Why are there blue-green stains in my bath?

Part of our tap water guide

A blue-green mark under the tap or round the plug hole. Here is what the official pages say about where it comes from, and how to check the copper at your own tap with a 20-in-1 test strip.

By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026

Why do blue-green stains appear in a bath?

The official pages we read point to copper plumbing. The Drinking Water Inspectorate says that on rare occasions a blue colour in water may be caused by corrosion of copper plumbing, that it indicates quite high levels of copper, and that it normally only happens where the copper pipework is relatively new. WaterSafe, the plumbers' accreditation register, says blue or green water is often due to new copper pipework settling in, in new houses or where pipework has been replaced in older ones.

Neither page is about bath stains as such, so linking the two is our inference. Water carrying copper can leave a blue-green deposit where it dries, such as the drip line under a tap. You can check the copper level at your own tap with a 20-in-1 test strip, which has a copper pad. We cover how below.

Could the colour be something other than copper?

Yes. The DWI says bright blue or pink water is more likely to be back siphoning from the toilet cistern, made noticeable by a coloured disinfectant block. It says to contact your water supplier as soon as possible, that some plumbing work is likely, and that a WaterSafe-registered plumber is the one to look for. WaterSafe adds that if your water is blue you should not drink it or use it for cooking until you have sought advice from your supplier.

Brown, black or orange water is a different problem. The DWI's discoloured water page ties it to disturbed sediment in pipes, and our brown or cloudy water guide covers it.

Why might your own tap differ from the area figure?

The area figure is an average. Your tap sits at the end of your own pipework, and a figure for the area does not tell you what comes out of it. The sources give three reasons a bath at home could behave differently from the house next door.

“The area figure only tells you about the supply, but your tap sits at the end of your own pipework, and things like new copper or a softener on the system can change what comes out. If you see a blue-green mark, I would check your own tap rather than assume the average applies to you.”

• New copper plumbing. The DWI says the problem normally only occurs with relatively new copper plumbing, and that the blue colour disappears when the tap is run. Our reading is that this points to water standing in your own pipes and not to the supply.
• Soft water. The DWI's hardness page says the World Health Organization's 2017 guidelines identified soft water, 100 mg/l as calcium carbonate or less, as more corrosive to pipework. Our guide to how hard your water is shows how to find your own figure.
• A water softener. The same DWI page says artificially softened water may be aggressive to plumbing, causing leaching of copper and lead, and that you should keep an unsoftened tap for drinking. Our water softener guide explains how they work.

That is why a reading at your own tap, for example with a 20-in-1 test strip, says more about your bath than any figure for the area.

What one reader found

In a 2020 Mumsnet housekeeping thread, one poster said their metal bath "has developed a blue-green stain running down from the tap". It would not rub off and they did not think it was mould. They lived in a hard-water area but used salt-softened water, and the thermostatic tap had recently been repaired, after which the staining seemed worse.

A reply said it was copper staining. It said bare copper is exposed inside the pipes after plumbing work and tarnishes, and that soft water may make it worse. It suggested a bathroom spray and a nylon scourer, and another reply suggested a mildly acidic cleaner such as vinegar, tried somewhere inconspicuous first. A forum reply is not official guidance, but it fits the DWI's point about softened water and copper. The thread does not say whether the cleaning worked. Shared on Mumsnet.

How can you check the copper at your own tap?

Dip a strip in a sample and read the copper and pH pads against the chart. Fill a clean glass or cup, dip one strip, wait 30 to 60 seconds and match the pads to the colour chart, or enter the colours in the free results checker. You can also scan the QR code on the colour chart in the pack, and no app or account is needed. We suggest two samples, which is our own method. Take the first from the bath cold tap before any tap has run that morning, and the second after the cold tap has run for about half a minute.

The copper steps on the chart are 0, 0.2, 0.5, 1, 2 and 5 mg/l. The limit in Schedule 1 of the 2016 Regulations is 2.0 mg/l at consumers' taps in England, so the 2 mg/l step sits at the limit. The bands below are WaterKit's reading of the chart, and the swatches are only a screen approximation, so match against the printed chart.

What the two samples indicate, in our reading, is as follows.

• First sample higher than the flushed one. This indicates copper picked up while water stands in your own pipework. The DWI says the blue colour disappears once the tap is run, which fits.
• Both samples at 2 mg/l or above. This indicates copper at or above the legal limit at that tap. The DWI says to contact your water supplier if the colour persists, and WaterSafe lists registered plumbers. If the pipework is new, WaterSafe says it is settling in, and the sources we read give no timescale, so re-test later.

The pH pad on the same strip runs in steps from 6.2 to 8.4. Schedule 2 lists pH as an indicator parameter of 6.5 to 9.5 at consumers' taps, and we read it only as context beside copper. Our pH guide explains it. The hardness pad also gives a rough comparison with the DWI's soft band of up to 100 mg/l, though its unit basis is unconfirmed.

The strip cannot show everything. It cannot read lead at the UK limit, so if you are wondering about older pipes, see our lead pipes guide. It also cannot show a cistern siphoning back, and a strip result is not a legal sample. A certified figure for a landlord or a dispute comes from a laboratory.

The area figure is an average, and the DWI says new copper plumbing and artificially softened water can change what reaches your bath, so your own tap may not match the supply. The copper pad is the one that matters here, with pH as context. A first sample higher than the flushed one indicates copper picked up in your own pipework, while both samples at 2 mg/l or above indicate copper at or above the limit at that tap. Best pack: 20 strips, enough to test first-draw and flushed samples at the bath and kitchen taps, then again after any plumbing work or cleaning, £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit

Blue-green stains: the questions people ask

Why is my bath going green?

Going by the DWI and WaterSafe pages above, the likely source is copper in your plumbing. Our inference is that water carrying it leaves a deposit where it dries. A cistern fault is the other cause the DWI names for coloured water.

Does a blue-green stain mean the water is over the copper limit?

Not by itself. A stain is a deposit and does not tell you a concentration. The limit is 2 mg/l at consumers' taps in England, and the DWI says blue water itself indicates quite high copper. Two strip readings indicate which side of 2 mg/l you are on.

Will it go away on its own?

The DWI says the blue colour disappears when the tap is run, and WaterSafe describes new pipes settling in. Neither gives a timescale or says what happens to a stain already on the bath. If the colour persists, the DWI says to contact your water supplier.

How do I remove the stain?

We found no UK official guidance on cleaning copper stains from baths in the sources we read (the DWI's blue water, hardness and discoloured water pages and WaterSafe's blue/green water page). Check your bath maker's care instructions before using any cleaner. Cleaning does not change what is in the water, so a reading at the tap is still worth taking.

Can soft water areas get this more?

The DWI says soft water, 100 mg/l as calcium carbonate or less, is more corrosive to pipework, and that artificially softened water may cause copper and lead to leach. Our hard water guide explains the hardness bands.

Sources

1. Drinking Water Inspectorate, Blue or Brightly Coloured Water (copper corrosion, relatively new copper plumbing, colour clears when the tap is run, cistern back siphoning, WaterSafe plumbers; undated page; read 29 October 2026).
2. Drinking Water Inspectorate, Water Hardness / Hard Water (soft water 100 mg/l or less more corrosive to pipework, softened water and copper leaching; undated page; read 29 October 2026).
3. Drinking Water Inspectorate, Brown, Black or Orange Water (sediment, internal plumbing and tanks; undated page; read 29 October 2026).
4. WaterSafe, Why is my water blue/green? (new copper pipework settling in, cisterns, do not drink or cook with blue water until advised; undated page; read 29 October 2026).
5. The Water Supply (Water Quality) Regulations 2016, Schedule 1 (copper 2.0 mg/l at consumers' taps; read 29 October 2026).
6. The Water Supply (Water Quality) Regulations 2016, Schedule 2 (pH indicator parameter, 6.5 minimum to 9.5 maximum at consumers' taps; read 29 October 2026).
7. Mumsnet, Blue staining under bath tap (housekeeping thread dated 18 August 2020; one reader's experience, not official guidance; read 29 October 2026).
8. WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, 20 strips £5.99 or £10.99 with a bacteria test, free 2nd Class delivery on every order, 30-day money-back guarantee; copper pad steps 0, 0.2, 0.5, 1, 2 and 5 mg/l; pH pad steps 6.2 to 8.4) and results checker. The bands and readings in the table and list are WaterKit's own reading of the chart.

Contains public sector information licensed under the Open Government Licence v3.0.