Whole House Water Filter Sizing: Flow Rate, Pressure Drop, and What Size You Actually Need

Most homes need a whole house water filter that can sustain 8 to 12 gallons per minute (GPM) at peak demand. That is the water running at your busiest moment, like two showers and a toilet filling at once, and it is the number to size the filter to.

Caller "What is the flow rate on that canister?"

Aidan "The non-backwashing is about 17 gallons per minute you can get through it. I have the same filter in my house, and you can still run two showers and have good pressure."

A man in Texas asked me that while planning our non-backwashing carbon tank with a Big Blue sediment filter ahead of it. This page is the long version of my answer.

How much water does a house need?

A house needs as many gallons per minute as its fixtures use when the most of them run at once. Match the tank to the peak, not the average. A busy morning, like the ones below, comes to 8 to 12 GPM.

The number of bathrooms matters mostly as the number of showers that can run at the same time. As I tell people who call me, it can be a mansion with two people, and the water usage is going to be for two people. It doesn't matter how big the house is.

To find your own peak, add up what runs together on a busy morning. These are the flows the plumbing code sizes each fixture's supply line for (Table 604.3 of the 2024 International Plumbing Code, as Maryland adopted it), with the federal limits on showerheads and faucets beside them.

Add them the way your house actually runs. Two showers and a toilet filling come to 8 GPM. Add the dishwasher and you are at 10.75; add the kitchen sink too and it is 12.5. A tub filling while someone showers and a toilet refills is 9.5, and the hose running while someone showers is 7.5.

What size filter does my house need?

For most homes I recommend the 2.5 cubic foot carbon filter. It's one size that fits all: it handles 2 to 10 people in the house and passes about 17 GPM. The 1.5 cubic foot tank is for 2 to 4 people and passes up to about 10 GPM, which a two-shower morning can reach.

In this industry, bigger is better: you get better flow rates, greater capacity and less maintenance. We always size bigger for most of our applications.

If you are putting a Big Blue ahead of the carbon tank, go with the 20, because it doesn't restrict the flow like the 10 does. The Big Blue is for sediment only and goes before the other filters. The carbon in either tank lasts about 4 to 5 years. Both carbon tanks are non-backwashing: no drain line and no electricity. If your water has heavy sediment or some iron, read backwashing vs non-backwashing carbon filters first, and every whole house carbon filter we sell is on one page.

Not sure which size fits?

Send me your household details

Text me how many people live there, how many showers run at once, your pipe size and whether you are on city or well water. I'll tell you which size I would put in.

Text Aidan443-277-2204 Call800-460-5810Aidan

8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.

How to tell your pipe size

Wrap a string once around your main water line and measure it: about 2 3/4 inches means 3/4 inch copper or PEX, and about 3 1/2 inches means 1 inch. Name the material first, because 3/4 inch PVC measures almost the same as 1 inch copper or PEX.

1. Find your main water line

   On city water it is the pipe coming in at the water meter or the main shutoff. On a well it is the line leaving the pressure tank, because the filter goes after the pressure tank.

2. Name the pipe material

   Copper is metal with a copper color. PEX is flexible plastic tubing. PVC and CPVC are rigid plastic pipe with glued joints.

3. Wrap a string around the pipe

   Wrap it once around a straight stretch of pipe, away from any fitting, and mark where it meets itself. Lay it flat and measure to the mark.

4. Match the string to the table

   Find your material and the measurement closest to yours in the table below.

The string length is the outside width times 3.14. Copper and PEX of the same size have the same outside width, 1/8 inch more than the size name. The copper and PEX flows are the plumbing code's sizing tables at 8 feet per second, the fastest speed those tables use, and the copper industry's handbook gives the same limit for cold water. Charlotte Pipe, a PVC maker, says to keep water in its Schedule 40 pipe under 5 feet per second, which works out to about 8 GPM in 3/4 inch and 13 GPM in 1 inch. You get more volume through a bigger line.

All our systems are done in 1 inch, either male or female. On a 3/4 inch line your plumber adapts the 1 inch connections with reducing bushings.

Will a filter lower my water pressure?

A filter sized to your peak should not lower your pressure in a way you notice. We use Vortech tanks, so the flow rate is not diminished by them, and I have the same filter in my house: you can still run two showers and have good pressure.

A sediment housing adds a small drop of its own. Pentair rates a clean Big Blue housing with 1 inch ports at 1 psi lost at 15 GPM, and one with 3/4 inch ports at 2 psi.

These are verified purchases of our non-backwashing Vortech tanks. The acid neutralizer (a tank that raises low pH) uses the same 13 by 54 inch tank as the 2.5 cubic foot carbon filter.

Sizing a filter on well water

On a well, your pump is the second ceiling: the house can never use more water per minute than the pump delivers. When a well owner calls, I ask for the flow rate in the house. A Connecticut caller with 3 bathrooms had a pump making a little over 7 GPM. I told him I'd probably go with the 2.0 cubic foot iron filter, or the 1.5 if only two or three people live there. The 2.5 would work too, but its backwash would have to run 25 minutes instead of 14.

Every filter goes after the pressure tank, because you can't obstruct the flow to the water tank. The Big Blue sediment filter goes first after the pressure tank, then the treatment equipment, with the carbon tank last on a well. An iron filter also needs enough water from the pump to backwash, and the iron filter sizing guide covers which tank fits which pump.

What you need to treat comes from a lab test, not from the flow rate. The well water test kit ($199) checks 53 contaminants; on city water the city water test kit ($199) checks 47.

Frequently asked questions