Reverse osmosis uses pressure and a membrane to separate treated water from a reject stream. It can be valuable for certain dissolved contaminants, but it is not automatically the right first choice for every household or every water concern.
Under-sink and whole-house RO are different projects
| Feature | Under-sink RO | Whole-house RO |
|---|---|---|
| Scope | One drinking-water faucet | Most or all household water |
| Typical complexity | Cabinet space, dedicated faucet, drain connection | Pretreatment, storage, pump, drain, whole-home plumbing, possible pH correction |
| Planning cost | $350–$1,800 installed for many systems | $4,800–$10,000+ for many projects; complex wells can exceed this |
| Best use case | Targeted drinking and cooking water | Documented need to treat the full household supply |
| Water use | Produces a reject stream | Can produce substantial reject water and needs careful system design |
Current cost benchmarks
Angi — Reverse Osmosis System Cost (2026) reports professional RO installation ranging from roughly $1,000 to $4,800 across point-of-use and whole-house projects, with wide variation by system type and number of stages. Angi also notes recurring filter replacement and membrane replacement every few years.
Retail under-sink systems may cost less when installed by the homeowner, while a high-capacity or difficult professional installation can cost more. Whole-house RO often requires a treatment train rather than a single appliance.
Do not buy based only on “number of stages”
A stage count does not prove performance. Review the model’s certified contaminant claims, production rate, recovery or efficiency information, feed-water requirements, operating pressure, and replacement schedule.
NSF/ANSI 58 is the principal standard for reverse-osmosis drinking-water systems. Search the exact model in NSF Certified Drinking Water Treatment Units Search and read the performance data sheet before treating a marketing claim as verified.
Budget for maintenance and water conditions
- Prefilters generally require periodic replacement; the interval depends on feed water and usage.
- The membrane has a longer but finite life and can be damaged by poor pretreatment or incompatible feed-water conditions.
- Low pressure may require a booster pump.
- Hardness, iron, sediment, or chlorine can require pretreatment depending on the membrane and design.
- Whole-house RO may require storage, repressurization, drainage, and post-treatment to stabilize water chemistry.
When a simpler system may be enough
For city-water taste and odor, a certified carbon filter may meet the goal with less complexity. For hardness, a softener addresses calcium and magnesium more directly. For microbial risk, validated disinfection and source remediation may be more important than RO. Match the technology to the test result.