margin
If realistic simultaneous demand approaches 10–12 GPM, a 9 GPM system has little margin for prefilter loading or additional fixtures.
Higher-flow examples
SpringWell CF4 is 12 GPM and RKIN CS10 is 15 GPM service flow. Very large homes can move to 20+ GPM tiers.
Check the whole plumbing path
The filter is only one restriction. Main-line size, meter, pressure regulator, softener and pipe length can all influence shower performance.