A whole-house filter has to do two jobs at once: treat the water for the claim you care about and pass enough flow without unacceptable pressure loss. Compare both before price.
1. Start with service flow
Use your simultaneous-flow target, not average daily gallons. A filter can have a large advertised gallon capacity and still be a poor hydraulic fit if its service flow is too low. Use the flow-rate calculator when you do not have a measured value.
2. Look for pressure-drop context
A flow number is more useful when the manufacturer tells you the corresponding pressure drop and test conditions. If that context is absent, treat the number as less comparable.
3. Separate media life from contaminant reduction
“1,000,000 gallons” is not the same thing as a certified reduction claim for every contaminant. Replacement life can depend on influent concentration, flow, media, fouling and the manufacturer’s end-of-life definition.
4. Verify the exact claim
NSF notes that NSF/ANSI 42 addresses aesthetic effects such as chlorine/taste/odor, while NSF/ANSI 53 addresses health-effect contaminant reductions. The standard number itself is not a promise that every claim under that standard applies to every product.
5. Price the replacement cycle
Compare replacement media/cartridge cost, expected life under your usage, prefilter demand and any service labor. Put recurring cost beside purchase/install cost using the ownership-cost calculator.
6. Check the actual product against your plan
Enter service flow and, for softening products, usable capacity in the Product Fit Checker. Then confirm installation, backwash/drain requirements and certification separately.