Total dissolved solids, or TDS, is a measure of everything dissolved in water: minerals, salts, metals, and trace amounts of organic matter. If you’ve seen a TDS reading on a water quality report or a home testing meter, that number (expressed in milligrams per liter, or mg/L) tells you the combined concentration of all those invisible dissolved substances. The U.S. EPA sets a recommended limit of 500 mg/L for drinking water, though this is a guideline for taste and aesthetics rather than a strict health standard.
What TDS Actually Includes
The dissolved solids in water are mostly inorganic salts. The most common ones are calcium, magnesium, sodium, and potassium on the positive side, paired with carbonates, bicarbonates, chloride, and sulfate on the negative side. Groundwater often picks up nitrate as well, particularly in agricultural areas where fertilizer use is heavy. Small amounts of organic matter round out the mix, but minerals and salts make up the bulk of any TDS reading.
These substances enter water naturally as it flows through rock and soil, dissolving minerals along the way. That’s why well water and spring water often have higher TDS than surface water from lakes or reservoirs. Human activity adds to TDS too: agricultural runoff, road salt, wastewater discharge, and industrial processes all push the number higher.
How TDS Differs From Suspended Solids
Not everything floating in water counts as “dissolved.” The dividing line is a filter with pores roughly 1.5 microns wide. Anything small enough to pass through that filter is classified as dissolved. Particles too large to pass through, things like sediment, algae, or silt, are called total suspended solids (TSS). You can often see suspended solids as cloudiness in water, while dissolved solids are invisible. A glass of water can look perfectly clear and still have a high TDS reading.
How TDS Is Measured
There are two main ways to measure TDS, and they differ significantly in accuracy. The gold standard is the gravimetric method: a lab evaporates a filtered water sample and weighs what’s left behind. This gives a direct measurement with roughly 15% uncertainty for samples above 100 mg/L.
The far more common approach, especially outside of labs, uses an electrical conductivity meter. Because dissolved minerals carry an electrical charge, water with more dissolved solids conducts electricity better. A handheld TDS meter (the kind you can buy for home use) actually measures conductivity and then converts it to an estimated TDS value. This is useful for quick checks and real-time monitoring, but it’s considerably less precise. A study of over 4,500 samples found that conductivity-based TDS estimates can be off by as much as 45% in either direction. For regulatory or legal purposes, the gravimetric lab method is the standard.
TDS Levels Across Different Water Sources
TDS varies enormously depending on the source:
- Fresh drinking water: typically under 500 mg/L, though many sources fall between 100 and 300 mg/L
- Brackish water: 1,000 to 10,000 mg/L, too salty for most uses without treatment
- Saline water: above 10,000 mg/L
- Seawater: typically above 35,000 mg/L
Municipal tap water in most U.S. cities falls well below the 500 mg/L guideline, but private wells can vary widely depending on local geology. Limestone-heavy regions tend to produce harder, higher-TDS water, while areas with granite bedrock often have lower readings.
How TDS Affects Taste and Your Plumbing
TDS is classified as a secondary water standard by the EPA, meaning it’s regulated for aesthetic reasons rather than direct health risks. The practical effects of high TDS are noticeable in daily life. Water above 500 mg/L often tastes bitter, salty, or metallic. It can leave white scale deposits inside pipes, water heaters, and kettles, reducing their efficiency and lifespan. You may also notice spots on dishes and glassware or staining on fixtures.
On the other end, water with very low TDS (below about 100 mg/L) tastes flat and bland. Most people prefer water somewhere in the middle range, where enough minerals are present to give it a clean, slightly crisp flavor without any off-putting saltiness.
Health Implications of High TDS
High TDS by itself isn’t considered a primary health hazard, but it works as a warning flag. When TDS is elevated, it may indicate the presence of specific ions that do pose health risks, including arsenic, lead, copper, nitrate, and various agricultural or industrial chemicals. The TDS number alone can’t tell you which specific contaminants are present, so a high reading is a reason to get a more detailed water test, not a diagnosis on its own.
People who aren’t used to high-TDS water may experience gastrointestinal irritation when they drink it. This is generally a short-term issue rather than a chronic health concern, but it’s worth noting if you’ve recently moved to an area with harder water or switched to a new well source.
The Risks of Very Low TDS Water
Extremely purified water with little to no mineral content carries its own set of concerns. Water stripped of minerals through distillation or aggressive filtration lacks calcium and magnesium that your body can readily absorb. Research suggests that even a diet adequate in these minerals may not fully compensate for their absence in drinking water, with magnesium being particularly difficult to make up through food alone.
Cooking with demineralized water makes the problem worse. Low-TDS water leaches minerals out of food during cooking, with losses reaching up to 60% for calcium and magnesium and even higher for trace minerals like copper (66%) and manganese (70%). Over time, this adds up. Studies have linked long-term consumption of very soft, low-mineral water to higher risks of cardiovascular problems, bone fractures in children, certain pregnancy complications, and some neurodegenerative diseases. An expert consensus group concluded that hard water’s association with lower cardiovascular risk is likely real, with magnesium as the most important contributing factor.
How to Lower TDS in Your Water
If your water tests high for TDS, reverse osmosis (RO) is the most effective home treatment option. A single-stage RO system typically reduces TDS by 80% to 85%. Two-stage systems push that to 98% or higher. Distillation also removes dissolved solids effectively by boiling water and collecting the steam, which leaves the minerals behind.
Standard carbon filters, the kind found in most pitcher-style filters, do very little to reduce TDS. They’re designed to remove chlorine, some organic compounds, and certain metals, but they don’t strip out the dissolved salts that make up most of the TDS reading. If TDS reduction is your goal, you need an RO system or a distiller.
One consideration with RO systems: because they remove nearly everything, the resulting water can fall into that very-low-TDS range linked to mineral deficiency concerns. Some RO units include a remineralization stage that adds back small amounts of calcium and magnesium. If yours doesn’t, you’re relying entirely on your diet for those minerals.