What size whole-house water filter for a 3, 4, or 5-bathroom home?
Whole-house filters are sized by peak flow rate — how much water moves when several fixtures run at once — so your pressure holds up. Here are the quick answers, then the formula.
Quick answers
- 3-bathroom home: ~10–11 GPM peak → a mid (8–12 GPM) class system.
- 4-bathroom home: ~14 GPM peak → top of the mid class, often into high-flow.
- 5-bathroom home: ~18 GPM peak → a high-flow (15–20 GPM) class system.
A 1–2 bathroom home generally lands in the entry (~7 GPM) class.
The formula
Peak flow ≈ full bathrooms × 3 fixtures × 2.0 GPM × 0.6 (a simultaneity factor, since not everything runs at once) — which simplifies to roughly 3.6 GPM per bathroom, with a 7 GPM floor for small homes. If you'd rather be precise, list the fixtures that could realistically run together and add their flow rates.
Why size up, never down
An undersized whole-house filter causes pressure drops and shortens media life. When your number falls between two classes, choose the larger one. The selector tool does this math automatically and pairs it with the right system type for your actual water — because flow rate tells you the size, but your contaminants tell you the type.