Soft or hard water in a copper bottle: what water hardness changes
Is your tap water soft or rather hard, and does that matter for a copper bottle? What hardness is, why tap water and copper get on well, how much copper ends up in your water and what you notice in the bottle.
Written and source-checked by the Kopero editors · Updated September 10, 2026
A question we hear a lot
"My water is soft. Can I still use a copper bottle?" It is a good question, because anyone reading about copper soon comes across the word corrosion, and that is usually about soft water in copper pipes. The short answer is yes. The longer answer shows why tap water and copper have got on well for decades, and what the hardness of your water really does change.
In short
- Hardness tells you how much calcium and magnesium is dissolved in your water. One millimole per litre equals 5.6 German degrees (°dH).
- European rules require drinking water not to be aggressive or corrosive, especially when it has been softened.
- In the Netherlands, softened drinking water may not go below 1.0 mmol per litre, so that copper does not easily dissolve from pipes.
- After sixteen hours in a copper pot, 0.18 milligrams of copper per litre was measured, about a tenth of the European limit for tap water.
- A full bottle therefore holds around 0.16 milligrams. EFSA considers 5 milligrams a day safe for adults.
- Soft water leaves hardly any limescale. Hard water sometimes leaves a light white film, which lemon takes off.
What hardness actually is
Hardness is the amount of calcium and magnesium dissolved in your water. Those minerals come from the ground the water has flowed through: limestone and chalk make water hard, sandy soil leaves it soft. Your body needs both, and you get most of them simply from your food.
Hardness is given in a few units. In the Netherlands, Germany and Scandinavia it is usually German degrees (°dH), in France and Belgium often French degrees (°f), and water companies like to work in millimoles per litre. One millimole per litre is 5.6 German degrees or 10 French degrees. In the UK you will also see milligrams per litre of calcium carbonate: one millimole is 100 of those.
| Category | °dH | mmol/l | °f |
|---|---|---|---|
| Very soft | 0 to 4 | 0 to 0.71 | 0 to 7 |
| Soft | 4 to 8 | 0.71 to 1.43 | 7 to 14 |
| Medium | 8 to 12 | 1.43 to 2.14 | 14 to 21 |
| Fairly hard | 12 to 18 | 2.14 to 3.21 | 21 to 32 |
| Hard | 18 to 30 | 3.21 to 5.35 | 32 to 54 |
In the Netherlands, where Kopero is based, tap water averages around 1.4 mmol per litre, just below the line between soft and medium.
Why tap water and copper get on well
In many homes across Europe, drinking water has run through copper pipes for decades, and the water is made for it. The European Drinking Water Directive requires water from the tap not to be aggressive or corrosive, and specifically mentions water that has been softened or desalinated. For copper itself, the same directive sets a limit of 2 milligrams per litre.
The Netherlands goes a step further. Its drinking water decree states that softened drinking water may not go below 1.0 mmol per litre, and gives the reason right alongside: water that is too soft dissolves metals such as copper more easily from pipes and taps. Water companies also adjust the acidity of the water so that it leaves the pipes alone. So even if your water is soft, you are drinking water that is set up to be in contact with copper.
A copper bottle works the same way. Over time a thin oxide layer forms on the inside by itself, the same layer that makes new copper pipes give off less copper after a while. That layer can simply stay. What it is and why it belongs there is in why copper turns green or black.
How much copper ends up in your water
This has been measured. In a study in India, drinking water stood in copper pots for sixteen hours, and afterwards it contained 0.18 milligrams of copper per litre. That is about a tenth of the European limit for tap water. In the same study, not a single bacterium could be found after those sixteen hours; the full story is in can copper really kill bacteria.
At that level a full 900 ml bottle holds around 0.16 milligrams of copper. Set that against what your body uses: the European Food Safety Authority (EFSA) works with 1.3 milligrams a day for women and 1.6 milligrams for men, and considers 5 milligrams a day safe for adults. That upper limit was confirmed again in 2023. Even if your water reached the European limit of 2 milligrams per litre, a full bottle at 1.8 milligrams would stay well below it. What copper does in your body is in is copper water healthy, and what may officially be said about it in Europe in the eight official EU claims.
We had our own bottle tested with distilled water, which is softer than any tap water: two hours at 40 degrees, repeated three times. The bottle met the European rules for materials that come into contact with food, and none of the nineteen heavy metals tested for could be detected in the water. The results are in the test file.
Soft or hard: what you notice in the bottle
With soft water, the inside of your bottle stays practically free of limescale. No white rim dries on and you never need to descale. On the inside, the copper takes on an even, warm colour at its own pace.
With hard water, a light white film can be left behind after draining. That is limescale drying on, not copper. A slice of lemon with a little salt dissolves it, just as it brings back the shine on the outside. How to do that, and which products to leave alone, is in cleaning copper: soda, vinegar, lemon or citric acid.
The same habit applies to both: fill the bottle with still water, leave it overnight, drink it in the morning and let the bottle drain upside down afterwards. Fizzy water, lemon juice and other acidic drinks go in a glass. How much to drink from it a day is in how often a day to drink from a copper bottle, and how the ritual fits into an ordinary week in tamra jal in practice.
How to look up your own water hardness
No need to guess. Water companies in the European Union are required to tell their customers how hard their water is, and most have a search box on their website where you enter your postcode. You then get the hardness in German degrees or in mmol per litre. A dishwasher or coffee machine often asks for the same number, so you may already have looked it up once.
How long your bottle lasts
A lifetime, whether your water is hard or soft. The bottle weighs around 300 grams and is solid copper, with no coating and no inner lining; what ends up in the water each day is a fraction of a milligram. What changes over the years is the colour, and that is up to you: let the patina deepen or polish it back. How to keep the shine is in how to keep copper shiny, and the full care routine in caring for a copper bottle.
Where our copper comes from and how pure it is, you can read in what our copper products are made of. Take a look at the water bottle, the cup or the jug, or put them side by side in the comparison.
