What happens if you use a water filter past its replacement date?

An expired filter stops removing contaminants effectively. Worse, a saturated carbon filter can release trapped contaminants back into your water, called "dumping", if it becomes overloaded. Bacteria can also grow in a wet, saturated filter over time. You may be drinking water that's worse than unfiltered.

How often should I replace a Brita filter?

Brita's standard pitcher filter should be replaced every 40 gallons or 2 months, whichever comes first. The Brita Longlast filter lasts 120 gallons or 6 months. These estimates assume average tap water quality, harder water or higher chlorine levels can shorten filter life. The indicator light on Brita pitchers tracks usage, but it measures time, not actual gallons filtered.

How long do RO filters last?

A reverse osmosis system typically has 3, 4 filter stages with different replacement schedules. Pre-filters (sediment and carbon) should be replaced every 6, 12 months. The RO membrane typically lasts 2, 5 years depending on water quality. Post-filters (polishing carbon) should be replaced annually. Check your specific system's manual, higher TDS or iron in your source water shortens all these intervals.

Can I tell if my filter needs changing by the taste of my water?

Sometimes. A filter that needs replacement often allows chlorine taste and odor to return, which is detectable. But a filter that's past its lead or PFAS capacity may still make your water taste fine while providing no protection. Taste is not a reliable indicator for health-effect contaminants. Track gallons and time, not taste.

How do I know how many gallons my filter has processed?

If your filter housing has a built-in counter or smart indicator, use that. For simple filters with no tracking, estimate based on household consumption. The average American uses about 1 gallon of drinking/cooking water per day. A family of 4 uses roughly 120 gallons per month from the kitchen tap. Compare that to your filter's rated capacity and calculate your expected replacement date.

When to Replace Your Water Filter (By Filter Type)

Most people change their water filter when they remember to. That’s the wrong approach.

An expired filter doesn’t just stop working, it can actively make your water worse. Carbon filter media has a limited capacity. Once it’s saturated, it can release previously captured contaminants back into your water. In a humid environment, bacterial growth inside an old filter is also a real risk.

Filter replacement isn’t maintenance. It’s part of how filtration works.

Why Filter Life Matters More Than You Think

Activated carbon, the media inside most household water filters, works by adsorption. Contaminant molecules stick to the carbon’s surface. When the surface is full, nothing more sticks.

But it’s not a clean binary. As a carbon filter approaches saturation, its performance degrades gradually. For chlorine (taste and odor), you’ll often notice the degradation, your water starts tasting like tap water again. For lead or PFAS, you won’t notice anything. The filter just quietly stops protecting you.

A study published in Water Research found that carbon filters that had exceeded their rated capacity began releasing VOCs back into filtered water. The effect was contaminant-specific, but the principle applies broadly: a saturated filter is not a neutral filter.

Replacement Schedules by Filter Type

Pitcher Filters

Standard activated carbon pitcher (Brita Standard, PUR Classic): Replace every 40 gallons or 2 months.

Extended-life pitcher (Brita Longlast, Clearly Filtered): Replace every 100, 150 gallons or 4, 6 months. Clearly Filtered’s filter is rated to 100 gallons; their included tracking sticker helps.

ZeroWater pitcher: ZeroWater uses a 5-stage ion exchange filter that’s effective but short-lived, typically 25, 40 gallons depending on source water TDS. ZeroWater includes a TDS meter with most pitchers; replace when your filtered water reads above 006 ppm.

For all pitcher filters: track gallons, not just calendar time. A single person using 1 gallon/day will replace a 40-gallon filter every 40 days. A family of 4 using 4 gallons/day replaces it every 10 days.

Under-Sink and Countertop Carbon Filters

Most single-stage under-sink carbon block filters carry a 6-month or 500-gallon rating.

At 4 gallons per day (household average for kitchen tap use), you hit 500 gallons in about 4 months. Set a reminder and replace at the gallon mark, not just the calendar mark.

Reverse Osmosis Systems

RO systems have multiple filter stages, each with its own replacement schedule:

Stage 1, Sediment pre-filter: Replace every 6, 12 months. High-sediment well water may require 3, 6 months.

Stage 2, Carbon pre-filter: Replace every 6, 12 months. This stage protects the RO membrane from chlorine damage.

Stage 3, RO membrane: Replace every 2, 5 years. Membrane life depends heavily on source water quality. High TDS, iron, chlorine exposure (if carbon pre-filter isn’t changed), or bacterial contamination shorten membrane life. A TDS meter can help you monitor: if your filtered water TDS climbs back toward source water TDS, the membrane is degrading.

Stage 4, Post-filter (polishing carbon): Replace annually.

Most RO system manufacturers include a filter replacement schedule in the manual. Follow the more conservative of the calendar recommendation or the gallon recommendation.

Whole-House Filters

Sediment filters: Replace when flow rate drops noticeably, or every 3, 6 months for active households on well water.

Whole-house carbon filters: Replace every 6, 12 months depending on chlorine levels.

Iron filters (oxidizing media like birm or greensand): Backwash as scheduled (usually weekly or biweekly). Replace media every 5, 8 years or when iron breakthrough occurs.

Water softener resin: Ion exchange resin lasts 10, 15 years under normal conditions. Chlorine exposure, iron fouling, and bacterial contamination degrade resin faster. If your softener is regenerating more frequently or hardness is breaking through, test the resin before replacing the whole unit.

UV Systems

UV bulbs should be replaced annually, regardless of hours operated. UV output degrades over time even when the lamp is still glowing, older bulbs emit UV at reduced intensity, which may not reach the 40 mJ/cm² required for Class A disinfection.

The quartz sleeve (the glass tube around the UV bulb) should be cleaned or replaced annually to prevent mineral scaling that blocks UV transmission.

Don’t wait for the lamp to burn out. A UV lamp that’s “still on” may not be delivering effective disinfection.

Practical Tracking System

The simplest approach: a permanent marker and a piece of tape on the filter housing.

Write the installation date and the replacement date (or gallon count) when you install a new filter. Make it visible. Don’t rely on memory or a calendar reminder that gets snoozed.

For households with multiple filter stages, keep a list on the inside of the cabinet door under the sink:

Stage 1 (sediment): installed [date], replace [date]
Stage 2 (carbon): installed [date], replace [date]
Stage 3 (RO membrane): installed [date], replace [date + 2 years]
Stage 4 (polish): installed [date], replace [date + 1 year]

A $2 investment in a label protects a $400 filtration system.

One Warning Sign to Watch For

If your filtered water suddenly tastes worse, more chlorine, more metallic, more chemical, don’t assume your source water got worse. Check your filter first. A filter that’s been overloaded for a while can suddenly dump trapped contaminants when disturbed (by temperature change, backflow, or vibration). Replace it and flush the system before drinking.

Related: What Water Filter Do I Need?, match your specific contaminant to the right filter type before you buy.