What Size Water Filter GPM Calculator

This calculator is designed to help you estimate what size water filter you need for your house, based on the number of people and appliances in your home.

How do I know what size water filter I need?

When you are shopping for a water filter, your biggest consideration is finding the right size filter so that water pressure remains uninterrupted during peak use.  The number of different appliances being used and the number of people using them at the same time have a big impact on water pressure.

This all varies by household.  If it’s common for someone to be showering while the dishwasher is running, the sink faucet is running, and another person is flushing a toilet, you should factor in all 3 of those appliances.  It goes without saying that the person taking the shower would not want to lose water pressure.

The average residential home may only need a filter capable of handling 12 gallons per minute, but a larger home or one with more residents may need as much as 25-40 GPM.

How To Calculate GPM Needed For Water Filter

The best way to guestimate what your gallons per minute requirement might be for a whole house water filter is to identify what might appliances might be used simultaneously (peak usage).  Each appliance has a different flow rate.

In 1992, the EPA mandated the maximum GPM for efficiency standards for toilets, showerheads and faucets.  Most newer toilets only need 1.6 gallons per flush, whereas older toilets might still need 5 gallons per flush.  Older homes with older appliances will likely have much higher flow rate requirements.

Common Flow Rates

• Faucet: 2.5 to 3 GPM
• Bathtub: 4 to 8 GPM
• Washing machine: 4 to 5 GPM
• Shower: 2.5 to 5 GPM
• Toilet: 1.6 to 5 GPM (gallons per flush)
• Dishwasher: 2 to 3 GPM

To use the water filter calculator, enter the number of people living in your home and select the quantity of each appliance that might be used simultaneously.  For example, on a busy evening when everyone is home, how many of each appliance would everyone be using at the same time.  If you frequently have a lot of guests, you might want to factor that in as well.