Filter replacement schedules

Replace a filter cartridge when it reaches its rated gallons or its rated months, whichever comes first. For the filters below, that means every 2 to 6 months for pitchers, 3 to 4 months for faucet filters, 6 months for under-sink cartridges and 6 to 24 months for reverse osmosis stages. Whole-house systems run from cartridges changed yearly to tanks rated for 10 years with a pre-filter changed every 2 months.

Why the schedule matters

Filters are only effective if they are maintained according to the manufacturer's instructions, and EPA says missing the replacement schedule can increase your risk of exposure to PFAS (EPA). CDC says filters need maintenance, including changes on the manufacturer's schedule, to keep working and to keep germs from growing in them (CDC). Portland's water bureau puts it plainly: a filter designed to remove 99 percent of lead can only do so for a specific volume of water (Portland Water Bureau). Certification listings record that volume: IAPMO lists the PUR PPF951K pitcher filter at a capacity of 40 gallons next to its lead and PFAS claims (IAPMO).

Rated life by filter type

Salt-free conditioners have media to replace as well: Watts says its OneFlow media should be replaced every 3 years (Watts). Salt-based softeners have no cartridge; see what size water softener do I need for salt use.

Gallons can run out before months

Replacements per year = the larger of (12 ÷ rated months) and (gallons you filter per year ÷ rated gallons).

Take a home that runs 1 gallon a day through a pitcher, 365 gallons a year:

• PUR PPF951K: 365 ÷ 40 is about 9 filters a year, not the 6 the 2-month limit suggests.
• Brita Elite OB06: 365 ÷ 120 is about 3 a year, not 2. Brita also says to process no more than 2 gallons a day (Brita).
• ZeroWater: 365 ÷ 15 is about 24 a year.
• Under-sink AQ-MF-1: even at 2 gallons a day, 730 ÷ 784 is less than one, so the 6-month limit decides: 2 a year.

For whole-house filters, the gallons are the whole household's. EPA says the average family uses more than 300 gallons a day at home, roughly 70 percent of it indoors (EPA WaterSense): about 77,000 gallons a year through a filter that treats indoor water only. That is under the iSpring's 100,000 gallons, so its 12-month limit comes first. At that rate the Rhino tank's 1,000,000 gallons would last about 13 years, so its 10-year limit comes first.

What shortens filter life

• Hard water. Brita says faucet filters may need changing more often if you have hard water (Brita).
• Slow flow. Brita's faucet data sheet says to replace the filter earlier if the flow drops (Brita data sheet).
• Manganese and sulfur on well water. Home Master says manganese and hydrogen sulfide reduce the effective life of its filter cartridges (Home Master).

To compare running costs between filter types, multiply the replacements per year for your household by the price of each replacement cartridge. For which type fits which problem, see whole-house vs under-sink vs pitcher filters.