People mix these up all the time, but they do different jobs.
Water softener (whole house)
A softener sits where water enters the house and treats every tap. It removes the hardness minerals (calcium and magnesium) that build scale in water heaters, fixtures and appliances. Most trade them for another ion, often sodium.1 Pick a softener if your problem is scale, spots, soap scum or appliance wear.
Reverse osmosis (usually one faucet)
An RO system pushes water through a fine membrane. Most homes put a point-of-use unit under the kitchen sink for drinking and cooking water. Whole-house (point-of-entry) RO exists, but it's a much bigger and costlier job.2,3 Pick RO if your concern is taste or dissolved solids in the water you drink.
Why many homes in hard-water areas have both
A softener protects the plumbing and the RO drinking unit polishes the water you drink. Some dealers sell both together. Desert Water Services in Tucson, for example, says many local homes pair whole-house treatment with RO at the kitchen sink. Ask whether a bundle is cheaper than buying them separately.
Quick decision guide
| Your problem | Usually fixed by |
|---|---|
| White scale, spotty dishes, short-lived water heater | Softener |
| Taste of drinking water, dissolved solids | Under-sink RO |
| Both | Softener + RO |
| Private well | Test first. See well water in Arizona |
See costs in How much does an RO system cost?
Sources
- Tucson Water: water quality parameter explanation table
- This Old House: reverse osmosis system cost (2026)
- Plumbing Picks: cost to install a reverse osmosis system (2026)
Last reviewed Oct 13, 2026. This is general information, not professional advice. Confirm figures with your utility and installer.