TDS meter readings explained: what is normal?

Reading time: approximately 13 minutes

What exactly does a TDS meter measure?

TDS stands for total dissolved solids: all inorganic salts and a small amount of organic material dissolved in the water. According to the background document of the World Health Organization, this primarily concerns calcium, magnesium, sodium, and potassium on one hand, and carbonate, bicarbonate, chloride, sulfate, and nitrate on the other. These are nine substances that together determine the majority of your measurement value.

Note what is not included in that. Bacteria are not listed, as they are not dissolved. Loose particles such as sand or rust flakes are not either; they are not dissolved but remain suspended. And PFAS, medication residues, and pesticides occur in such low concentrations, usually in nanograms or micrograms per liter, that they completely disappear on a scale measured in milligrams per liter. A TDS meter is therefore not a safety meter, but a composition meter.

At PureAqua, we notice that almost no one starts with a TDS question. People want to know if their water is clean, and the meter is the first affordable instrument they can get their hands on. That is not a bad reflex, provided you know what the number does and does not mean.

How does a TDS meter work?

A TDS meter actually measures something other than what it displays. It applies a small voltage between two electrodes and measures how well the water conducts electricity. Dissolved salts split into charged ions which conduct; pure water barely does this. The WHO describes that this conductivity measurement is then converted into a TDS value using a factor that varies by water type.

The latter is the most important footnote of this entire article. Two meters measuring the same conductivity can show a different number because they work with a different conversion factor. In practice, manufacturers use factors ranging from 0.5 to 0.7 and rarely put them on the display. Therefore, never compare your ppm value with that of someone else using a different device; only compare measurements from the same meter with each other.

According to the WHO, the practical detection limit of this method is approximately 10 mg/l. Below that value, the difference between 4 and 8 ppm is meaningless, even if your pen dutifully shows it on the unit. With very pure water, you are primarily measuring the noise of your own instrument.

What a meter therefore cannot demonstrate

• Bacteria and viruses: not dissolved, therefore invisible to the meter.
• PFAS and medication residues: concentrations too low to influence the total.
• Which specific substance: 250 ppm of calcium and 250 ppm of sodium result in the same number.
• Whether the water is safe: there is no TDS limit value for that.

That exact distinction between "what is in it" and "what is harmful" is often blurred when it comes to filters. We have detailed this in our article about what an activated carbon filter does not remove from your water.

Which TDS values are normal?

Natural sources vary enormously. The WHO mentions measured TDS levels from less than 30 mg/l to as much as 6,000 mg/l, depending on which minerals are soluble in the soil. Water from a limestone-rich subsoil is therefore by definition higher than water from a sandy soil, without making either one unsafe.

In terms of experience, there is a guideline. Taste panels evaluated drinking water according to the WHO as follows: below 300 mg/l excellent, between 300 and 600 good, between 600 and 900 fair, between 900 and 1,200 poor, and above 1,200 unacceptable. Water below 1,000 mg/l is generally acceptable for consumers. On the other end of the scale, the same applies: extremely low TDS can taste bland and featureless.

For Dutch tap water, there is no national TDS figure, because no one reports it that way. The Drinking Water Decree sets standards per substance and not for the total, so your water company publishes hardness, chloride, sodium, and dozens of other parameters, but rarely a TDS value. If you want a hard figure, look at your own supplier's water quality report. Vitens takes approximately 200,000 samples annually from source to tap and processes about 750 samples per day in the lab, which, according to the company, makes tap water the most strictly controlled food product in the Netherlands.

The fact that your meter rarely shows an alarming number in the Netherlands also has a technical reason: water companies soften hard water almost everywhere, so calcium and magnesium are already partially removed from the water before it reaches your tap. Hard water therefore only occurs in a few areas.

What is a good TDS value for drinking water?

A good TDS value for drinking water does not exist as a legal figure. The WHO concludes literally that no health-based guideline value is proposed for TDS, because there are no reliable data on the health effects of total dissolved solids. What does exist: guidelines for individual components such as boron, fluoride, and nitrate.

There is, however, a practical limit that has nothing to do with health. According to the WHO, above approximately 500 mg/l, excessive scale builds up in water pipes, boilers, and household appliances such as kettles and steam irons, which shortens the lifespan of those devices. This is the reason why people with a high measurement value often see limescale rings in their kettle: not dangerous, but expensive in the long run.

Anyone wanting to tackle limescale in the whole house will not be helped by a drinking water filter. You lower limescale on the water supply side, and we explain in a separate article why that requires a water softener rather than a filter pitcher. By the way, the TDS meter is the wrong tool for this, as it also measures sodium and chloride, which have nothing to do with limescale formation.

How do you read a TDS meter correctly?

Most incorrect measurements do not come from the device but from the way of measuring. Four things make the difference between a usable number and a random one.

1. Let the tap run first

Water that has stood in the pipe overnight has had more time to pick up substances from the pipe. Let the tap run for half a minute to a minute before collecting. At Vitens, there is normally only 24 to 48 hours between the production of the drinking water and the delivery from your tap, so freshness mainly affects the last section of the pipe.

2. Pay attention to the temperature

Conductivity increases with temperature. If you measure warm water, you will read a higher value even though nothing has changed in the composition. Better pens automatically compensate to 25 degrees; if "ATC" is on your meter, it does this. If not, always measure at room temperature.

3. Stir and wait for a stable number

Submerge the electrodes to the indicated line, move the pen briefly through the water to remove air bubbles, and wait until the display remains still for a few seconds. If the value is still fluctuating, you are measuring an air bubble or a temperature gradient, not water.

4. Always measure two samples in succession

First measure the unfiltered tap water and immediately after the filtered water, using the same pen and the same glass. Only this way will you know if your filter is still doing what it is supposed to do. Write down both numbers with the date, because the trend over the months is more interesting than today's measurement.

What do you measure after reverse osmosis?

This is where the meter becomes truly useful. Reverse osmosis forces water under pressure through a semi-permeable membrane that largely blocks dissolved salts. Exactly the substances that a TDS meter measures are therefore removed, which is why the value drops sharply after the membrane. At PureAqua, The Source removes up to approximately 99 percent of, among other things, PFAS, microplastics, heavy metals, and limescale from your tap water, with the performance documented in the NSF/ANSI 58 standard. How that step technically works is described on our page about how reverse osmosis works.

A strongly decreased measurement value is therefore the best evidence that your membrane is healthy. If the value slowly increases over the months while your tap water remains the same, the membrane is weakening or a pre-filter is clogging. Many people discover in this way that they need to replace their filter set, well before they notice anything in the taste.

There is a downside that you see happening on the display. The membrane removes calcium and magnesium just as well as the unwanted parts, and the WHO points out that water with extremely low TDS can taste bland. That is not a defect but physics, and it is exactly why a mineral filter exists after the membrane: it adds minerals back and lifts your measurement value up a bit again. We have honestly listed what a membrane costs in terms of minerals and rinse water.

By the way, do not confuse osmosis water with demineralized or distilled water, even if their measurement values are in the same neighborhood. The difference lies in the production process and what you use it for; we explain how osmosis water relates to demineralized water.

Is a TDS meter nonsense?

No, but it is often used for the wrong purpose. A TDS meter is nonsense as a sales argument and useful as a control instrument. That distinction is the whole story.

Nonsense is the demonstration in which someone measures your tap water, shows a number of a few hundred ppm, and concludes that your water is contaminated. The number largely consists of calcium and magnesium: minerals that are touted as a selling point on the labels of mineral water. The same measurement on a bottle of spring water often yields a higher value than on tap water.

Another reason not to base your safety on it: Dutch tap water is safe and healthy according to the RIVM, and the drinking water sector tests for thousands of substances. A pen costing a few euros does not add any judgment to that. How that control works in practice is described in our article on which thousands of substances your tap water is tested for.

The meter is useful, however, at three moments: upon the installation of a new filter system, as a monthly check to see if the membrane is still performing, and to determine whether a mineral filter adds enough back for your taste. On the first page of Google, TDS meters in the Netherlands cost about 2.95 to 42.99 euros; PureAqua provides one itself for 5.95 euros. For that money, it is a fine maintenance indicator and a poor judge of water quality.

TDS is not the same as water hardness

This is the most common confusion. Water hardness, according to Vitens, is determined by the amount of calcium and magnesium in the water and is expressed in German degrees or millimoles per liter. TDS also counts calcium and magnesium, but simply adds sodium, potassium, chloride, sulfate, nitrate, and carbonate to that.

As a result, two things can go wrong. Two houses with the same hardness can have different TDS values because one water contains more chloride or sodium. And a water softener lowers the hardness by replacing calcium and magnesium with sodium, so the TDS value barely drops and sometimes even rises slightly. Those who measure with a TDS meter after installing a softener then wrongly conclude that the device is not working.

If you want to know whether softening is useful, use the hardness provided by your water company and not your ppm value. If you want to know if your osmosis system is still working, use the TDS meter and not the hardness. Two questions, two instruments.

TDS values per application side-by-side

The WHO classification concerns drinking water and taste. Other considerations apply to other applications, which explains why you read such different advice online.

Two notes regarding this table. The scale concerns taste perception, not safety: water of 900 mg/l can also fully meet legal requirements. And the 500 mg/l limit concerns your appliances, not your health.

What do you do with the result?

A number without a follow-up step is a curiosity. Here is how you turn it into a decision.

You measure a low value and your water tastes good

Then there is nothing wrong, and you do not need a filter for taste. If you still want to filter, the reason is PFAS, nitrate, lead, or drug residues, and those are exactly the substances your meter does not see. Let your choice be guided by what you want to remove, not by the ppm value.

You measure a high value and see a lot of limescale

Request the hardness from your water company. If it is high, it concerns limescale throughout your entire house and a water softener is the logical technology. A filter on the kitchen tap will not solve the limescale in your bathroom and washing machine.

You measure an increasing value behind your filter

Record the measurement of your tap water and your filtered water in the same week. If the tap water remains the same while the filtered water rises, it is time for the filter set or the membrane. At PureAqua, a filter set for The Source 50 costs from 75 euros; in the subscription of 29 euros per month with a 299-euro startup fee, the annual filter set is already included.

You find your osmosis water too bland

Then you are likely measuring a very low value, and that corresponds with what you taste. A mineral filter after the membrane brings back calcium and magnesium and costs 45 euros at PureAqua. Your measurement value will then intentionally rise a bit, and that is not a setback.

Frequently asked questions

Conclusion

A TDS meter is an inexpensive and honest instrument as long as you ask it the right question. It tells you how many dissolved salts and minerals are in your water, and on the WHO taste scale, below 300 mg/l means excellent and above 1,200 mg/l unacceptable. It tells you nothing about bacteria, PFAS, or drug residues, and there is no health standard for TDS.

Therefore, use it as a comparison tool, not as a judge. Measure your tap water, measure your filtered water, and track the trend. For the question of whether softening is useful, use the hardness from your water company. And for the question of whether your membrane is still performing, that ppm value is the quickest answer you can give yourself, long before the taste tells you anything.