Journal

What is TDS in water, and what does the number tell you?

Turn a bottle of mineral water around and you will usually find a short analysis on the label: calcium, magnesium, bicarbonate, and one figure for total dissolved solids. That last figure, TDS, is the quickest way to know how a water will feel in the mouth before the cap comes off. Here is what it measures, how it is produced, how to read it, and how it varies across the still and sparkling waters we sell by the case.

TDS in plain terms

Total dissolved solids is the weight of everything dissolved in a liter of water, once the water itself is taken away. It is reported in milligrams per liter (mg/L). Some labels and meters use parts per million (ppm) instead; for drinking water the two give effectively the same number.

Most of that weight comes from a short list of ions. Calcium and magnesium are picked up from limestone and dolomite. Bicarbonate forms as slightly acidic rainwater works its way through carbonate rock. Sodium, potassium, sulfate, chloride and silica make up most of the rest, in proportions set by the ground a spring drains. TDS is the total. The list behind it is what gives a water its personality.

How the figure is measured

Laboratories measure TDS by weight. A measured sample is filtered, evaporated in a dish at a fixed temperature (180 °C in the standard US method), and the dry residue left behind is weighed. It is slow work, and it is the method behind the analysis printed on a bottle.

Handheld TDS pens take a shortcut. Dissolved ions conduct electricity, so a pen measures conductivity and multiplies it by a conversion factor to estimate mg/L. That is useful for checking a home filter, but the result is an estimate: two pens can disagree, and substances that carry little or no charge barely register. If a meter reading differs from a bottle's published analysis, trust the analysis.

Reading the number on a label

There is no single official scale for describing TDS, but a few reference points help. In the United States, the FDA reserves the name mineral water for water from a protected underground source with at least 250 mg/L of dissolved solids, and no minerals added. A mineral water under 500 mg/L may be labeled low mineral content, and one above 1,500 mg/L high mineral content. Using our own range as a guide:

Under 50 mg/L, very light: Voss at 22, S. Bernardo at 22, Sant'Anna at 23.
50 to 250 mg/L, light to moderate: Saratoga at 116, Fiuggi at 120, Acqua Panna at 130, Tŷ Nant at 165, Mountain Valley at 220.
250 to 500 mg/L, medium: Hildon at 312, Evian at 357, Perrier at 475.
500 to 1,500 mg/L, firmly mineral: Topo Chico at 700, S.Pellegrino at 970, Galvanina at 1,100.
Above 1,500 mg/L, high: Vilajuïga at 2,900, Vichy Catalan at 3,052, Radenska at 3,500, and Donat Mg at around 13,000, an outlier usually poured in small glasses.

What TDS does to taste

As a rule, weight follows the number. A water under 50 mg/L feels thin and nearly neutral, closer to the idea of water than to any particular water. Around 100 to 300 mg/L there is enough mineral to give a soft, rounded body. Past 500 mg/L the water takes on a firmness you can feel on the tongue, and in the thousands it starts to taste savory or salty.

Composition explains the rest. Calcium with bicarbonate tends to read smooth and faintly chalky. Sulfate brings a dry, slightly bitter edge; S.Pellegrino carries 460 mg/L of it, which is a large part of its character. Sodium and chloride taste salty. Compare Perrier and Topo Chico: both are firm sparkling waters, but Topo Chico carries 100 mg/L of sodium against Perrier's 12, so it lands more savory. Vichy Catalan, with more than 1,000 mg/L of sodium and 2,100 mg/L of bicarbonate, is frankly saline.

Carbonation adds its own bite. Dissolved carbon dioxide forms carbonic acid, which is why most sparkling waters show a pH between about 5.5 and 6.5 while their still counterparts sit near neutral. That acidity sharpens the finish whatever the TDS.

Choosing a water by its TDS

For delicate cooking, a light white wine or simply a glass you want to drink all afternoon, a still water under about 250 mg/L stays out of the way. For a long dinner with several courses, a medium-mineral water has enough body to hold its own without taking over. For rich, salty or fried food, or an aperitivo, a firmly mineral sparkling water cuts through. The very high waters are best treated as something to taste in a small glass rather than pour by the liter.

For tap water, the US EPA sets a non-enforceable secondary guideline of 500 mg/L for TDS. It is an aesthetic benchmark, concerned with taste and appearance, rather than a health limit, and many bottled mineral waters sit well above it by design.

Where a water comes from shapes how much it carries. Our guide to spring, mineral and purified water explains the labels, and our sparkling water comparison puts the numbers side by side.

Frequently asked questions

Is a higher TDS better?

No. TDS describes style, not quality. A light water and a heavily mineral one are simply different things to drink, and the right choice depends on the food, the setting and your own taste.

Are ppm and mg/L the same?

For drinking water, effectively yes. A liter of water weighs about a kilogram, so one milligram per liter is close to one part per million.

Why does my TDS meter disagree with the bottle?

Meters estimate TDS from electrical conductivity using a conversion factor, while published analyses weigh the dried residue in a laboratory. The laboratory figure is the reference.