Total Dissolved Solids
Everything dissolved in the water added together. A slow-moving indicator that the water is getting tired.
- Safe range
- 0 – 1500 mg/L
- Ideal range
- 500 – 1200 mg/L
- Measured in
- mg/L
What it is
Total dissolved solids is the sum of everything dissolved in the pool: salts, minerals, metals, and the residue of every chemical ever added. It only rises. Water leaves by evaporation and leaves its dissolved content behind.
Why it matters
TDS is best read as the pool's age rather than a problem in itself. As it climbs, chemicals become less effective, water can look dull and feel flat, and adjustments stop producing the response you expect. In a saltwater pool, note that the salt itself is a large part of the TDS reading, so the useful number is how much it has risen above the salt baseline.
How to measure it
A TDS pen, and understand what it does: it measures how much is dissolved in the water, never what.
Digital meter or pen
- How trustworthy
- Dependable
- Cost
- Mid range
- Effort
- Seconds to read, minutes to calibrate
A probe on a handheld body that reads the water electrically and prints a number. No colour to judge, which removes the commonest source of error and introduces a new one: a meter that has drifted looks exactly as confident as one that has not.
Dip the pen in a cup of pool water and read it immediately. Cheap, quick, and the only practical home test for this parameter.
Watch out: The pen measures electrical conductivity and multiplies by a generic factor. In a saltwater pool the salt dominates the reading, so compare your own pool with itself over time rather than against a figure quoted for a chlorine pool.
Where to find it: Online and in pool and aquarium shops. Buy the calibration solution at the same time; a meter without it is a guess with a decimal point.
Free test at a pool shop
- How trustworthy
- Dependable
- Cost
- Free
- Effort
- A trip, and a sample carried carefully
Take a sample in and they will run it, usually on a photometer, usually free, and print you a full panel. The catch is that the printout ends in a list of things to buy.
A shop panel usually includes TDS and puts it beside everything else, which is the only way this number means much: rising TDS is context for water that has stopped responding to chemicals.
Where to find it: Most pool and spa retailers. Ask which parameters they cover before driving over.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • Take a baseline the day the pool is filled, and write it down. TDS is a trend rather than a verdict, and without the starting number the trend has nothing to start from.
If it is too low
Nothing to correct. Low TDS is fresh water and is not a fault.
- • No action needed.
If it is too high
Chemicals become less effective and the water may look persistently dull or cloudy. Partial draining and refilling is the usual remedy.
- • Partially drain and refill with fresh water. There is no additive that reduces TDS.
- • In a saltwater pool, subtract your salt level before judging the number.
Common questions
What is a high TDS level in a pool?
Past the top of the range shown above, in a conventional pool, is usually where people start noticing that chemicals are less responsive. Saltwater pools run far higher by design, because the salt counts toward the total; judge them by the rise above their salt baseline instead.
Can I lower TDS without draining?
Not practically. Dissolved solids stay behind when water evaporates, so replacing some of the water is the only straightforward way to bring the number down.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.