Tap water TDS: what does it really tell you?

Reading time: ~12 minutes

What is the TDS value of water?

A TDS meter does not measure those substances directly. The little device measures the conductivity of the water: dissolved salts conduct current, pure water barely does. The meter converts that conductivity into an estimate in mg/l. The WHO calls that conversion usable as an estimate, not as an analysis.

That difference is the core of this whole article. A TDS meter gives a number for the amount of dissolved substances. It says nothing about which substances they are. Two glasses of water can both show 350 mg/l while one is full of harmless calcium and the other of sodium and chloride.

Most of those dissolved substances in Dutch tap water are simply limescale. It is the same limescale you see on your tap and in your kettle, and it determines how you measure your water hardness at home. In the Netherlands, hardness and TDS largely run in parallel, but they are not the same thing: hardness counts only calcium and magnesium, TDS counts everything.

What is a good TDS value?

The most frequently cited classification comes from the WHO background document on dissolved solids in drinking water. Pay close attention to what that classification measures: taste, not safety.

And now the sentence that is missing from almost every Dutch article on this subject. The WHO writes literally: "No health-based guideline value is proposed for TDS." So there is no health-based limit value for TDS. The whole scale above is about how the water tastes.

At PureFilter we see that customers who start with a TDS meter are often looking for confirmation that their water is "clean". That is understandable, but the number cannot answer that question. If you want to know which filter type suits which concern, you will get further with which filter type removes dissolved substances and which does not.

What TDS value does Dutch tap water have?

Here is a detail that surprisingly often goes wrong: Dutch drinking water rules have no TDS standard at all. What is standardised is the conductivity. Vitens (a Dutch drinking water company) gives a maximum of 125 mS/m at 20 degrees as the standard.

If you convert that to dissolved substances using the usual factor of 0.5 to 0.7, you arrive at roughly 625 to 875 mg/l as an upper limit. That is the ceiling, not the practice. Most Dutch tap water sits well below it, in the "good" category of the WHO table.

Hardness gives a better indication

Because limescale supplies the lion's share of the dissolved substances, the hardness published by your water company is more useful than a single TDS measurement. According to WMD (the water company for Drenthe), the legal range lies between 1 and 2.5 mmol/l, or 5.6 to 14 degrees German hardness. In Drenthe the water is around 8 °dH.

Those differences are considerable by region. If you want to know where your town stands, you will find it in the overview of water hardness per municipality. A higher hardness almost always means a higher TDS reading too, without anything being wrong with the water.

Also good to know: according to the RIVM (the Dutch National Institute for Public Health and the Environment), sixty per cent of Dutch drinking water comes from groundwater. Groundwater picks up minerals from the soil on its way, and that is exactly why deeply extracted groundwater shows a higher TDS than water from the dunes or from a river.

The blind spot: what a TDS meter cannot see

This is the point where a TDS meter wrong-foots people. It measures in steps of 1 mg/l. Anything present in much smaller quantities falls entirely outside its range, however harmful it might be.

Take PFAS. The RIVM reports that 100 nanograms per litre is legally permitted for twenty different PFAS, and itself advises a much stricter limit of 4.4 nanograms per litre, expressed as PFOA equivalents. Four point four nanograms is 0.0000044 milligrams. That is more than two hundred thousand times smaller than the smallest step a TDS meter can display.

In other words: water that substantially exceeds the RIVM advisory value gives exactly the same number on a TDS meter as water that stays well below it. The same goes for bacteria, medicine residues and most pesticides.

If you are worried about PFAS specifically, you will get more from the overview of where PFAS is found than from a meter that by definition cannot see the substance. And if you want to know which filter answers which health question, you will find that in what the healthiest water filter is.

Measuring TDS yourself: how to do it properly

A TDS meter costs roughly 10 to 25 euros. If you want a usable measurement, stick to this order.

1. Let the tap run first. Water that has stood in the pipe overnight has had more time to absorb substances. Flush for half a minute.
2. Use cold water. Warm water from the boiler is not drinking water and gives a different value.
3. Collect it in a clean glass. Soap residue in a washing-up glass pushes the measurement up.
4. Let the water reach room temperature. Conductivity is temperature-sensitive; most meters compensate up to about 25 degrees.
5. Measure two to three times. If the measurements differ by more than ten per cent, the meter needs calibrating.

Combine the result with the hardness published by your water company. If your measurement differs greatly from it, that is more often down to the meter or the pipework in your home than to the water supplied. The same practical approach is covered in more detail in the explanation of measuring water hardness at home.

And be careful with comparisons between types of water. A glass you have stirred, water that has been in a bottle for hours or water you have chilled yourself all give slightly different values. Also take a look at how much water you need per day if you are scrutinising your water intake anyway.

What does a water filter do to the TDS value?

This is where the technologies diverge sharply, and where the TDS meter is most often used as a sales argument. A lower TDS, after all, looks impressive on a display.

Why our own TDS value deliberately does not drop

That is not sloppiness but a choice. The PureFilter Mineral+ purifies all 40 chemicals tested, from 75% to 99.97%, from your tap water in five steps, and then adds calcium, magnesium and potassium back via Swedish Mineral Rock™. The order matters: purify first, then add calcium, magnesium and potassium via Swedish Mineral Rock™. The result is that the TDS reading behind our filter does not fall to zero and sometimes even rises slightly.

Anyone going purely by a TDS meter would conclude from this that nothing is happening. That is precisely the wrong conclusion, and it is the reason we are writing this article. The substances you do not want in it are gone; the minerals you do want in it are back.

With a reverse osmosis installation it is different: it removes everything, including the minerals. Whether that matters depends on your situation and is explained in the question of whether osmosis water is healthy or unhealthy. The water jug with a built-in TDS meter, where you see it most often, we discuss in our review of ZeroWater in the Netherlands.

If you are torn between technologies, the overview of minerals in drinking water gives you something to hold on to when weighing them up, and in the fixed filters for the kitchen sink you will find what we supply for that.

You do not need a plumber for it: for the installation you use a single pair of water pump pliers, and later you change the cartridge with the key supplied. How that looks step by step is covered in the guide to installing a water filter.

Frequently asked questions

Conclusion

The TDS value of tap water is a useful number as long as you know what it is: an estimate of how much is dissolved, derived from conductivity. In the Netherlands there is no TDS standard; there is, however, a limit on conductivity of 125 mS/m, which comes down to roughly 625 to 875 mg/l as a ceiling. The WHO links the scale to taste and expressly sets no health standard.

What the number does not do is tell you whether your water is safe. The substances most concerns are about are present in quantities that lie hundreds of thousands of times below the measuring range of such a device. That is why a falling TDS is no proof of cleaner water, and an unchanged TDS is no proof that a filter does nothing. With the PureFilter Mineral+ the latter is literally the case: it purifies first and then deliberately returns the minerals.