Water treatment · Buying guides · Updated November 2026
Under-Sink Water Filters: Replacement Filters and Total Ownership Cost
An under-sink filter is a modest upfront purchase, but you will buy replacement cartridges for as long as you own it. Comparing the five-year cost, not the box price, gives a truer picture.
Common types
| Type | What it does | Ongoing parts |
|---|---|---|
| Single-stage carbon | Improves taste and odor; some reduce specific contaminants | One cartridge |
| Multi-stage carbon | Sediment plus carbon stages | Two or three cartridges, often on different schedules |
| Reverse osmosis (RO) | Removes a wide range of dissolved substances | Pre-filters, a membrane and a post-filter; most also need a drain connection and storage tank |
Estimate the five-year cost
- Note the price of each replacement cartridge and its rated life (in months or gallons).
- Work out how many of each you need in five years at your household's water use.
- Add them up, then add the unit price and any installation.
- For RO, add the membrane replacement and any wasted water, which affects water bills.
Example method: a $40 cartridge changed every 6 months is about $400 over five years, before the unit itself. Use the actual prices and schedules of the systems you are comparing.
What to check before buying
- Certification: look for NSF/ANSI certification for the specific contaminants you care about, not just "filters water."
- Fit: cabinet space, especially with a garbage disposal; RO tanks take room.
- Faucet: whether a separate faucet is needed and whether your sink has a spare hole.
- Cartridge availability: proprietary cartridges can get expensive or discontinued.
- Leak protection: consider a leak detector under the sink (leak detector guide).
Not sure what you need to remove? Start with understanding a home water test, and compare with whole-house options.