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A TDS water test at home can give you a useful first reading on what is dissolved in your tap water. It is quick, affordable and reassuring when used correctly. But a number on a handheld meter is not a complete water-quality report, and it should not be treated as one.
For London households in particular, TDS readings often reflect the minerals that make local water hard. That may explain limescale around taps, cloudy glassware and a dry feeling after washing. It does not, by itself, prove that water is unsafe, nor can it identify specific contaminants. Knowing the difference helps you make a sensible decision about filtration rather than buying equipment based on a number alone.
What TDS means in household water
TDS stands for total dissolved solids. It is the combined amount of dissolved material in water, usually expressed as parts per million (ppm) or milligrams per litre (mg/L). A TDS meter estimates this total by measuring electrical conductivity: water with more dissolved ions conducts electricity more readily.
Those dissolved solids can include naturally occurring calcium, magnesium, potassium and sodium, along with small amounts of salts and minerals picked up as water travels through pipes. Calcium and magnesium are the main contributors to hard water in many parts of London and the South East.
That is why a higher reading is not automatically a bad reading. Mineral-rich water can produce a higher TDS result while remaining suitable for drinking. Conversely, a low reading does not guarantee that water is free from unwanted substances. Some contaminants are present at levels a basic TDS meter cannot distinguish, while others do not affect conductivity in a meaningful way.
Think of TDS as a broad indicator of dissolved content, not a verdict on purity or safety.
How to do a TDS water test at home
A basic digital TDS meter is easy to use, but small details make the result more reliable. Test cold water rather than hot water, as temperature can affect the reading and hot water may have passed through a boiler or cylinder.
Start with a clean glass. Run the cold tap for around 30 seconds so you are testing water from the supply pipe rather than water that has been standing in the tap or household plumbing. Fill the glass, switch on the meter and rinse the probe in the sample. Immerse it to the indicated level without touching the sides or bottom of the glass.
Gently stir the meter through the water, then hold it still and wait for the display to settle. Record the reading in ppm. Rinse and dry the probe after use.
For a more useful comparison, take readings from the kitchen tap, a filtered drinking-water tap if you have one, and water after any treatment system. Test at similar times and in the same way each time. One isolated result can be influenced by the sample, the meter’s condition and even a recently fitted cartridge.
Check that your meter is trustworthy
Inexpensive meters are helpful for household comparisons, but they are not laboratory instruments. If your model supports calibration, follow the manufacturer’s instructions using the appropriate calibration solution. Keep the probe clean, replace batteries when needed and avoid storing it wet unless the instructions specifically say otherwise.
A sudden change in a reading is often more useful than chasing a supposedly perfect number. For example, if a reverse-osmosis system normally produces water with a much lower TDS reading than the incoming supply, then the filtered reading begins to rise steadily, the membrane or filters may need inspection. The exact point at which this matters depends on the system, incoming water and its maintenance schedule.
How to interpret your TDS result
There is no universal household TDS figure that tells every family what to do. The source of the dissolved solids, the age and condition of plumbing, local supply conditions and your intended use of the water all matter.
As a general guide, a reading below 50 ppm is often seen in water treated by reverse osmosis, although it depends on the incoming supply and membrane performance. Readings from around 100 to 300 ppm are common in many drinking-water supplies and can reflect ordinary dissolved minerals. In hard-water areas, readings may be higher because of calcium and magnesium.
Rather than judging a number in isolation, ask practical questions. Does the water have an unpleasant taste or smell? Is limescale building up quickly on the kettle, shower screen and heating elements? Have you noticed sediment, discolouration or changes after building work or plumbing repairs? Are you comparing untreated water with the output from a filtration system?
If the answer is yes, a TDS reading can be one useful part of a wider assessment.
High TDS and limescale are related, but not identical
TDS and hardness are often confused. Hardness is primarily about calcium and magnesium. TDS measures all electrically conductive dissolved solids. Water can have a high TDS result without being especially hard, and hard water will not always have the same TDS reading from one area to another.
If limescale is your main concern, consider a hardness test alongside the meter reading. A properly specified filtration or treatment solution may use resin-based media to reduce scale-forming minerals, while drinking-water systems can use reverse osmosis, ultrafiltration or other stages according to the household’s priorities. These technologies do different jobs, so the right choice depends on what testing shows.
What a TDS meter cannot tell you
A handheld TDS meter cannot identify what the dissolved solids are. It cannot confirm the presence or absence of bacteria, viruses, lead, pesticides, PFAS, chlorine by-products or other specific substances. It also cannot measure turbidity, which is the cloudiness caused by suspended particles rather than dissolved material.
This matters when a household has a particular concern. Older properties may require attention to internal plumbing and potential metal contamination. Homes supplied by private water sources need a more detailed approach than a quick meter test. A strange odour, persistent discolouration, visible particles or a sudden change in taste should not be dismissed because a TDS result looks ordinary.
Where there is a specific concern, professional analysis or a targeted laboratory test is the appropriate next step. That process identifies the issue before a treatment system is selected, rather than assuming one filter will solve every water-quality question.
Comparing filtered and unfiltered water
Testing before and after filtration is one of the best uses of a TDS meter, provided you understand the technology installed.
Reverse osmosis is designed to reduce a broad range of dissolved solids, so its output should usually show a clear reduction compared with the incoming water. Ultrafiltration and UV systems serve different purposes. UV treatment targets microorganisms, while ultrafiltration focuses on fine particles and certain contaminants; neither is designed simply to produce a dramatically lower TDS figure. A carbon filter may improve taste and reduce chlorine-related compounds while leaving much of the mineral content, and therefore much of the TDS reading, in place.
A lower number is not always the goal. Some households prefer water that retains beneficial minerals, while others want highly purified drinking water from a dedicated reverse-osmosis tap. Whole-home treatment also needs to account for bathing, washing, appliances, water pressure and flow rate, not just the water in one glass.
When to arrange a professional water assessment
Home testing is an excellent starting point, particularly if you want to understand hard-water symptoms or check how an existing system is performing. It becomes less useful when you need an answer about a specific contaminant, an unexplained change in water quality or the right treatment for every tap in the property.
A professional assessment considers the incoming water, your plumbing, household size, daily water use and the concerns you want to address. It can distinguish between a drinking-water solution under the sink and a whole-home approach that protects showers, appliances and bathroom fittings from sediment or limescale. It also creates a clear baseline for post-installation testing and future maintenance.
For homeowners who want practical reassurance rather than guesswork, Mountain OV can assess the water in the context of the home and recommend filtration that fits how the household actually lives. The most useful reading is not simply the lowest one on a screen, but the one that helps you choose water treatment with confidence.
