How to Replace an Under-Sink Water Filter
Updated March 2, 2026
Change the filter cartridges in your under-sink water filtration system -- single-stage, multi-stage, and reverse osmosis filter replacement step by step.
Overview
Under-sink water filters need cartridge replacement every 6-12 months depending on your water quality and usage. A clogged filter reduces flow to a trickle and can release trapped contaminants back into the water. Reverse osmosis membranes last longer -- 2-3 years -- but the pre-filters and post-filters still need changing every 6-12 months. The replacement process takes 10-20 minutes: turn off the water, swap the cartridges, flush the new filters, and you are done. No plumber needed.
What You'll Need
Safety First
- Turn off the cold water supply valve under the sink before opening any filter housings. The system is pressurized -- opening a housing without shutting off the water sprays water across the cabinet.
- New carbon filters produce black water for the first 1-2 minutes. This is loose carbon fines and is harmless. Flush until the water runs clear before using for drinking.
Step-by-Step Instructions
- Turn Off Water and Depressurize
Close the cold water supply valve that feeds the filter system (the valve on the inlet line to the first filter housing). Open the filtered water faucet to release pressure and drain water from the system. The faucet will flow briefly and then stop -- that means the system is depressurized. Place a towel or shallow pan under the filter housings to catch drips when you open them.
Tip: If your RO system has a storage tank, close the tank valve (on the line between the tank and the faucet) before opening the housings. Otherwise the tank drains backward through the open housings. - Remove and Replace the Cartridges
Standard cartridge filters: use the filter wrench (included with the system) to unscrew the housing sump (the bottom canister) counterclockwise. The old cartridge is inside -- pull it out. Insert the new cartridge with the same orientation. Thread the sump back on hand-tight, then snug with the wrench (do not overtighten -- the O-ring seal does the work). RO systems: the pre-filters and post-filter use the same housing-and-cartridge method. The RO membrane slides into a separate horizontal housing -- unscrew the cap, pull the old membrane out by the stem, push the new one in until it seats.
Tip: Check the O-ring on each housing sump before reassembling. If it is cracked, flat, or missing, replace it ($2-$3). A bad O-ring causes the housing to leak under pressure. Apply a thin coat of food-grade silicone grease to the O-ring before reinstalling for a better seal. - Flush the New Filters
Close the housing sumps. Turn the supply valve back on slowly. Open the filtered water faucet. For carbon filters: let the water run for 3-5 minutes until it runs clear (the initial black water is carbon fines). For sediment filters: flush for 1-2 minutes. For RO membranes: fill and drain the storage tank twice (the first two tanks of water may taste off from the new membrane preservatives). After flushing, the water should run clear with no taste or odor from the new filters.
Tip: Write the installation date on the new filter with a permanent marker. This tells you exactly when to replace it next. Most filters last 6-12 months -- set a calendar reminder.
Pro Tips
- Buy filters in bulk -- a 4-pack of replacement cartridges costs 20-30% less per filter than buying singles. APEC, iSpring, and Express Water all sell multi-packs of their replacement filters.
- If flow rate from the filtered faucet drops noticeably, the filters are clogging and need replacement regardless of the calendar date. High-sediment water clogs filters faster than the manufacturer's estimate.
- RO systems have 3-5 stages, each with different replacement intervals. Pre-filters (stages 1-3): every 6-12 months. RO membrane (stage 4): every 2-3 years. Post-carbon filter (stage 5): every 12 months. Keep a replacement schedule posted inside the cabinet door.
- Third-party replacement filters ($8-$15) work in most name-brand systems and cost 30-50% less than OEM filters. Verify the size (length and diameter) and connection type match your housing before buying.
- Some whole-house and under-sink systems have a filter change indicator -- a pressure gauge that shows when the filter is clogging. If yours has one, use it instead of the calendar. It accounts for actual water quality and usage.
When to Call a Pro
You do not need a plumber for filter replacement. It is the most basic water treatment maintenance task. Call a plumber only if a housing is cracked and leaking (housing replacement needed) or if the inlet/outlet connections are leaking (fitting repair).
Frequently Asked Questions
When should I call a plumber to replace an under-sink water filter?
You do not need a plumber for filter replacement. It is the most basic water treatment maintenance task. Call a plumber only if a housing is cracked and leaking (housing replacement needed) or if the inlet/outlet connections are leaking (fitting repair).
What are some expert tips to replace an under-sink water filter?
Buy filters in bulk -- a 4-pack of replacement cartridges costs 20-30% less per filter than buying singles. APEC, iSpring, and Express Water all sell multi-packs of their replacement filters. If flow rate from the filtered faucet drops noticeably, the filters are clogging and need replacement regardless of the calendar date. High-sediment water clogs filters faster than the manufacturer's estimate. RO systems have 3-5 stages, each with different replacement intervals. Pre-filters (stages 1-3): every 6-12 months. RO membrane (stage 4): every 2-3 years. Post-carbon filter (stage 5): every 12 months. Keep a replacement schedule posted inside the cabinet door. Third-party replacement filters ($8-$15) work in most name-brand systems and cost 30-50% less than OEM filters. Verify the size (length and diameter) and connection type match your housing before buying. Some whole-house and under-sink systems have a filter change indicator -- a pressure gauge that shows when the filter is clogging. If yours has one, use it instead of the calendar. It accounts for actual water quality and usage.
What tools do I need to replace an under-sink water filter?
You will need: Filter wrench (included with most systems -- specific to the housing size), Replacement filter cartridges ($15-$60 per set depending on system type), Towel or shallow pan (to catch drips when opening housings), Food-grade silicone grease (optional, for lubricating O-rings), Permanent marker (to write the installation date on the new filter).
When should I call a professional instead of doing this myself?
You do not need a plumber for filter replacement. It is the most basic water treatment maintenance task. Call a plumber only if a housing is cracked and leaking (housing replacement needed) or if the inlet/outlet connections are leaking (fitting repair).
How difficult is this project?
This project is rated easy. Most homeowners with basic tools can complete it.
How long does this take?
Plan for approximately 10 - 20 minutes. First-timers may need extra time.
How much will this cost?
The estimated cost is $15 - $60 for replacement filters. Costs vary by location and materials chosen.