TDS is commonly expressed in parts per million (ppm) or milligrams per liter (mg/L). For dilute water solutions, 1 ppm is approximately equivalent to 1 mg/L.
TDS is one of the most commonly discussed measurements when choosing a water purifier, particularly an RO purifier. However, TDS alone does not tell you whether water is safe to drink. It does not identify which substances are responsible for the reading.
A water sample can have a relatively low TDS and still contain a specific contaminant, while another sample with higher TDS may contain mostly naturally occurring minerals.
What Does TDS Mean in Water?
Total Dissolved Solids means the total amount of dissolved material in water.
These dissolved substances can include:
- Calcium
- Magnesium
- Sodium
- Potassium
- Chlorides
- Sulfates
- Bicarbonates
- Carbonates
- Nitrates
- Other dissolved substances
The exact composition depends on the source of the water.
Groundwater, municipal water, well water, tanker water, bottled water and surface water can all have different TDS levels.
What Is a Normal TDS Level in Drinking Water?
There is no single TDS number that can be used as a universal definition of safe drinking water.
TDS is primarily a measurement of the total concentration of dissolved substances. It does not tell you whether those substances are beneficial minerals or potentially harmful contaminants.
For this reason, a TDS meter should not be treated as a complete drinking-water safety test.
For household decision-making, TDS can nevertheless be a useful screening measurement, especially when comparing untreated and RO-treated water.
What TDS Levels Mean for Water Purifiers
| TDS Level | General Interpretation | Purifier Consideration |
|---|---|---|
| Very low | Low concentration of dissolved solids | RO may not be necessary solely because of TDS |
| Low to moderate | Relatively low dissolved-solids concentration | UV, UF or other filtration may be suitable depending on water quality |
| Moderate to high | Higher concentration of dissolved substances | Consider whether RO is appropriate based on source and contaminants |
| Very high | High dissolved-solids concentration | RO is often considered, subject to water testing and purifier specifications |
These categories are intended as practical guidance rather than universal safety limits.
The correct purifier depends on what is actually dissolved in the water, not simply the TDS number.
What Is the Unit of TDS?
TDS is commonly expressed as:
- ppm — parts per million
- mg/L — milligrams per liter
For water with relatively low concentrations of dissolved substances, ppm and mg/L are approximately equivalent.
For example:
300 ppm ≈ 300 mg/L
How Is TDS Measured?
TDS can be measured using a handheld TDS meter.
Most inexpensive TDS meters do not directly measure every dissolved substance individually. Instead, they generally measure the water’s electrical conductivity and use a conversion factor to estimate TDS.
Therefore, a TDS-meter reading should be understood as an estimate rather than a complete chemical analysis of the water.
How Does a TDS Meter Work?
Dissolved ions allow water to conduct electricity.
A TDS meter measures electrical conductivity and converts that measurement into an estimated TDS value.
The conversion depends on the relationship between conductivity and the dissolved substances in the water.
Because different dissolved substances have different electrical properties, two waters with identical TDS values can have somewhat different conductivity characteristics.
Can a TDS Meter Tell If Water Is Safe?
No.
A TDS meter cannot identify individual contaminants such as:
- Arsenic
- Lead
- Fluoride
- Nitrate
- Specific bacteria
- Specific viruses
- Pesticides
- Many other chemical contaminants
A laboratory water test is much more informative when you need to determine whether water is safe or identify a particular contaminant.
Is High TDS Water Safe to Drink?
Not necessarily—and not necessarily unsafe either.
High TDS means that water contains a relatively high concentration of dissolved substances. It does not identify what those substances are.
High TDS can result from naturally occurring minerals, salts or other dissolved substances.
When TDS is unusually high, it is sensible to investigate the source and, where appropriate, test for specific contaminants.
Is Low TDS Water Always Safe?
No.
Low TDS does not guarantee microbiological or chemical safety.
For example, water can contain a particular contaminant at a concentration that does not contribute substantially to the overall TDS reading.
Therefore, low TDS should not be interpreted as proof that water is safe.
Does TDS Mean Water Quality?
TDS is one indicator of water characteristics, not a complete measure of water quality.
It can be useful for:
- Comparing source water and RO output
- Monitoring changes in dissolved-solids concentration
- Identifying unusually high dissolved-solids levels
- Understanding why RO may be considered for some water sources
But TDS cannot tell you the exact chemical or microbiological composition of water.
What Causes High TDS in Water?
High TDS can have many causes.
Groundwater
Water moving through soil and rock can dissolve minerals and salts. This can produce relatively high TDS, particularly in some groundwater sources.
Borewell Water
Borewell water can have high TDS because it may remain underground for long periods and interact with mineral-rich geological formations.
Hard Water
Calcium and magnesium contribute to TDS. Therefore, hard water can have elevated TDS, although TDS and hardness are not the same measurement.
Salty or Brackish Water
Water containing high concentrations of dissolved salts can have very high TDS.
Tanker Water
Tanker water can have widely varying TDS depending on where it was sourced.
Is TDS the Same as Hardness?
No.
TDS and hardness measure different things.
TDS represents the total concentration of dissolved substances.
Hardness is primarily associated with dissolved calcium and magnesium.
Hardness contributes to TDS, but TDS also includes many other dissolved substances.
Therefore, two water samples with the same TDS can have different hardness levels.
TDS vs Hardness
| Characteristic | TDS | Hardness |
|---|---|---|
| What it measures | Total dissolved substances | Primarily calcium and magnesium |
| Unit commonly used | ppm or mg/L | Often mg/L as CaCO3 |
| Indicates scale-forming potential | Not directly | Yes, more directly |
| Can be measured with basic TDS meter | Estimated | No |
Does RO Reduce TDS?
Yes.
RO, or reverse osmosis, uses a semi-permeable membrane to reduce many dissolved substances in water.
As a result, RO-treated water generally has substantially lower TDS than the incoming water.
The actual TDS reduction depends on:
- Incoming-water TDS
- RO membrane condition
- Water pressure
- Water temperature
- Water chemistry
- Membrane rejection performance
- Purifier design
How Much Does RO Reduce TDS?
There is no single percentage that applies to every RO system.
A properly functioning RO membrane can provide substantial reduction of dissolved solids, but the actual rejection rate varies according to membrane specifications and operating conditions.
If you want to evaluate an RO purifier, compare the actual input and output TDS and the manufacturer’s membrane specifications rather than relying on a generic percentage.
Does UV Reduce TDS?
No.
UV treatment is primarily used to inactivate microorganisms.
It does not remove dissolved salts and therefore does not significantly reduce TDS.
Does UF Reduce TDS?
No, generally.
UF, or ultrafiltration, uses a membrane to physically retain particles and larger microorganisms.
It does not normally remove dissolved salts, so it does not significantly reduce TDS.
Does a Gravity Water Purifier Reduce TDS?
Most conventional gravity purifiers do not significantly reduce TDS.
The exact performance depends on the filter technology used.
Gravity systems may remove sediment, particles and some other impurities, but they generally do not provide the dissolved-solids reduction associated with RO.
Does Boiling Reduce TDS?
No.
Boiling kills or inactivates many microorganisms when done appropriately, but it does not remove dissolved salts and minerals.
In fact, if water is boiled and some water evaporates, the concentration of dissolved substances in the remaining water can increase.
Boiling therefore should not be considered a TDS-reduction method.
Does a Water Filter Reduce TDS?
It depends on the filter technology.
Common sediment and activated-carbon filters generally do not significantly reduce TDS.
RO is specifically capable of substantial dissolved-solids reduction.
Specialized technologies can also target particular dissolved substances, but their performance should be evaluated according to the specific contaminant and filter specification.
Can Activated Carbon Reduce TDS?
Generally, no.
Activated carbon is highly useful for reducing chlorine, odors, tastes and certain organic compounds, but it is not a general-purpose TDS-removal technology.
A carbon filter can therefore improve taste without producing a major change in the TDS reading.
Can TDS Be Reduced Without RO?
Yes, depending on what is causing the TDS.
Specialized treatment technologies can target particular dissolved substances.
However, ordinary sediment filters, carbon filters, UV systems and conventional UF membranes do not normally provide the broad dissolved-solids reduction associated with RO.
The appropriate alternative depends on the contaminant that needs to be removed.
How Can You Reduce TDS in Water?
Possible methods include:
- Reverse osmosis: broad reduction of many dissolved substances.
- Distillation: can remove many dissolved substances but is generally impractical for normal household drinking-water production.
- Specialized ion-exchange treatment: useful for specific dissolved ions and water-quality problems.
- Blending: can adjust final TDS but does not remove contaminants by itself.
The correct method depends on the composition of the water.
Does a TDS Controller Reduce TDS?
No.
A TDS controller in an RO purifier is generally used to regulate the final output TDS rather than remove dissolved solids.
In blending-type systems, the controller can allow some water with retained minerals to mix with RO-treated water, increasing the final TDS relative to pure RO permeate.
This is why a TDS controller should not be confused with the RO membrane.
What Is the Difference Between TDS Controller and TDS Meter?
| Feature | TDS Meter | TDS Controller |
|---|---|---|
| Purpose | Measures/estimates TDS | Regulates output TDS in some RO systems |
| Removes contaminants | No | No |
| Measures water | Yes | Not necessarily |
| Changes output composition | No | Yes, depending on design |
What Is the Difference Between TDS Controller and Mineralizer?
A TDS controller regulates the final TDS, often through controlled blending.
A mineralizer is designed to add or restore selected minerals after RO treatment.
They can be separate components in the same purifier.
A mineralizer is therefore not simply another name for a TDS controller.
What TDS Is Good for Drinking Water?
There is no single TDS number that should be used by itself to determine whether drinking water is safe.
For purifier selection, a TDS reading can provide useful information about dissolved-solids concentration, but it should be combined with knowledge of the source and, when appropriate, laboratory testing.
In particular, water from borewells and other variable sources may warrant testing for specific contaminants rather than relying exclusively on a TDS meter.
Is 50 TDS Water Safe?
A reading of 50 ppm indicates relatively low dissolved-solids concentration, but it does not prove that the water is microbiologically or chemically safe.
Safety depends on the actual contaminants present.
Is 100 TDS Water Safe?
100 ppm indicates relatively low TDS, but the number alone cannot establish safety.
Specific contaminants and microbial quality need to be considered separately.
Is 200 TDS Water Safe?
A TDS reading of 200 ppm does not by itself determine whether water is safe or unsafe.
It indicates the approximate concentration of dissolved substances, not their identity.
Is 300 TDS Water Safe?
300 ppm is a measure of dissolved-solids concentration, not a complete safety assessment.
Whether the water is suitable for drinking depends on its overall chemical and microbiological quality.
Is 500 TDS Water Safe?
500 ppm indicates a considerably higher dissolved-solids concentration than 100 ppm, but TDS alone still cannot identify whether the water contains harmful contaminants.
If the TDS is unusually high for your local source, water testing can help determine what is causing it.
Is 1000 TDS Water Safe?
1000 ppm represents a high concentration of dissolved substances.
At this level, investigating the source and composition of the dissolved solids is particularly important before selecting a drinking-water treatment system.
RO is commonly considered for high-TDS water, but the decision should still take the actual water chemistry into account.
Why Does RO Water Have Low TDS?
The RO membrane rejects a large proportion of dissolved substances.
Consequently, water passing through the membrane contains substantially fewer dissolved solids than the incoming feed water.
If a TDS controller or mineralizer is present, the final output may have a higher TDS than the water directly leaving the RO membrane.
Why Is My RO Water TDS Increasing?
An increase in RO output TDS can have several possible causes:
- RO membrane deterioration
- Incorrect TDS-control or blending settings
- Changes in source-water TDS
- Insufficient water pressure
- Incorrect purifier operation
- Membrane fouling or scaling
- Changes in water temperature
If the increase is significant or unexpected, the purifier should be checked according to the manufacturer’s service procedure.
Why Is My RO Water TDS Too Low?
Very low output TDS can simply indicate strong RO rejection.
However, the desired output depends on the purifier design and water source. There is no universal requirement that every RO purifier should produce the same TDS level.
If your purifier includes a TDS controller or mineralizer, the output can also depend on how that component is configured.
Does Higher TDS Mean More Minerals?
Not necessarily.
TDS represents the combined concentration of dissolved substances.
Some dissolved substances are minerals, but TDS does not tell you how much of the reading comes from calcium, magnesium, sodium, chloride or potentially undesirable contaminants.
Therefore, saying that “higher TDS means more healthy minerals” is an oversimplification.
Does Low TDS Mean Water Has No Minerals?
No.
Low TDS means that the overall concentration of dissolved substances is low. It does not necessarily mean that every mineral has been removed.
The exact mineral composition can only be determined through appropriate water analysis.
Does RO Remove Beneficial Minerals?
RO can reduce many dissolved minerals along with other dissolved substances.
Some RO purifiers therefore use mineralizers or controlled blending to alter the final mineral content.
Whether mineral removal is desirable depends on the overall source-water composition and the reason RO treatment is being used.
Can You Increase TDS After RO?
Yes.
Some RO purifiers can increase final TDS through controlled blending or mineralization.
However, increasing TDS should not be viewed as automatically improving water quality.
If blending is used, the water being blended back into the RO stream must itself be appropriately treated.
Should You Choose RO Based Only on TDS?
No.
TDS can be an important indicator, but it should not be the only criterion.
Consider:
- Water source
- TDS
- Hardness
- Microbial quality
- Fluoride
- Arsenic
- Nitrate
- Iron
- Lead and other relevant contaminants
- Water taste and odor
- Storage and distribution conditions
This is particularly important for borewell, tanker and other variable water sources.
When Should You Consider an RO Purifier?
RO is generally worth considering when:
- The water has elevated dissolved solids.
- The water is saline or brackish.
- Water testing identifies dissolved contaminants that RO can effectively reduce.
- The source water has substantial dissolved hardness.
- The water source varies significantly and requires broad dissolved-contaminant treatment.
However, the exact purifier should be selected based on the source water and manufacturer’s specifications.
When Might You Not Need RO?
You may not need RO simply because you have a water purifier requirement.
If your source water is already treated, has relatively low dissolved solids and does not contain a dissolved-contaminant problem requiring RO, a UV, UF, UV+UF, activated-carbon or gravity-based system may be more appropriate depending on the actual water quality.
This can also reduce electricity consumption, maintenance requirements and RO reject-water production.
Does TDS Matter for Water Purifier Selection?
Yes, but it is only one part of the decision.
TDS is particularly useful for determining whether dissolved-solids reduction may be relevant.
But the purifier should ultimately be selected based on the specific contaminants and characteristics of the source water.
Final Verdict
TDS, or Total Dissolved Solids, is a measurement of the combined concentration of dissolved substances in water.
It is commonly measured in ppm or mg/L and is useful for understanding the dissolved-solids characteristics of water.
RO can substantially reduce TDS, while UV, conventional UF and most sediment or carbon filters generally do not significantly reduce it.
However, TDS is not a complete measure of drinking-water safety. A TDS meter cannot tell you exactly which substances are dissolved in the water and cannot replace appropriate laboratory testing when specific contamination is a concern.
The best way to choose a water purifier is therefore to combine TDS measurement, source-water information and contaminant testing where appropriate.
Do not choose an RO purifier simply because the TDS number is high, and do not assume low TDS automatically means safe water.
Frequently Asked Questions
What is TDS in water?
TDS stands for Total Dissolved Solids. It represents the combined concentration of dissolved substances in water.
What is TDS measured in?
TDS is commonly expressed in ppm or mg/L.
Is TDS the same as hardness?
No. Hardness mainly refers to calcium and magnesium, while TDS represents the total concentration of dissolved substances.
Does RO reduce TDS?
Yes. RO can substantially reduce many dissolved substances and therefore usually lowers TDS.
Does UV reduce TDS?
No. UV is primarily a microbial-disinfection technology and does not significantly reduce dissolved salts.
Does UF reduce TDS?
Generally no. UF does not normally remove dissolved salts and therefore does not significantly reduce TDS.
Does boiling reduce TDS?
No. Boiling does not remove dissolved salts and minerals. Evaporation can actually increase their concentration in the remaining water.
Is high TDS water unsafe?
Not necessarily. TDS does not identify the substances responsible for the reading. High TDS should be investigated rather than interpreted as a complete safety assessment.
Is low TDS water safe?
Not automatically. Low TDS does not rule out microbial contamination or specific chemical contaminants.
Is 500 TDS safe for drinking?
TDS alone cannot establish whether water is safe. The actual dissolved substances and microbiological quality need to be considered.
What TDS is best for drinking water?
There is no single TDS value that should be used as a universal safety target. Water quality should be assessed more broadly.
Does a TDS controller remove TDS?
No. A TDS controller generally regulates the final output TDS rather than removing dissolved solids.
Does a TDS meter test water quality?
It provides an estimate of dissolved-solids concentration, but it cannot identify individual contaminants and is not a complete water-quality test.
How can I reduce TDS in water?
RO is the most common household technology for broad reduction of dissolved solids. Other specialized technologies can target particular dissolved substances.
Should I buy RO if my water has high TDS?
RO is often considered for high-TDS water, but the source and actual contaminants should also be considered before making the final decision.
About the Author
ManBearPig is a part-man, part-bear, part-pig creature and a full-time expert on global warming—mostly because he has personally experienced the consequences of being extremely, extremely, super cereal.
When he isn’t roaming the wilderness, terrorizing unsuspecting humans, or demanding that people take climate change seriously, ManBearPig studies rising temperatures, melting ice, extreme weather, carbon emissions, and other inconvenient signs that the planet may not be particularly happy with us.
His research philosophy is simple:
“I’m super cereal.”
ManBearPig believes climate change is too important to discuss without facts—and too ridiculous to discuss without a little humor. Through this website, he explores global warming, climate science, environmental changes, energy, and the strange ways humans respond when their own planet starts sending them warning signals.
He has no university degree, no laboratory, and absolutely no intention of becoming a respectable academic.
But he does have hooves, claws, a snout, and an unusually strong opinion about melting ice.