A TDS meter gives you a number in seconds, but what does that number actually tell you? This guide explains what a TDS meter reading means, what kind of figures are commonly seen in UK tap water, why temperature and calibration matter, and where TDS is useful for aquariums, hydroponics and pools.
What Is TDS?
TDS stands for total dissolved solids: the combined amount of minerals, salts and other substances dissolved in water. Typical contributors include calcium, magnesium, sodium, potassium, bicarbonates, chlorides and sulphates.
Most handheld TDS meters do not measure dissolved solids directly. They measure electrical conductivity (EC), because dissolved ions let water conduct electricity, and then convert that to an estimated TDS figure using a conversion factor. That is why many testers show both EC and TDS.
TDS is usually shown in ppm (parts per million), which is roughly the same as milligrams per litre (mg/L) for water.
How to Take a TDS Meter Reading
- Rinse the probe with clean water (ideally distilled) and shake off the excess.
- Use a clean container. Residue from soap or a previous sample will skew the result.
- Immerse the probe to the level shown in the manual, without touching the sides or bottom.
- Stir gently to remove air bubbles from the electrode.
- Wait for the reading to settle, then note the value. Use the hold function if your meter has one.
- Rinse and dry the probe after use, and replace the cap.
What Is a Normal TDS Reading for UK Tap Water?
There is no single figure for the UK. TDS depends largely on where your water comes from:
- Soft-water areas, often supplied from upland reservoirs and moorland catchments (much of Scotland, Wales and northern England), tend to give lower readings.
- Hard-water areas, often supplied from chalk and limestone aquifers (much of southern and eastern England, including London), tend to give noticeably higher readings because of dissolved calcium and magnesium.
As a rough guide only:
| TDS reading (ppm) | What it generally suggests | |---|---| | Very low (close to 0) | Distilled, deionised or reverse osmosis water | | Low | Soft water or filtered water | | Moderate | Typical of many mains supplies | | Higher | Hard water with more dissolved minerals |
A higher reading does not, on its own, mean your water is unsafe. TDS does not identify what is dissolved, and it cannot detect bacteria or many contaminants. UK public water supplies are regulated and tested by water companies. If you have concerns, your water supplier can provide a water quality report for your postcode.
Using TDS to Check a Filter
One of the most practical uses is comparing water before and after a filter or reverse osmosis system. Measure the tap water, then the filtered water. A reverse osmosis system should produce a much lower reading than the tap. If the filtered reading creeps upwards over weeks, the membrane or cartridge may need changing.
How Temperature Affects TDS Readings
Conductivity rises as water gets warmer, so the same water reads differently at different temperatures. Most meters apply automatic temperature compensation (ATC), adjusting the result to a reference temperature, commonly 25 °C. For consistent results:
- Test samples at a similar temperature each time.
- Allow the meter and sample a minute or so to reach the same temperature.
- Record the temperature alongside the reading if your meter shows it.
How to Calibrate a TDS Meter
Calibration keeps readings trustworthy over time:
- Buy a fresh calibration solution of known value, as specified in your meter's manual.
- Bring the solution to room temperature.
- Rinse the probe, then immerse it in the solution.
- Enter calibration mode as described in the manual and adjust or confirm the value.
- Rinse the probe again before testing samples.
Check calibration regularly, and recalibrate if readings drift or after the meter has been stored for a long time. Never pour used solution back into the bottle.
TDS for Aquariums, Hydroponics and Pools
- Aquariums and ponds: tracking TDS helps you spot mineral build-up between water changes and check the water used for top-ups.
- Hydroponics: growers use EC/TDS to monitor nutrient strength in the solution.
- Pools and hot tubs: a steadily rising TDS can indicate that it is time for a partial water change.
Water Quality Testers at EkoDeluxe
- EZ-9909 5-in-1 Water Quality Tester Pen: a pocket-sized pen that checks pH, TDS, EC, salinity and temperature, suited to aquarium and pond keepers, hydroponic growers, pool owners and home water testing.
- PH-W3988 6-in-1 WiFi Water Quality Monitor: installed permanently in a pool, pond or aquarium, it tracks pH, EC, TDS, salinity, specific gravity and temperature around the clock and uploads readings to a smartphone app every 15 seconds. Supplied with a UK plug power adapter.
Frequently Asked Questions
What does ppm mean on a TDS meter?
Parts per million. For water it is roughly equivalent to milligrams of dissolved solids per litre.
Is a high TDS reading dangerous?
Not necessarily. TDS shows the total amount dissolved, not what it is. Hard water often reads higher. Contact your water supplier if you have concerns about safety.
Why do my TDS readings change?
Temperature, air bubbles, a dirty probe or drifting calibration can all affect results. Rinse the probe, stir gently and test at a similar temperature.
How often should I calibrate a TDS meter?
Check it regularly against a calibration solution, and recalibrate whenever readings drift or the meter has been stored for a while.
Can a TDS meter test for bacteria or lead?
No. A TDS meter estimates dissolved ions from conductivity. It cannot identify bacteria, lead or specific contaminants; a laboratory test is needed for that.
Cover photo: “Tds-meter” by Spotless property, CC BY-SA 3.0, via Wikimedia Commons. Cropped; the crop is shared under the same licence. No endorsement implied.
