How to decide what size whole house filter you need.

Pure Water Occasional Technical Wizard Pure Water Annie

by Pure Water Annie

More good advice from the Pure Water Occasional  Technical Department


One of the critical factors in determining the size of a “whole house” water treatment system, whether you’re removing iron from well water or chemicals from city water, is the rate of service flow you need. To work effectively, the filter must be large enough to handle the volume of water, in gallons per minute, that you plan to run through it.

Here’s a chart that will help you take an educated guess at how much water you would expect to use. The numbers are based on Annie’s years of experience in water treatment and some clever theft from other sources.

Number of Residents 1-2 Bathrooms 2-3 Bathrooms 3-4 Bathrooms 4-5 Bathrooms
1-2 5 GPM 7 GPM 10 GPM 12 GPM
2-4 7 GPM 10 GPM 12 GPM 14 GPM
5-6 10 GPM 12 GPM 14 GPM 18 GPM
7-8 10 GPM 12 GPM 14 GPM 18 GPM
9-10 12 GPM 14 GPM 18 GPM 20 GPM

GPM means Gallons Per Minute of Service Flow.

This chart is intended as a suggestion only. The nature of the building and the individuals who live there must be taken into consideration. It is intended for use in sizing for standard residential dwellings. Mansion dwellers must look elsewhere for advice.