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The best TDS value for water is not one exact number. For drinking water, a TDS below about 500–600 mg/L is generally a practical range for taste and household use, but a low TDS reading does not prove that water is safe to drink.
The World Health Organization says water with less than about 600 mg/L TDS is generally considered palatable, while water above about 1,000 mg/L can become increasingly unpleasant to drink. WHO does not set a health-based guideline value for TDS.
In the United States, the EPA lists 500 mg/L as a secondary drinking-water standard for TDS. This standard mainly addresses taste, staining, deposits, and similar problems rather than defining whether water is microbiologically or chemically safe.
What Does TDS Mean?
TDS stands for total dissolved solids. It is the amount of dissolved material in water.
These dissolved materials can include:
- Calcium
- Magnesium
- Sodium
- Potassium
- Bicarbonates
- Chlorides
- Sulfates
- Other dissolved substances
TDS is usually shown as milligrams per liter (mg/L) or parts per million (ppm). For ordinary freshwater measurements, meters commonly display these units in roughly equivalent numerical values.
For example, a meter reading of 250 ppm is commonly treated as about 250 mg/L TDS.
A handheld TDS meter normally does not measure each dissolved substance directly. It measures how well the water conducts electricity and estimates TDS from that reading.
That distinction matters because the meter tells you roughly how much dissolved material is present, not what that material is.
What Is a Good TDS Level for Drinking Water?
There is no health-based "ideal" TDS number that makes water safe.
For practical household use, readings can be viewed roughly like this:
| TDS reading | Practical interpretation |
|---|---|
| Below 50 mg/L | Very low dissolved mineral content |
| 50–300 mg/L | Low to moderate dissolved mineral content |
| 300–500 mg/L | Moderate TDS; often acceptable for taste |
| 500–600 mg/L | Higher mineral content; taste or scaling may become more noticeable |
| 600–1,000 mg/L | Taste and mineral deposits may become increasingly noticeable |
| Above 1,000 mg/L | Often less pleasant to drink and worth investigating |
These ranges are useful for understanding a meter reading. They are not a drinking-water safety scale.
WHO's current drinking-water guidance, published in 2026, says water below about 600 mg/L TDS is generally considered good in terms of palatability. It also notes that high TDS can contribute to scaling in pipes, water heaters, boilers, and appliances. WHO has not proposed a health-based TDS guideline because TDS itself does not tell you which individual substances are present.
Is Lower TDS Always Better?
No.
A reading of 20 mg/L is not automatically better or safer than a reading of 250 mg/L.
Low-TDS water simply contains fewer dissolved substances that affect electrical conductivity. It can still contain microorganisms or contaminants that a normal TDS meter cannot identify.
For example, a TDS meter cannot reliably tell you whether water contains harmful levels of:
- Bacteria
- Viruses
- Protozoa
- Some pesticides
- Other organic contaminants
- Specific metals
- Nitrate
- Other substances that require dedicated testing
A water sample could therefore show a very low TDS reading while still being unsuitable for drinking.
The reverse is also true. Water can have a moderate TDS because it contains naturally occurring calcium and magnesium. The TDS number alone does not tell you whether those dissolved solids are harmful.
Why 500 mg/L Is Often Mentioned
The number 500 mg/L appears frequently because it is the EPA's secondary standard for TDS in drinking water.
A secondary standard is different from a primary health-based contaminant limit.
EPA associates higher TDS with issues such as:
- Mineral deposits
- Hardness
- Staining
- Changes in color
- Salty taste
So 500 mg/L should not be interpreted as a sharp line where water is safe at 499 mg/L and unsafe at 501 mg/L.
It is better viewed as a useful point for investigating water quality, taste, scaling, and the substances that are creating the high reading.
What Is a Good TDS Reading for Rainwater?
Rainwater often has relatively low dissolved mineral content because it has not spent much time passing through soil and rock.
However, low-TDS rainwater should never be assumed to be safe to drink.
Roof-collected rainwater can pick up contaminants from:
- Roofing materials
- Gutters and downspouts
- Dust
- Bird and animal droppings
- Leaves and organic debris
- Airborne pollution
- Storage tanks
- Pumps and plumbing
Many of these hazards cannot be evaluated with a TDS meter.
For garden irrigation, toilet flushing, cleaning, or other non-potable uses, TDS can still help identify changes in the stored water. Non-potable means water that is not intended for drinking.
For drinking-water use, roof runoff needs to be treated as a whole system. Collection surfaces, debris control, a first-flush arrangement where appropriate, storage hygiene, treatment, maintenance, and current laboratory testing all matter.
A first flush diverts some of the initial roof runoff so that part of the dirt and debris washed from the roof does not immediately enter the tank.
Do not use a low TDS reading as evidence that collected rainwater is potable.
What If Your TDS Is Very Low?
Very low TDS is common with reverse-osmosis water, distilled water, and some rainwater sources.
A low reading does not automatically create a problem. It mainly indicates that there are relatively few dissolved ions in the water.
Very low-TDS water may taste flat to some people. Water chemistry can also affect plumbing and treatment equipment, so TDS should be considered with other measurements such as pH, alkalinity, hardness, and the materials in the system when those factors matter.
Review interpreting TDS levels in drinking water to assess which conclusions the measured water-quality values support.
Do not add minerals to water simply because a TDS meter displays a low number unless you have a specific treatment reason and understand the water chemistry involved.
What If Your TDS Is Above 500 mg/L?
A reading above 500 mg/L is a reason to find out what is contributing to the dissolved solids rather than assuming the water is unsafe.
Start by considering the source.
High TDS in well or groundwater may come from naturally occurring minerals. In other situations, salts or other dissolved substances may contribute.
You may also notice practical symptoms such as:
- White scale around fixtures
- Deposits in kettles or water heaters
- Salty or mineral taste
- Staining
- Changes in water treatment performance
If the water is intended for drinking, laboratory testing is more useful than simply trying to lower the TDS number.
The correct treatment depends on what is actually in the water.
For example, a treatment system chosen for hardness may not solve excessive sodium. A sediment filter will not significantly reduce ordinary dissolved salts because those materials are dissolved rather than suspended as particles.
TDS Is Not the Same as Hardness
TDS and hardness are related, but they are not the same measurement.
Water hardness mainly refers to dissolved calcium and magnesium.
TDS includes calcium and magnesium but can also include sodium, chloride, sulfate, bicarbonate, and many other dissolved substances.
This means two water samples can have the same TDS reading while having very different hardness levels.
A TDS meter should not be used as a substitute for a hardness test when choosing a water softener or investigating scale.
Can a TDS Meter Tell Whether a Filter Works?
Only in limited situations.
Some treatment methods significantly change dissolved ion levels. Reverse osmosis is one example. Comparing the feed-water and treated-water TDS can therefore help you notice changes in how such a system is operating.
But many useful filters are not intended to lower TDS.
For example, depending on their design, sediment filters and some other treatment stages may remove unwanted material without causing a large change in the TDS reading.
So a filter should not be judged as good or bad simply by whether the TDS number falls.
You also cannot use a TDS meter to confirm that a purifier, UV system, or drinking-water treatment setup has made contaminated water safe.
Use TDS as One Clue, Not a Pass-or-Fail Test
TDS is most useful for:
- Comparing water sources
- Tracking changes over time
- Watching some treatment systems
- Investigating taste
- Investigating scale and mineral deposits
- Spotting an unexpected change that deserves further testing
It is much less useful for determining drinking-water safety.
If your normal rainwater tank reads 40 mg/L and suddenly reads 250 mg/L, for example, the change may tell you that something in the water or system has changed. It does not tell you exactly what changed.
For drinking water, use testing that identifies the hazards relevant to your source and local conditions. Private wells and rainwater systems can require different tests than treated municipal water.
Frequently Asked Questions
Is 100 TDS good for drinking water?
A TDS of 100 mg/L is relatively low and may provide acceptable-tasting water. However, the number does not prove the water is safe to drink. Microbial and specific chemical contaminants require appropriate testing.
Is 300 TDS safe to drink?
A reading of 300 mg/L is within a range that many people find acceptable for taste. WHO considers water below about 600 mg/L generally palatable. Safety still depends on which contaminants are present, not the TDS number alone.
Is 500 TDS too high?
Not necessarily. The EPA lists 500 mg/L as a secondary standard related mainly to aesthetic and household effects such as taste and deposits. A reading around or above this level is a good reason to investigate the mineral and chemical makeup of the water.
Is zero TDS the best drinking water?
No. Zero or near-zero TDS means very little dissolved mineral material is present. It does not mean the water is automatically safer, and a TDS meter cannot rule out every type of contamination.
Does boiling water reduce TDS?
Generally, no. Boiling removes water as vapor while many dissolved minerals remain behind. This can actually increase their concentration in the water that remains. Boiling can be useful for microbial treatment in appropriate circumstances, but it is not a general TDS-removal method.
Can a TDS meter detect bacteria?
No. A standard TDS meter estimates dissolved ion content from electrical conductivity. It cannot determine whether water is free from harmful bacteria or other disease-causing organisms.
What TDS should rainwater have?
There is no required TDS value that makes collected rainwater safe or suitable. Rainwater often has low TDS, but roof runoff can contain microorganisms, debris, metals, and other contaminants that a TDS meter cannot identify. Intended drinking use requires suitable collection and treatment plus appropriate laboratory testing.
