Water with a TDS (total dissolved solids) level below 300 mg/L is rated excellent for drinking, and water up to 600 mg/L is considered good. The U.S. EPA sets a recommended limit of 500 mg/L, while the World Health Organization considers anything above 1,200 mg/L unacceptable. But the story isn’t as simple as “lower is always better,” because water stripped too low in minerals carries its own risks.
What TDS Actually Measures
TDS is the total concentration of dissolved substances in water, measured in milligrams per liter (mg/L), which is the same as parts per million (ppm). These dissolved substances include calcium, magnesium, sodium, potassium, bicarbonates, chlorides, and sulfates. Some amount of these minerals is normal and even beneficial. The number on your TDS meter doesn’t tell you which specific minerals or contaminants are present, only the combined weight of everything dissolved in the water.
Most handheld TDS meters actually measure electrical conductivity and convert it to an estimated TDS reading using a conversion factor, typically around 0.65. This means the number you see is an approximation. Two water samples with identical TDS readings could have very different compositions: one might be rich in harmless calcium and magnesium, while another could contain lead or arsenic at concerning levels.
The WHO Palatability Scale
The World Health Organization rates drinking water taste by TDS level:
- Excellent: below 300 mg/L
- Good: 300 to 600 mg/L
- Fair: 600 to 900 mg/L
- Poor: 900 to 1,200 mg/L
- Unacceptable: above 1,200 mg/L
These ratings are based on taste panels, not direct health thresholds. Water in the “fair” range is still safe to drink in most cases, but you’ll start noticing a mineral or slightly salty flavor. Above 1,000 mg/L, many people describe the taste as distinctly unpleasant.
The EPA’s secondary standard of 500 mg/L is not a legally enforceable limit. It’s a guideline based on taste, appearance, and the potential for mineral deposits and staining in your plumbing. Municipal water systems aim to stay below this level but aren’t penalized for exceeding it.
When High TDS Becomes a Health Concern
Drinking water with moderately elevated TDS (say, 700 to 900 mg/L) from natural mineral sources is generally not dangerous. The health risks increase when TDS climbs well above 1,200 mg/L or when the dissolved solids include harmful contaminants rather than benign minerals.
Chronically high TDS water has been linked to kidney stones, gastrointestinal problems like stomach pain and diarrhea, and cardiovascular strain. The risk depends heavily on what’s dissolved in the water. If the high reading comes from heavy metals like lead, arsenic, or elevated nitrates, even moderate TDS levels can pose serious long-term risks. Excess calcium and magnesium at high concentrations can also cause dry skin, scalp irritation, and hair thinning over time.
Water that tastes bad also creates an indirect health problem: people drink less of it. If your tap water tastes salty or metallic, you’re more likely to reach for sugary drinks or simply not hydrate enough.
Why Very Low TDS Water Isn’t Ideal Either
There’s a widespread assumption that the purest possible water is the healthiest. Research suggests otherwise. Water with very low TDS, below about 50 to 80 mg/L, has been stripped of the minerals your body expects to get from drinking water.
A review published in the Medical Journal of Armed Forces India found that long-term consumption of demineralized water is associated with real health consequences. Even in countries where people eat a balanced diet, food alone may not fully compensate for the absence of calcium and especially magnesium in drinking water. The concern grows when you cook with demineralized water, which leaches minerals out of food. Losses can reach up to 60% for calcium and magnesium, and even higher for trace minerals like copper (66%), manganese (70%), and cobalt (86%).
Studies have linked consumption of very soft, low-mineral water to higher risk of bone fractures in children, certain pregnancy complications including preeclampsia, and increased cardiovascular risk. An expert consensus group concluded that the protective effect of harder water against heart disease is likely real, with magnesium being the most probable contributor to those benefits.
The practical takeaway: a TDS reading of 50 to 150 mg/L from a home filtration system isn’t dangerous in the short term, but it’s not optimal for years of daily consumption.
How Home Filters Affect TDS
Standard carbon filters (like a Brita pitcher) remove chlorine, some organic compounds, and certain heavy metals, but they don’t significantly change TDS. If your concern is overall mineral content, a carbon filter won’t move the number much.
Reverse osmosis (RO) systems are a different story. A single-pass RO system typically reduces TDS by 80% to 85%. If your tap water starts at 400 mg/L, an RO system will bring it down to roughly 60 to 80 mg/L. Two-stage systems can push removal above 98%, producing near-pure water with TDS in the single digits.
This is why many newer RO systems include a remineralization stage that adds back small amounts of calcium and magnesium after filtration. If you’re using an RO system without remineralization, your water likely falls in that very low TDS range that researchers have flagged as suboptimal for long-term health. Adding a remineralization cartridge or mineral drops is a simple fix that brings TDS back up to the 50 to 100 mg/L range.
The Sweet Spot for Daily Drinking
Pulling together the WHO ratings, EPA guidelines, and the research on demineralized water, the ideal TDS range for drinking water falls between about 50 and 600 mg/L. Water in the 100 to 300 mg/L range hits the sweet spot: it tastes clean, contains meaningful amounts of beneficial minerals, and sits well within every guideline.
If your TDS reading is between 300 and 500 mg/L, your water is perfectly fine. You’re getting more minerals per glass, which is a net positive unless specific contaminants are driving the number up. If your reading exceeds 900 mg/L, it’s worth getting a detailed water quality test to find out what’s actually dissolved in it. A high TDS number alone isn’t a diagnosis; it’s a signal to look closer.
Keep in mind that TDS is a blunt instrument. A reading of 250 mg/L from a natural spring is very different from 250 mg/L in water contaminated with industrial runoff. If you have concerns about specific contaminants, a TDS meter won’t give you the answer. You’ll need a lab test that breaks down the individual substances in your water.