What Is a Normal TDS Level for Tap Water in California? Testing & Fixes
You bought a cheap TDS meter, dipped it in a glass of tap water, and now you're staring at a three-digit number with no idea whether it's fine or a problem. Total dissolved solids (TDS) is one of the most misunderstood readings in home water testing — it's genuinely useful, but it doesn't mean what most viral videos claim. Here's what the number actually measures, what's normal across California, and what to do if yours is high.
What TDS actually measures
TDS is a measurement of everything dissolved in your water that isn't H₂O — calcium, magnesium, sodium, potassium, bicarbonates, sulfates, chlorides, and trace minerals. A TDS meter doesn't identify any of these individually; it just measures the water's ability to conduct electricity, since dissolved minerals carry a charge, and converts that reading into parts per million (ppm), which is functionally the same as milligrams per liter (mg/L). It's a general indicator of mineral content, not a contaminant test. A high number doesn't automatically mean unsafe water, and a low number doesn't guarantee purity — TDS meters can't detect bacteria, lead, PFAS, or most pesticides at all.
What counts as normal for California
The EPA sets 500 mg/L as a secondary standard for TDS — a non-enforceable guideline tied to taste, staining, and scale buildup rather than health. The World Health Organization breaks palatability down further:
- Below 300 ppm — rated "excellent," tastes clean with no mineral edge
- 300–600 ppm — "good," most people find this pleasant
- 600–900 ppm — "fair," a noticeable mineral or slightly flat taste
- 900–1,200 ppm — "poor," often tastes salty, bitter, or metallic
- Above 1,200 ppm — "unacceptable" for most people's taste, though still not automatically a health hazard
Where your water lands depends heavily on the source. Cities drawing from Sierra snowmelt (much of the Bay Area and parts of Northern California) often test well under 300 ppm. Water pulled from Central Valley groundwater or the Colorado River aqueduct — common through much of Southern California — regularly runs 300–700 ppm because it's naturally mineral-rich before it ever reaches a treatment plant. Neither extreme is inherently a problem; it's mostly a taste and appliance-scaling issue, which is the same underlying story as our guide to hard water in California — TDS and hardness overlap heavily since calcium and magnesium are usually the biggest contributors to both.
When a high reading actually matters
The EPA and WHO agree on one important point: it's the composition of dissolved solids that determines risk, not the raw total. A reading of 450 ppm made up mostly of calcium and bicarbonate is a nuisance. A reading of 450 ppm that includes elevated nitrate, arsenic, or heavy metals is a health question — and TDS alone can't tell the two apart. That's why a high or rising TDS reading is a reason to order a real lab test, not a reason to panic on its own. If you're on a private well, this matters even more, since well water isn't monitored by a public utility; see our guides on what to test for in well water and specific contaminants like nitrate or arsenic. City water customers can also check their utility's free Consumer Confidence Report, which lists actual lab-tested contaminant levels rather than a single conductivity number.
What raises TDS in a home's plumbing
Sometimes the source water is fine but the reading climbs somewhere between the meter and the treatment plant:
- Old galvanized or corroding pipe. Corrosion byproducts dissolve into the water as it sits in the line, pushing TDS up along with a metallic taste.
- A failing water softener. Softeners trade calcium and magnesium for sodium — they don't lower TDS, and a malfunctioning brine cycle can actually raise it. See water softener repair costs if yours is dumping excess salt into the line.
- Irrigation or seasonal supply shifts. Utilities that blend groundwater and surface water seasonally can show real TDS swings of 100+ ppm through the year — nothing wrong with your plumbing, just a different source mix.
- A dying reverse osmosis membrane. If you already run an RO system and the post-filter TDS creeps up toward your tap water's level, the membrane has likely worn out and needs replacing.
How to actually lower it
Boiling, freezing, or letting water sit does nothing to TDS — those methods don't remove dissolved minerals, only some volatile compounds. The filtration types that measurably lower TDS are narrower than most people expect:
- Reverse osmosis (RO) is the only common residential technology that meaningfully strips TDS, typically removing 90–99% of it. An under-sink RO system runs about $300–$950 installed, plus roughly $10–$25 a month in filter and membrane replacement. Our reverse osmosis cost guide breaks down installation pricing in detail.
- Distillation also removes nearly all TDS but is impractical at whole-house scale and mostly used for small-volume needs.
- Carbon filtration (pitcher filters, standard under-sink or whole-house carbon systems) removes chlorine taste, some organics, and certain contaminants — but it does not meaningfully lower TDS, since dissolved minerals pass right through carbon media. If your goal is specifically a lower TDS number, a carbon-only system won't get you there; compare options in our whole-house filtration cost guide and RO vs. whole-house filtration comparison.
- Water softeners address hardness and scale but, again, don't reduce TDS — they just swap which minerals make up the total.
The bottom line
A TDS reading is a quick, cheap screening tool — useful for catching a sudden change or judging taste, but not a substitute for a real water quality test. This is exactly the kind of taste-driven threshold research that goes back decades: a 1969 study by William Bruvold and Henry Ongerth, conducted for the California State Department of Public Health and published in the Journal AWWA, tested how panels of California residents actually rated mineralized water at different concentrations, and its findings on palatability thresholds still underpin the EPA and WHO guidance cited above (see the study via Google Scholar). If your number is elevated and you want to know why, a licensed plumber or a certified lab can test for the specific minerals and contaminants a meter can't distinguish, and recommend whether an RO system, a softener, or nothing at all is the right fix.
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