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Why TDS is a poor measure of water quality

Why TDS is a poor measure of water quality

Many water amateurs and companies that want to promote water filters for tap water use TDS (Total Dissolved Solids) as the key water quality testing method. The reason is that it’s very easy and cheap to measure. The problem is that this is not a good measure and it’s very limited in terms of water quality parameters.

Below is a simplified explanation of what TDS is and to what extent it should be used as a water quality measure.

Also read our article about why you probably want to be using a water filter independent of TDS. 

What is TDS in tap water?

TDS comprise of inorganic salts and small amounts of organic matter that are dissolved in water. The principal constituents are usually the cations calcium, magnesium, sodium and potassium and the anions carbonate, bicarbonate, chloride, sulphate and, particularly in groundwater, nitrate. TDS is expressed in units of mg per unit volume of water (mg/L) or also referred to as parts per million (ppm).

Typically, natural mineral water and tap water can have a TDS value of 100-200 mg/l. In areas with high concentration of minerals, the natural tap water could however be considerably higher. Water filtering technologies such as reverse osmosis, water distillation and ion exchange can reduce this to close to zero, whereas active carbon filters will not filter out TDS.


So is a higher level of TDS good or bad?

Just like most bottled mineral water contains higher TDS (e.g. Evian 300 mg/l; San Pellegrino 850 mg/l), this can also be positive for tap water. WHO and most other institutions that regulate water quality consider values up to 600 mg/l to be entirely safe and 2,000 mg/l safe for temporary consumption in case no other water is available. Read more about the health benefits of mineral water.

For values below 600 mg/l there is no scientific evidence that TDS makes any difference, and no health impact has been identified below 2,000 mg/l. To complicate this a little bit there is however evidence that high concentrations of specific constituents such as calcium may have an impact, but TDS is a very vague indicator of this, since it does not specify the different parameters that constitute the final number.

In a study by the World Health Organization, a panel of tasters came to the following conclusions about the preferable level of TDS in water (mg/l):

  • 50 – 300: Excellent*
  • 300 – 600: Good
  • 600 – 900: Fair
  • 900 – 1,200: Poor
  • Above 1,200: Unacceptable

* The testers in the report by WHO noted that water with less than 50 might taste flat but this is aesthetic rather than a health concern.

For example, tap water in Barcelona varies between 200-600 mg/l, Madrid 50 mg/l, Palma de Mallorca 400-900 mg/l, Berlin 260-400 mg/l, Paris 200-300 mg/l, Stockholm 73-93 mg/l, London 260 mg/l… and all of them are considered to have safe and healthy drinking water.

It’s worth noting that higher TDS waters usually have a heavier taste and a much more prominent “mouthfeel,” a term used by water connoisseurs to describe the overall sensory impression.

Why TDS is a poor measure of water quality

Why do water filter companies and amateurs use high TDS as an argument for water filtration?

The simple reason is that it’s cheap and easy to test. A TDS meter can be purchased online for about €50 ($55) or less, and then anyone can easily perform a test in seconds at home.

Real tests that measure chlorine, pesticides, heavy metals and pharmaceuticals, which are important to determine water quality, require sophisticated and expensive labs. Your local water company tests the water every day for contaminants and is obliged to provide a water quality report, at least, yearly. This provides a scientific analysis of the water quality.

TAPP Water filters have been tested for TDS in tap water with values up to 1000. The water taste remains good and the water is safe to drink.

What do I do to get accurate water quality testing?

In conclusion, water quality testing is important, but think twice before using TDS as an indicator or complement it with other technologies. Generally, the belief that high TDS in tap water is bad is based on misconceptions about the negative health impact of minerals in the body.

Learn more about common contaminants in tap water.

For Spain, you can check your local water quality report on

If you would like to do a test yourself in order to assess potential leakages of heavy metals from the pipes of your building (such as calcium, magnesium, chlorine, nitrate, sulfate, fluoride, sodium, copper or nickel) you can request a test from a local water lab. 

What about limescale?

High TDS is often directly correlated to hard water which means that you have a greater risk of limescale. Read more about limescale and how to filter limescale from tap water.


Unless your tap water as a TDS value that is vastly outside of the recommended range you have no need to worry. High TDS in tap water basically means that there is mineral water coming out of your tap. You should be happy.


WHO, EU Water Framework, EPA

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12 thoughts on “Why TDS is a poor measure of water quality”

  1. whoah this blog is magnificent i really like reading your posts.
    Keep up the great work! You realize, many individuals are looking round for this info, you could aid them greatly.

  2. Hi. Interesting article. As you mentioned TDS is not good measure. Its only indicator. For example tap water can have 300ppm same as evian. But in tap water you can have metals and chloride what you havent got in evian. Just wanted add a bit

  3. Article is really useful. I want know about if there any cheap technology/meter is available for chlorine, pesticides, heavy metals and pharmaceuticals contamination testing.

    1. Hi!
      Thank you for your message. The most reliable way of testing is through a certified laboratory.
      However, if you want to have a rough indication of these parameters, there are some testing stripes that you can buy online (eg. through Amazon).
      The TAPP Water team

  4. Graham Bennett

    This article is nowhere near as good as it could be, and at worst, is dangerously misleading. I’m not saying that it is factually or logically wrong. TDS does not tell you exactly what is in the water, but at least it can tell you when it has gone. I reckon most folks filter their water want pure water and do not have time to worry what was filtered out. I worry about radionuclides, but if 0 TDS means that the arsenic, pesticides and mercury are gone too – then fine. 0 TDS is fine for me, thanks. If, for some reason or other it is not, then you need to say exactly what dangerous substances TDS cannot measure, and why you would not want 0 TDS anyway. Taking out beneficial minerals is not really a problem, if we eat good food or replace them. Worst comes to the worst you can filter down to 0 and add the good minerals you bought on Amazon or Ebay.

    I, for instance, have concerns for the levels of radioactive radon, radium, caesium, strontium, plutonium, particularly if they are dissolved in the water, as TDS can measure. Tiny quantities of these substances in water are immensely dangerous because they cause cancers. They are mistaken by the human and animal metabolism for calcium potassium and other non-radioactive chemical look-alikes. They then get stored in our bones, liver, heart muscle and other tissues. Caesium, for instance, is a chemical analogue for (looks like) potassium and is stored in the muscles (sudden heart attacks), strontium is an analogue for calcium and is stored in the bones (leukemia) and so on.

    Most folks will not know, as I did not know, the levels of radioactive materials in the water are NOT measured by most water authorities in the UK, despite continuing leaks from Fukushima, Chernobyl, Sellafield and 500 nuclear plants the world over. Some of this pollution cannot be removed even by the power plants and is routinely released into the environment all the time the plant produces electricity, so it makes eminent good sense to both measure your water for it and to filter it out with a 0 TDS water filter. You can buy these things.

    1. Hi Graham,

      Thank you for your comment. The parameters provided in the blog post are not our personal recommendation, they are the official ones given by the World Health Organization. According to them, the preferable level of TDS in water (mg/l) are:
      – Less than 300: Excellent
      – 300 – 600: Good
      – 600 – 900: Fair
      – 900 – 1,200: Poor
      – Above 1,200: Unacceptable
      You can find more information on this topic in the report they published:

      Best regards,
      The TAPP Water team

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