Water Softener vs Reverse Osmosis (and the Best Softener + RO Combos)
Quick answer: they do different jobs. A water softener treats hardness in all the water entering your house. A reverse osmosis (RO) system usually sits under the kitchen sink and purifies just your drinking water, removing dissolved solids, including the sodium a softener adds. Many homes with hard water use both: the softener first, then RO at the kitchen tap. Cornell Cooperative Extension says softeners are used ahead of RO when water is very hard. Our favorite pair is the SpringWell salt-based softener with the Waterdrop G3P600 tankless RO.
Our picks:
- Best tankless RO: Waterdrop G3P600 (certified to NSF/ANSI 58)
- Best value RO: iSpring RCC7AK (certified to NSF/ANSI 58, with remineralization)
- Softener to pair with RO: SpringWell salt-based softener, or the Fleck 5600SXT on a budget
How we chose: we haven’t tested these systems ourselves. We picked RO systems with certifications we could confirm on the makers’ own pages, and softeners we’ve already compared in our other guides.
What each one does
| Water softener | Reverse osmosis (RO) | |
|---|---|---|
| Treats | All the water entering the house | Usually one tap (most home RO units sit under the kitchen sink) |
| Hardness | Removes it | Reduces it at that tap, along with other dissolved solids |
| Sodium | Adds about 8 mg/L for every grain per gallon removed | Removes it |
| Lead, nitrate, fluoride and similar | No | Reduces them, up to each system’s certified claims |
| Chlorine | No | Yes, through its carbon pre-filters |
| Water sent to the drain | Some, during regeneration | Some with every glass (for example, 1 gallon per 2 gallons of pure water on the Waterdrop G3P600) |
| Upkeep | Salt refills | Filters and membrane on a schedule |
Sources: Cornell Cooperative Extension (RO), North Dakota State University Extension (sodium), and the makers’ own pages. For softeners vs other filters, see water softener vs water filter.
RO isn’t perfect. Cornell notes it doesn’t remove dissolved gases and isn’t very effective against some organic chemicals, such as trihalomethanes, some pesticides and solvents. It also recommends feeding RO only water that’s microbiologically safe, so test a well for bacteria first.
Do you need both?
- Hard water, and you’re happy with how it tastes: a softener alone. See the best water softeners.
- Soft water, but you want purer drinking water (lower dissolved solids, lead, nitrate or fluoride): RO alone.
- Hard water, and you want purer drinking water or less sodium: both. The softener protects your plumbing and the RO membrane, and the RO removes the sodium the softener adds. See is softened water safe to drink?
Which goes first?
The softener goes first, on the main water line, and the RO system goes under the kitchen sink on the softened cold-water line. Cornell gives two reasons this works. Softeners are used ahead of RO when water is very hard, and RO can follow a softener to reduce the sodium the softener adds.
On city water, chlorine is the other thing to know about. Cornell says chlorine and similar disinfectants damage thin-film composite RO membranes, so an activated carbon filter must remove them first. RO systems include carbon pre-filters for this reason; Waterdrop’s G3P600, for example, uses a carbon block filter. If you also want chlorine removed for the whole house, see our filter and softener combos.
Can a whole-house RO system replace a softener?
Rarely worth it. Cornell notes that most home RO systems are point-of-use units under the kitchen sink, and that whole-house (point-of-entry) RO costs more to buy and run. RO also sends some water down the drain with every gallon it makes. A softener is the efficient way to treat hardness for the whole house, and an under-sink RO handles the drinking water.
Our picks: the best softener + RO pairs
Waterdrop G3P600
A tankless under-sink RO system that makes up to 600 gallons a day, so there’s no storage tank to take up space. Its faucet shows the water’s TDS and tells you when to change filters.
- Certification: tested and certified by IAPMO R&T to NSF/ANSI 58 and NSF/ANSI 372 (lead-free)
- Capacity and efficiency: 600 gallons per day, 8 stages, 2:1 pure-to-drain ratio
- Filter life: up to 6 months for the composite filter, 12 months for the carbon block and 24 months for the RO filter, per Waterdrop
- Needs: electricity; 1-year warranty
Good to know: check the Performance Data Sheet for the specific contaminants it’s certified to reduce. Waterdrop’s page lists its certifications slightly differently in different places.
Check price on AmazoniSpring RCC7AK
A 6-stage, tank-based RO system with an alkaline remineralization stage that adds some minerals back after the membrane. iSpring says it’s tested and certified by NSF International to NSF/ANSI 58.
- Capacity: up to 75 gallons a day, with a storage tank (iSpring lists 3.2 gallons)
- Certified reductions: iSpring lists TDS, fluoride, lead, barium, cadmium, chromium, copper and selenium, among others
Good to know: we didn’t find an official drain ratio for this model. The tank takes more space under the sink than a tankless system.
Check price on AmazonSpringWell salt-based water softener (SS)
Sized by bathroom count: SS1 for 1–3 bathrooms (32,000 grains), SS4 for 4–6 (48,000) and SS+ for 7 or more (80,000). It comes with a lifetime warranty against defects and a 6-month money-back guarantee. More detail in our salt-based vs salt-free guide.
See it at SpringWellAFWFilters Fleck 5600SXT 48,000-grain
A widely used metered Fleck valve with 1.5 cubic feet of resin, on Amazon. Pair it with the iSpring RCC7AK for a budget softener and RO setup. Check your size with our size calculator.
Check price on AmazonFrequently asked questions
Does reverse osmosis remove hardness?
At the tap it serves, yes. RO reduces dissolved solids, including hardness minerals. But it treats only that tap, and on very hard water a softener ahead of it helps protect the membrane.
Do I need a water softener if I have reverse osmosis?
If your water is hard, yes: RO treats your drinking water, while a softener protects your pipes, water heater and appliances and gives soft water at every tap.
Does RO remove the sodium from softened water?
Yes. Cornell notes RO can follow a softener to reduce the sodium it adds, which makes the pair a good fit for anyone limiting sodium.
How much water does RO waste?
It depends on the system. Cornell says older home systems were often designed to recover only 20–30% of the water, while Waterdrop rates its G3P600 at a 2:1 pure-to-drain ratio.
Related guides
- Is softened water safe to drink?
- Best water filter and softener combos
- Water softener vs water filter
- Best water softeners (all types)
Sources
- B. Kneen, A. Lemley and L. Wagenet, Cornell Cooperative Extension, Reverse Osmosis Treatment of Drinking Water (republished in Water Conditioning & Purification, 2005).
- T. Scherer, North Dakota State University Extension, Water Softening (Ion Exchange) (revised May 2022).
- Waterdrop, G3P600 Under Sink RO System.
- iSpring, RCC7AK reverse osmosis system.
- SpringWell, salt-based water softener.