Short answer
Yes. Hard water carries more dissolved minerals, and when a drop dries on glass the water goes and the minerals stay behind as a white spot. That’s why tap water can mark windows. Pure water has the minerals taken out first, so there’s less to leave behind. It’s not a promise nothing will ever mark the glass.
What makes water hard?
Water picks up minerals, mainly calcium and magnesium, as it moves through the ground. Water with more of them is called hard. You may have noticed the same minerals as limescale in a kettle or on a tap.
Water companies publish their test results. For Rayleigh, Essex & Suffolk Water’s 2026 drinking water report for its Rayleigh supply zone says "The water in your area is hard." Its 12-month average, for January to December 2025, was 272.99 mg/l of calcium carbonate, or 19 degrees Clarke. Those figures come from the water company’s own tests. The report doesn’t say which streets sit in that zone, so check your own postcode with the water company.
| Rayleigh supply zone (Essex & Suffolk Water, Jan to Dec 2025) | Figure |
|---|---|
| Described as | Hard |
| Average hardness | 272.99 mg/l calcium carbonate |
| In degrees Clarke | 19 |
Why does it mark glass?
When a drop of tap water sits on a window, the water evaporates, and the minerals can’t. They’re left as a small ring or spot. The more minerals in the drop, the more visible the spot. If it’s left for a long time, or dried by sun and wind, the marks can be harder to shift.
It isn’t dirt, which is why wiping a cloth over it doesn’t always help. It’s a thin mineral deposit.
What does pure water change?
Pure water is tap water with the dissolved minerals taken out, usually by reverse osmosis followed by deionisation. I make mine in the van. If the drops on the glass have no minerals in them, there is nothing left to dry onto the window, so the glass dries clear. Is pure water window cleaning better? goes into the detail.
When can spots still happen?
Even with pure water, you can still get marks:
- Old residue from soap or detergent cleans, which can run out of frames on the first few cleans. Spots after the first clean has the detail.
- Dirt from frames, sills and vents that runs onto the glass.
- Water that isn’t pure enough, if a system isn’t maintained, or a poor rinse.
- Rain, which can carry dirt.
If your own tap water is hard, a hose or sprinkler that sprays your windows can leave spots too. Not much to be done about that, other than to know what it is.
How can I tell if it’s hard-water spots?
Hard-water spots usually look like small white rings or dots, often on the lower part of a pane where water runs and dries. They feel slightly rough if you run a fingernail over them. Dirt streaks, by contrast, usually look grey or brown and smear when you wipe them.
What should I do about it?
If you’ve noticed spots that you can’t wipe off, it’s worth telling me, ideally with where they are on the window. On the outside window cleaning round, pure water is what I use for exactly this reason. If you’re in Rayleigh, there’s more on what that means locally in window cleaning in Rayleigh: hard water and older frames.
Common questions
Will vinegar remove hard-water spots?
Some people use diluted vinegar on spotted glass. Acids can harm some frames and seals, so test a small area and keep it off frames.
Do spots mean my window is damaged?
Not usually. Light mineral spots sit on the surface. Very old, baked-on deposits can etch some glass over a long time, which is why I’d rather deal with spots sooner. I can’t judge etching without seeing it.
Are spots worse on south-facing windows?
Sun can dry drops faster, so spots may form quicker.
Is softened water any better for washing windows?
A water softener swaps hardness minerals for others, so the water still has dissolved solids in it and can still leave marks as it dries. Pure water, from reverse osmosis and deionisation, has the dissolved solids taken out.
