Current SpringWell tiers use roughly 9-inch, 10-inch and 13-inch tanks for 9, 12 and 20 GPM service. RKIN similarly scales tank width from 8/9/10 to 13 inches as service flow rises from 9 to 28 GPM.
9-inch-class tanks
SpringWell CF1 uses a 9-inch tank at 9 GPM service. RKIN CS9 uses a 9-inch tank at 11 GPM service.
10-inch-class tanks
SpringWell CF4 uses a 10-inch tank at 12 GPM. RKIN CS10 uses a 10-inch tank at 15 GPM.
13-inch tanks
SpringWell CF+ and RKIN CS13 move to 13-inch tanks for much larger homes and higher service-flow requirements.
Why diameter is not enough
Valve design, media type, bed depth and connection size also affect usable flow. Use tank diameter as context, not the sole sizing rule.
Whole-house sizing and evidence checks for Whole-House Carbon Tank Size: 9-Inch vs 10-Inch vs 13-Inch Tanks
Whole-house treatment is a flow problem as well as a contaminant problem. Identify whether the home needs sediment removal, chlorine or chloramine reduction, hardness treatment, iron treatment or another specific process, then compare the exact system's evidence for that job. Carbon media type alone is not a substitute for model-level performance data when a health-related contaminant drives the purchase.
Size the system for peak service flow. Bathroom count can be a rough shortcut, but simultaneous showers, laundry, high-flow fixtures and irrigation connections can produce very different demand in two homes with the same number of bathrooms. A system with insufficient service flow can create pressure complaints even when its media capacity looks generous.
Installation, maintenance and lifecycle cost
Plan for bypass valves, drain or backwash requirements, prefiltration and access for future service before choosing equipment. Large media tanks need floor space and may require enough incoming pressure and flow for backwashing. Cartridge systems need room below the housings to remove sumps without dismantling nearby plumbing.
Compare lifecycle cost rather than the tank's headline gallon rating. Include sediment cartridges, media replacement, backwash water, electricity where applicable and professional service. Long media life is valuable only when the system remains correctly sized and the replacement media or parts are realistically available.