How do you test water hardness at home?
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Part of our water testing guide
Your kettle is furring up and the dishwasher wants a hardness setting. Do you need a kit, a postcode, or just a squirt of washing-up liquid?
Short answer: Ask your water company for your supply zone figure, because the Drinking Water Inspectorate (DWI) says that is how to get an accurate measurement. The DWI calls 200 to 300 mg/l (as calcium carbonate) hard and over 300 very hard. A figure for the area does not tell you what comes out of your own tap.
By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026
How do you test water hardness at home?
Ask your water company for the figure for your supply zone, then check your own tap with a 20-in-1 strip from the WaterKit Water Test Kit to see what reaches it. The DWI says to ask the company directly for an accurate measurement, because its map of typical hardness leaves out local variation. None of the sources we read gives a step-by-step method for home strips, drop-count kits or soap-shake tests, so the last column shows what each does say.
| Method | What it gives you | What the sources say |
|---|---|---|
| Water company lookup, zone report or helpline | The typical hardness of your supply zone | The DWI says to ask your water company. Welsh Water has a postcode tool, and Scottish Water publishes hardness data by supply zone, with a postcode search to find your zone |
| Signs in the home | A hint, with no number | The DWI, United Utilities, Welsh Water and Scottish Water list scale, poor lather and scum |
| Soap lather | A hint, with no number | The DWQR says hard water takes more soap to form a lather. None of the pages we read describes a shake test |
| Test strip or drop-count kit | A reading at your own tap, on the maker's chart or count | The pages we read give no method, so follow the maker's instructions |
Our postcode guide covers each company's lookup.
What is water hardness measured in?
Usually milligrams per litre of calcium carbonate (mg/l CaCO3), which United Utilities says is the same as parts per million. You may also see degrees Clark, French or German degrees, or mg/l as calcium. The table follows United Utilities' six bands, which split the DWI's soft band at 50 mg/l, and United Utilities says there are no universally agreed criteria.
| Band | mg/l calcium carbonate | Degrees Clark | German degrees (°dH) |
|---|---|---|---|
| Soft | 0 to 50 | 0 to 3.5 | 0 to 2.8 |
| Moderately soft | 51 to 100 | 3.6 to 7.0 | 2.9 to 5.6 |
| Slightly hard | 101 to 150 | 7.1 to 10.5 | 5.7 to 8.4 |
| Moderately hard | 151 to 200 | 10.6 to 14 | 8.5 to 11.2 |
| Hard | 201 to 300 | 14.1 to 21 | 11.3 to 16.8 |
| Very hard | Over 300 | Over 21 | Over 16.8 |
One °dH is about 17.8 mg/l and one degree Clark about 14.3 mg/l, and a figure in mg/l as calcium is multiplied by 2.5. These are our rounded figures, worked from the factors in the United Utilities factsheet.
How do you check water hardness for a dishwasher?
Find your zone figure, then convert it to the unit in your dishwasher's manual. Scottish Water says dishwasher manufacturers use different units for their settings. For example, 250 mg/l of calcium carbonate is 14 °dH (250 × 0.056) or 17.5 degrees Clark (250 × 0.07), our arithmetic.
Why might your tap not match the area figure?
Because the area figure is an average. The DWI describes its map as showing typical hardness and says some areas have local variation the map does not show.
- Different sources. Scottish Water says supplies drawn from boreholes in rock or gravel tend to be harder than surface sources, and Welsh Water says its tap water comes from upland reservoirs to boreholes, which affects hardness.
- A softener in your home. The DWI says most domestic softeners replace calcium and magnesium with sodium, and advises an unsoftened tap for drinking and cooking. A softened tap and your kitchen cold tap can give different answers.
- A private supply. The DWI's glossary defines a private supply as any supply not provided by a statutory water undertaker, so a water company's zone figure does not describe it. The DWI says your local authority advises on testing.
“If your tap result and the water company's area figure don't line up, don't assume one of them is wrong. The area figure is an average for the zone, and things like your water source, a softener in the house or a private supply can all change what reaches your own kitchen cold tap.”
What one reader found
A fishkeeper posted on a forum after a liquid drop-count kit arrived. They counted 23 drops for general hardness and worked out 409.4 ppm, adding that the kit's chart only went up to 400 ppm. They then used their water company's postcode checker (Yorkshire Water), which gave 110.8 mg/l as calcium, equal to 277 mg/l as calcium carbonate, well below the kit's figure. They tested the aquarium and the tap and got 23 drops for each, and planned a second tap test the next evening. Replies in the thread said that kit's hardness test is not reliable.
On the DWI's table, 277 is hard (200 to 300) and 409 would be very hard, so the two results fall in different bands. The thread does not say why they differed. The lesson we take is to check the unit, repeat the test and compare like with like. Shared on Fish Forums.
What does the DWI say about doorstep water tests?
It advises against buying a treatment device from a door-to-door salesperson, particularly one offering to test your water to show it is contaminated, and says such scams should be reported to your local trading standards officer. Once you know your band, our guide to softening hard water at home compares the routes.
How do you test your own tap with a WaterKit strip?
Dip one strip in a sample from your cold kitchen tap and match the hardness pad to the chart after 30 to 60 seconds. The DWI says drinking and cooking water should come from a cold tap fed directly off the mains, nearly always the kitchen one, and that hot and bathroom taps usually draw from a loft tank. Our routine follows.
- Fill a clean glass or cup from the cold kitchen tap.
- Dip one strip.
- Wait 30 to 60 seconds, then hold the hardness pad against the chart in daylight.
- Enter the colour in the results portal, which names the band it indicates. The free checker needs no app or account, and the QR code on your chart opens it.
The pad reads in steps of 0, 25, 50, 100, 250 and 425 mg/L. The last column is our reading against the DWI's bands, taking the pad's mg/L as calcium carbonate, which we have not checked with the supplier. The portal's band names are WaterKit's own.
| Chart step | Results portal band | Against the DWI bands (our reading) |
|---|---|---|
| 0, 25 or 50 mg/L | Soft | Soft |
| 100 mg/L | Moderately soft | The top of the DWI's soft band (up to 100) |
| 250 mg/L | Hard | Hard (200 to 300) |
| 425 mg/L | Very hard | Very hard (over 300) |
- Band, not a figure to the milligram. There is no step between 100 and 250 or between 250 and 425, so if your pad sits between two chart colours, enter the nearer step and read it as a range.
- Match the unit. If your company gives mg/l as calcium, multiply by 2.5 before comparing bands.
- One reading. The pad does not split temporary from permanent hardness, which the DWI's glossary says together make up total hardness.
- No limit, and no legal weight. The DWI says UK drinking water regulations set no standard for hardness, so the bands are labels, and a strip result is not a legal sample.
The area figure is an average for the zone, and the DWI says its map leaves out local variation, so your own tap may differ, for example through a borehole source, a softener in the house or a private supply. The pad that matters here is the hardness pad. A reading in a different band from your company's figure, or a different reading at a softened tap and at your kitchen cold tap, would indicate that something between the zone and your tap is changing the water. Best pack: 20 strips, enough to test your kitchen cold tap now and again after any softener or plumbing change, £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit
Your water hardness testing questions
How to test water hardness?
Get your supply zone's figure from your water company, as the DWI advises, then dip a strip in cold kitchen tap water and match the hardness pad after 30 to 60 seconds.
How do you know if you have hard water?
The DWI lists scale in kettles and appliances, less lather from soap, tide marks on basins, baths and toilets, and a scum on tea. These show hardness is present, not how much.
How is water hardness measured?
As the total amount of calcium and magnesium in the water, the DWI's glossary says, usually given as mg/l of calcium carbonate (see the units table above).
How to check water hardness in my area?
In Wales, use Welsh Water's hardness page and its postcode tool. In Scotland, Scottish Water's water quality search finds your zone. In Northern Ireland, NI Water has a water quality results page. For England, our postcode guide covers each company. On a private supply, the DWI says your local authority will advise on testing.
What is dH in water hardness?
German degrees of hardness. United Utilities gives the factor, 0.056 per mg/l of calcium carbonate.
How to read water hardness test strips?
Read the hardness pad inside the maker's time window, which is 30 to 60 seconds on our strip, and match it to the nearest step on the chart.
Sources
- Drinking Water Inspectorate, Water Hardness / Hard Water (Table 1 bands in mg/l CaCO3; signs of hard water; "for an accurate measurement ask your water company directly"; map of typical hardness and local variation; scale shortens element life; no standards for hardness in the regulations; softeners and an unsoftened drinking tap; undated page; read 29 October 2026).
- Drinking Water Inspectorate, Private water supplies glossary (hardness, total hardness, temporary and permanent hardness, private supply; read 29 October 2026) and Chlorine (cold mains tap, usually the kitchen cold tap; hot and bathroom taps usually from a loft tank; read 29 October 2026).
- Drinking Water Inspectorate, Domestic water filters and softeners (door-to-door salespeople offering to test water; report to trading standards; softeners replace calcium and magnesium with sodium; read 29 October 2026) and Water Testing (local authority advises on and arranges testing for a private supply; read 29 October 2026).
- United Utilities, Water hardness factsheet, reference 09/25/SD/10586-6335 (units; Table 1 conversion factors 0.4, 0.07, 0.1 and 0.056; Table 2 six bands in mg/l CaCO3, Clark, French and German degrees and mg/l calcium; no universally agreed criteria; read 29 October 2026). The PDF's text layer is scrambled, and its factors match those in Scottish Water's factsheet. The 17.8 (the figure our hardness guides use for one German degree; 1 divided by 0.056 is 17.86), 14.3 (1 divided by 0.07 is 14.29), 2.5, 14 °dH and 17.5 degrees Clark conversions are our arithmetic.
- Welsh Water, Water hardness (sources from upland reservoirs to boreholes; signs of hard water; units; postcode tool; read 29 October 2026). Scottish Water, Factsheet 12, Hardness in drinking water explained (reference SWFact WH12 05/20; range 10 to 500 mg/l; borehole supplies tend to be harder; hardness of any zone by helpline; dishwasher manufacturers use different units; read 29 October 2026). Scottish Water, Water Quality Search (postcode search for your supply zone; hardness data by zone; read 29 October 2026). Drinking Water Quality Regulator for Scotland, Water hardness (most Scottish drinking water classified as soft; more soap needed to form a lather; read 29 October 2026).
- NI Water, Water quality results (page for checking the water quality in your area; the text we could read was cut short; read 29 October 2026).
- Searches and reading for a home-strip, drop-count or soap-test method, 29 October 2026. We read the DWI, United Utilities, Welsh Water, Scottish Water, DWQR and NI Water pages above and found no step-by-step method in any of them.
- Fish Forums, Water hardness (a public forum thread; a fishkeeper's drop-count result against a water company postcode figure, paraphrased; read 29 October 2026).
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, 20 strips from £5.99, free 2nd Class delivery on every order, 30-day money-back guarantee) and results portal (hardness pad steps 0, 25, 50, 100, 250 and 425 mg/L, unit given as mg/L; band names Soft, Moderately soft, Moderately hard, Hard and Very hard). The strip table and routine are WaterKit's own reading.
Contains public sector information licensed under the Open Government Licence v3.0.