Size from simultaneous fixture demand, then choose a filter with continuous service flow above that demand. Treat peak-flow ratings as short-duration headroom, not the normal operating target.
Start with simultaneous fixtures
Estimate which fixtures can realistically run together: showers, faucets, washer, dishwasher, tub filler and irrigation if it passes through the filtered line.
Use service flow, not marketing capacity
A “1 million gallon” media-life claim says nothing about how many gallons per minute the system can treat without a noticeable pressure penalty.
Manufacturer reference points
SpringWell publishes 9, 12 and 20 GPM service tiers for CF1, CF4 and CF+. RKIN publishes 9, 11, 15 and 28 GPM service tiers for CS8, CS9, CS10 and CS13.
Leave margin
A system sized exactly to the calculated demand has little buffer for pressure variation, aging prefilters or a future fixture upgrade.