Water softener for very hard or mineral-heavy water: what it can and can’t do
A water softener removes hardness, not minerals in general: it exchanges calcium and magnesium for sodium or potassium, so total dissolved solids stay the same [1]. Each grain per gallon of hardness removed adds about 7.9 mg/L of sodium [2], while EPA advises no more than 20 mg/L for people on very low-sodium diets [3]. For salty, high-TDS water the tool is reverse osmosis, at least for drinking water [4].
Updated 2026-09-29 · every regulation and price links to its source at the bottom of the page. A guide, not a construction design.
Why doesn’t a softener lower TDS?
Because it swaps one dissolved mineral for another. As a Montana State University fact sheet puts it: “Water softening does not reduce total dissolved solids, it simply exchanges the calcium and magnesium for sodium or potassium.” [1] Chloride and sulfate pass through a softener as they were.
EPA’s secondary levels are 500 mg/L for TDS and 250 mg/L each for chloride and sulfate. They are not enforceable — the regulation calls them “reasonable goals” — and they apply to public water systems, not to your well [5] [6].
How hard is “very hard” water?
There is no federal limit on hardness. The US Geological Survey uses these reference classes [7]; the grains-per-gallon column is our conversion at 17.1 mg/L per grain.
| USGS class | mg/L as CaCO₃ | Grains per gallon |
|---|---|---|
| Soft | 0–60 | 0–3.5 |
| Moderately hard | 61–120 | 3.6–7.0 |
| Hard | 121–180 | 7.1–10.5 |
| Very hard | over 180 | over 10.5 |
How much sodium does a softener add?
About 7.9 mg/L of sodium for every grain per gallon removed, or about 23 mg/L per milliequivalent per liter; at 10 grains per gallon that is roughly 79 mg/L [2]. The US has no mandatory sodium limit for drinking water. EPA advises no more than 20 mg/L for people on a very low-sodium diet (500 mg a day) and 30–60 mg/L to avoid taste problems [3].
| Hardness removed | Sodium added — example calculation |
|---|---|
| 5 gpg (86 mg/L) | about 40 mg/L |
| 10 gpg (171 mg/L) | about 79 mg/L |
| 20 gpg (342 mg/L) | about 158 mg/L |
| 30 gpg (513 mg/L) | about 237 mg/L |
Does a softener work on high-TDS water?
Less and less well. A trade-journal analysis (industry data, not a standard) found that at 2,000 mg/L TDS it takes about 9 lb of salt per cubic foot of resin just to hold leakage to 1 grain per gallon, and about 17 lb at 3,000 mg/L; above 3,000 mg/L a single household softener can hardly soften deeply [8].
Penn State Extension recommends softeners for hardness of 3–20 grains per gallon (50–350 mg/L), pH above 6.7 and dissolved iron below 5 mg/L [9].
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How soft should the water be?
We found no US rule on residual hardness, and no requirement to keep an unsoftened tap. Montana State University Extension recommends a practical fix: keep a separate tap that bypasses the softener, or install reverse osmosis at the kitchen sink, so you don’t drink the added sodium [1].
When do you need reverse osmosis instead?
When the problem is salt rather than scale. High TDS, salty taste, chloride, sodium, fluoride, nitrate and arsenic call for reverse osmosis or distillation [4]; RO also removes calcium and magnesium in part [10].
Most households use RO only for drinking and cooking, at an under-sink faucet. Whole-house RO costs much more to buy, install and run, turns only 20–30% of the water into product, and its water is more aggressive toward pipes. On hard water, a softener ahead of the membrane extends its life [11].
Approximate prices, as of September 2026: under-sink RO $150–$1,300, whole-house RO $1,000–$4,800 or more [12].
How much salt and water does a softener use? Example calculation
NSF/ANSI 44 has a voluntary efficiency class: at least 3,350 grains of hardness removed per pound of salt and no more than 5.0 gallons of water per 1,000 grains; California requires at least 4,000 grains per pound. An average softener uses about 25 gallons (95 L) a day for regeneration [13].
- Assumptions (ours): 250 gallons of softened water a day, hardness 20 gpg (342 mg/L).
- Hardness to remove: 250 × 20 = 5,000 grains a day.
- Salt at the efficiency threshold: 5,000 ÷ 3,350 ≈ 1.5 lb a day, about 45 lb (20 kg) a month.
- Regeneration water: no more than 5,000 ÷ 1,000 × 5 = 25 gallons (95 L) a day.
- Sodium added: about 158 mg/L.
Frequently asked questions
Will a water softener lower TDS?
No. It exchanges calcium and magnesium for sodium or potassium, so TDS stays about the same [1].
Is softened water safe to drink?
It depends on the sodium. At 20 gpg removed a softener adds about 158 mg/L [2], far above EPA’s 20 mg/L advice for very low-sodium diets [3]. Keep an unsoftened tap or RO for drinking.
Softener or reverse osmosis for salty well water?
Reverse osmosis: salty water, high TDS, chloride and sodium are RO problems [4]. A softener still helps protect the RO membrane on hard water [11].
How much salt does a water softener use?
An efficient softener removes at least 3,350 grains per pound of salt [13]; in our example with 20 gpg water and 250 gallons a day, that is about 45 lb a month.
What hardness level needs a softener?
USGS calls water over 180 mg/L (about 10.5 gpg) very hard [7]; Penn State recommends softeners for 3–20 gpg [9].
Sources
- Montana State University Extension Water Quality — Hardness fact sheet, Treating Hard Water (US) · recommendation · checked 2026-09-29 · source
- products.geappliances.com (US) · recommendation · checked 2026-09-29 · source
- US EPA Drinking Water Advisory: Consumer Acceptability Advice and Health Effects Analysis on Sodium (EPA 822-R-03-006, 2003), Executive summary / Consumer acceptability (US) · recommendation · checked 2026-09-29 · source
- University of Nebraska–Lincoln Extension EC703 «Drinking Water Treatment: An Overview» (rev. 2014), Table I (problem → treatment); Table II (methods) (US) · recommendation · checked 2026-09-29 · source
- 40 CFR 143.3 Secondary maximum contaminant levels (National Secondary Drinking Water Regulations), § 143.3 (US) · recommendation · checked 2026-09-29 · source
- US EPA — Private Drinking Water Wells (landing page) (US) · regulation · checked 2026-09-29 · source
- USGS Water Science School — Hardness of Water, General guidelines for classification (US) · recommendation · checked 2026-09-29 · source
- WC&P International — C.F. «Chubb» Michaud, «Total Dissolved Solids Limits for Effective Softening» (01.02.2024), Sodium-to-Hardness Ratio; Figure 3 (US) · recommendation · checked 2026-09-29 · source
- Penn State Extension — Iron and Manganese in Private Water Systems, Water Softeners (US) · recommendation · checked 2026-09-29 · source
- CDC — A Guide to Drinking Water Treatment Technologies for Household Use (2012), Reverse Osmosis; Ultraviolet; Water Softeners; Filtration (US) · recommendation · checked 2026-09-29 · source
- University of Nebraska–Lincoln Extension G1490 «Drinking Water Treatment: Reverse Osmosis» (rev. 2014), Treatment principles; Equipment (US) · recommendation · checked 2026-09-29 · source
- This Old House — How Much Does a Reverse Osmosis System Cost in 2026?, System type (US) · price · checked 2026-09-29 · ✓ · source
- epa.gov, III. Water Efficiency; Salt Efficiency (US) · regulation · checked 2026-09-29 · source
- thisoldhouse.com (US) · price · checked 2026-09-29 · ✓ · source
- Homewyse — Cost to Install Water Softener (2026 calculator), Item details (US) · price · checked 2026-09-29 · ~ · source
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