What Is TDS in Tap Water, and Should You Worry?

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Total dissolved solids (TDS) is everything dissolved in your water: minerals, salts and other substances. It affects taste and can leave residue.

The 500 mg/L guideline

EPA's secondary standards set TDS at 500 mg/L.^1,2 Secondary standards are non-enforceable guidelines for things like taste, odor and appearance, not health limits.¹

An example: Phoenix

Phoenix's 2025 Water Quality Report lists TDS from 464 to 716 ppm next to the 500 ppm secondary guideline.³

Bringing TDS down

A softener swaps hardness minerals for sodium or potassium, so it doesn't meaningfully lower TDS. A point-of-use reverse osmosis system does. TDS reduction is one of the things NSF/ANSI 58 tests.⁴ See RO costs.

Sources

1. US EPA: secondary drinking water standards
2. 40 CFR 143.3: secondary maximum contaminant levels
3. City of Phoenix 2025 Water Quality Report
4. NSF: NSF/ANSI 58 reverse osmosis systems

Last reviewed Oct 13, 2026. This is general information, not professional advice. Confirm figures with your utility and installer.