What is distilled water?
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Part of our bottled and other water guide
Many people only meet the word when an appliance handbook asks for it. What is it, and how does it differ from what comes out of your tap? You can see part of that difference at your own tap with a 20-in-1 test strip.
Short answer: Distilled water is water boiled into steam and condensed back to a liquid, which leaves dissolved solids behind. The Royal Society of Chemistry says it should contain none, whereas tap water carries some. The Drinking Water Inspectorate says hardness comes from dissolved calcium and magnesium, and calls water hard from 200 mg/l calcium carbonate.
By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026
What is distilled water?
It is water that has been turned into steam and condensed again, so what was dissolved in it stays behind. The Royal Society of Chemistry (RSC) defines simple distillation as separating a liquid from a solution through evaporation followed by condensation. In its school practical, learners boil a coloured solution gently, lead the steam down a delivery tube into a test tube standing in ice water, and collect the liquid, which the RSC calls the distillate. The aim is to obtain pure water from the solution.
A second RSC practical shows the result. Learners evaporate a little tap water, distilled water and seawater on watch glasses and look for solid residue. The teaching notes say distilled water should contain no dissolved solids, tap water some (those causing hardness, for example) and seawater more. A 20-in-1 test strip can show a similar contrast pad by pad, which we come back to below.
How is distilled water different from tap, boiled and reverse osmosis water?
Each is made differently, and only distillation collects and condenses the steam. A kettle does not condense its steam, so boiled tap water is not distilled. Our boiled versus distilled water guide goes through it.
| Type | How it is made | What the sources say |
|---|---|---|
| Distilled | Boiled, steam condensed | RSC says it should contain no dissolved solids |
| Tap water | Supplied by the water company | RSC says it holds some dissolved solids. The DWI says hardness comes from dissolved calcium and magnesium |
| Boiled tap water | Heated in a kettle | The DWI says boiling can remove temporary hardness, the calcium and magnesium carbonates and bicarbonates, by forming scale |
| Reverse osmosis (RO) | Forced through a membrane | The DWI says RO removes most dissolved solids and that the units send 20 to 30% of the incoming flow to waste. See our reverse osmosis guide |
| Deionised | Treated to remove ions | The RSC lists distilled or deionised water together in its practical. See our deionised versus distilled guide |
The DWI sets out the RO details on its filters and softeners page and the boiling point on its hardness page.
How pure is distilled water?
Pure in the school chemistry sense. The RSC defines "pure" as a substance made of only one element or compound, and treats the distillate from its practical that way. In the sources we read, no one gives a measured figure for a bottle of distilled water, so we do not give one either.
The nearest comparison is the legal ceiling for tap water. The Water Supply (Water Quality) Regulations 2016, which apply to water companies in England, list conductivity as an indicator parameter with a ceiling of 2,500 µS/cm at 20 °C. It is a ceiling and not a typical tap reading.
Is distilled water meant for drinking?
In the sources we read, no UK body gives a yes or no on drinking distilled water as such. The nearest official comment is on RO water, which is also low in minerals. The DWI says RO water is not generally considered suitable for drinking and cooking, and that WHO recommends minerals be added back when it is drunk.
WHO has two relevant publications. In 2003 it assembled nutrition, medical and scientific experts to look at the long-term consumption of water manufactured to add or delete minerals, and separate WHO guidance covers drinking-water from desalination. We have not summarised their health findings, because they sit with WHO and your doctor and not with us.
Why might my own tap water differ from the area's typical water?
The area figure is an average. Your tap can differ for reasons the DWI gives, and a figure for the area does not tell you what comes out of your own tap.
- Local hardness. The DWI's map shows typical hardness in England and Wales, and the DWI says to ask your water company for an accurate figure because some areas have local variation the map does not show.
- Taste. The DWI says the taste of tap water varies between regions because of differing hardness and the residual chlorine used to keep the network clean.
- Your own pipes. The DWI says problems with colour or particles can come from faults in the pipes and fittings the building owner is responsible for, and that a plumber should find the root cause.
- Where it is measured. In the 2016 Regulations, conductivity is checked at the supply point, whereas pH and coliform bacteria are checked at consumers' taps.
“If you've only ever judged your water by the area average, I'd compare your own tap with a clean sample before deciding anything. Local hardness, the pipes in your building and where a reading is taken can all make your glass differ from the figure for your postcode.”
Hardness is the difference most people can act on. The DWI says hard water scales on heating elements in hot water systems, kettles, electric irons and other appliances, which shortens their life and makes them less efficient. Our guides cover why kettles scale and what water to use in a steam iron.
| DWI hardness band | mg/l calcium carbonate |
|---|---|
| Soft | Up to 100 |
| Slightly hard | 100 to 150 |
| Moderately hard | 150 to 200 |
| Hard | 200 to 300 |
| Very hard | More than 300 |
To see where you sit, our hardness guide explains how to find out.
What can a test strip show about distilled and tap water?
It can show, pad by pad, how your tap compares with a bottle of distilled water. Fill a clean glass from the tap and another from the bottle, dip one strip in each, wait 30 to 60 seconds and match the pads to the chart. You can also enter the colours in our free results checker by scanning the QR code on the chart. The right-hand column is our own reading of the chart steps, set against the RSC's statement that distilled water should hold no dissolved solids. We have not run strips on bottles of distilled water, so it is a reading of the chart and not a test result.
| Pad | Chart steps (mg/L) | What to expect on a distilled sample |
|---|---|---|
| Hardness | 0, 25, 50, 100, 250, 425 | The zero step. A higher step indicates dissolved calcium and magnesium, and the unit basis is unconfirmed, so setting it beside the DWI bands is a rough comparison |
| Total alkalinity | 0, 40, 80, 120, 180, 240 | The zero step |
| Sodium chloride | 0, 100, 250, 500, 1000, 2000 | The zero step, which means below the first step of 100 and not none at all |
| Free and total chlorine | 0, 0.5, 1, 3, 5, 10, 20 | The zero step |
| Copper | 0, 0.2, 0.5, 1, 2, 5 | The zero step |
| Nitrate | 0, 10, 25, 50, 100, 250, 500 | The zero step, which means none detected at the first step of 10 |
Several limits apply.
- A zero on any pad means none detected at that pad's first step. It does not show that nothing is there.
- We do not give a pH reading for distilled water, because we have not tested the pH pad on it.
- The strip cannot read lead at the UK limit, and our lead pipes guide covers that instead.
- The separate rapid bacteria test is a presence screen for drinking-water samples, and we do not claim it for distilled water or for water in a humidifier or other appliance.
- A strip result has no legal effect.
Best pack: 20 strips, enough to test your tap and a bottle side by side and repeat the check later, £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit
Your distilled water questions
What is distilled water used for?
In the sources we read, the clearest use is teaching, where the RSC lists distilled or deionised water as a material for its practical. The DWI describes RO water, which is a different product, as suited to specialist uses such as photographic processing or hydroponic culture. For an iron, battery, humidifier or steriliser, we found no UK official list, so your handbook is the reference.
What is distilled water UK?
In the sources we read, no UK regulation defines it. The RSC definition above is the one we use. Tap water in England is covered by the 2016 Regulations, which set no category for distilled water.
What does distilled water mean?
It means water collected as condensed steam, which the RSC calls the distillate. The method leaves dissolved solids in the flask, and the RSC says distilled water should contain none.
Is distilled water a pure substance?
In school chemistry, yes. The RSC defines pure as one element or compound and describes distillation as a way to obtain pure water. We found no measured purity figure for bottles in the sources we read.
Does distilled water conduct electricity?
We found no conductivity reading for distilled water in the sources we read. The Regulations list conductivity as an indicator parameter for tap water, with a ceiling of 2,500 µS/cm at 20 °C.
Does distilled water go bad, and how long does it last?
We found no shelf-life guidance for distilled water in the sources we read. Our bottled water guide covers dating for bottled water in general, and a bottle's label or your appliance handbook is the reference.
Is still water the same as distilled water?
We found no definition of "still" in the sources we read, so we cannot say it means distilled. Distilled describes how water is made, and the RSC says it should hold no dissolved solids.
Is reverse osmosis water the same as distilled water?
No, though both are low in dissolved solids. RO pushes water through a membrane, while distillation boils and condenses it. The DWI says RO removes most dissolved solids.
Is purified water the same as filtered water?
We found no definition of either as a drinking-water label in the sources we read. Our distilled, filtered and purified water guide compares them. The DWI says activated carbon filters are the type sold for taste and odour.
Do window cleaners use distilled water?
We found nothing on window cleaning in the sources we read for this guide.
What is distilled water for coolant?
We found nothing on engine coolant in the sources we read. Your vehicle handbook or the coolant maker's label is the reference.
Why only distilled water in CPAP?
We found nothing on CPAP machines in the sources we read, so your sleep service or the maker can say which water your machine needs. The DWI does say hard water scales on heating elements, shortening their life and making appliances less efficient.
Can you use distilled water in a fish tank?
We found no guidance on distilled water in a fish tank in the sources we read. One RSC teaching note says dissolved oxygen in water is vital for fish. Our fish tank tap water guide covers what the UK sources do say.
Sources
- Royal Society of Chemistry, Simple distillation, practical videos, 14 to 16 years (method, "distillate", definitions of simple distillation and "pure"; undated page; read 29 October 2026).
- Royal Society of Chemistry and Nuffield Foundation, Dissolved substances in tap water and seawater (teaching notes on distilled, tap and seawater, and on dissolved oxygen and fish; undated page; read 29 October 2026).
- World Health Organization, Nutrients in drinking-water (overview page only, which describes the expert meeting on demineralised and modified water; undated page; read 29 October 2026).
- World Health Organization, Safe drinking-water from desalination (overview page only; guidance for desalinated drinking-water; undated page; read 29 October 2026).
- The Water Supply (Water Quality) Regulations 2016, Schedule 2 (conductivity 2500 µS/cm at 20 °C and its point of compliance; pH and coliform bacteria points of compliance; undated page; read 29 October 2026).
- Drinking Water Inspectorate, Domestic water filters and softeners (reverse osmosis section, taste and pipes sections; undated page; read 29 October 2026).
- Drinking Water Inspectorate, Water hardness and hard water (hardness bands, local variation and the map, scaling in appliances, temporary hardness removed by boiling; undated page; read 29 October 2026).
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, optional rapid coliform test read after 24 to 48 hours, 20 strips £5.99 or £10.99 with a bacteria test, free 2nd Class delivery on every order, 30-day money-back guarantee) and results portal. The strip table is WaterKit's own reading of the colour chart steps.
Contains public sector information licensed under the Open Government Licence v3.0.