When testing water quality, two common terms appear on every report: pH and TDS (Total Dissolved Solids). Both are key indicators of water safety and taste, yet they measure very different things. While pH determines how acidic or alkaline your water is, TDS measures the amount of dissolved minerals, salts, and metals in it.
Understanding the difference between pH and TDS is essential for maintaining safe drinking water, whether you rely on a reverse osmosis (RO) system, well water, or municipal supply.
A Must Know
- pH measures acidity or alkalinity on a scale of 0–14, with 7 being neutral.
- TDS measures the concentration of dissolved solids in water, expressed in parts per million (ppm).
- Ideal drinking water has a pH of 6.5–8.5 and TDS between 80–150 ppm.
- Both parameters affect taste, appliance performance, and health.
- Regular testing helps ensure your RO system or water purifier works optimally.
What Is pH in Water?
The pH level (potential of hydrogen) indicates how acidic or alkaline the water is. A lower pH means higher acidity, while a higher pH means more alkalinity. Pure water at room temperature typically has a neutral pH of 7.
pH Range Meaning:
- 0–6.9: Acidic water (may corrode pipes and affect taste)
- 7.0: Neutral (pure water)
- 7.1–14: Alkaline water (may taste smooth but too high can cause scaling)
In natural water sources, pH depends on minerals, dissolved gases like carbon dioxide, and human activities such as industrial discharge or chlorination.
The Environmental Protection Agency (EPA) recommends drinking water with a pH between 6.5 and 8.5, as anything outside this range may affect corrosion, taste, and even how efficiently chlorine disinfects water.
For more, see how to check pH level of RO water.
What Is TDS in Water?
TDS (Total Dissolved Solids) measures the concentration of dissolved minerals, salts, and organic matter in water. These can include calcium, magnesium, sodium, potassium, bicarbonates, and trace metals.
Unlike pH, which measures a chemical balance, TDS measures quantity — the total amount of dissolved content. TDS levels are expressed in ppm (parts per million) or mg/L (milligrams per liter).
| TDS Range (ppm) | Water Quality | Taste Description |
|---|---|---|
| 0–50 | Too pure (flat taste, may lack minerals) | Slightly acidic, bland |
| 80–150 | Excellent for drinking | Balanced taste |
| 150–300 | Acceptable | Mild mineral flavor |
| 300–500 | Hard water | Noticeably salty or bitter |
| 500+ | Unsafe for long-term drinking | Poor taste, scaling issues |
For healthy daily drinking, RO systems are often set to maintain TDS around 100 ppm, which ensures a balance between purity and taste.
To understand more, visit what should be the TDS of RO water.
The Main Difference Between pH and TDS
Though both are water quality indicators, pH and TDS measure completely different aspects. pH reflects chemical balance, while TDS reflects mineral concentration.
| Parameter | Definition | Ideal Range | Impact on Water | Controlled By |
|---|---|---|---|---|
| pH | Measures acidity/alkalinity | 6.5–8.5 | Affects corrosion, taste, disinfection | Carbon filters, alkaline cartridges |
| TDS | Measures dissolved solids in ppm | 80–150 ppm | Affects taste, hardness, appliance lifespan | RO membranes, mineral filters |
A simple way to remember:
- pH = “How acidic or alkaline is my water?”
- TDS = “How much stuff is in my water?”
High TDS doesn’t always mean unsafe water — many natural mineral waters have high TDS but are perfectly healthy. However, extremely low TDS (<50 ppm) may make water taste flat or slightly acidic.
You can balance both by using a remineralization filter, as discussed in how to make RO water healthy.
Why Monitoring Both Matters
- Taste and Comfort
Water that’s too acidic or too pure often tastes unpleasant. Balancing pH and TDS improves mouthfeel and flavor. - Appliance Protection
Incorrect pH or high TDS can cause scale buildup in kettles, pipes, and RO membranes. - Health Balance
Both pH and mineral levels influence hydration and nutrient absorption. Very low-mineral water can slightly reduce electrolyte intake over time. - System Efficiency
pH affects the lifespan of filters and membranes, while TDS reflects the RO system’s overall performance.
Factors That Affect pH and TDS
- Source of water: Groundwater has naturally higher TDS due to dissolved minerals.
- Industrial contamination: Can alter both acidity and dissolved solids.
- Chlorination: Raises pH levels slightly.
- RO filtration: Reduces TDS and can slightly lower pH.
When you use a reverse osmosis system, both TDS and pH are modified. RO membranes remove almost all dissolved solids, which reduces TDS drastically but can also cause water to become mildly acidic.
If you want to restore balance, see how to make RO water less acidic.
Dr. Maya Andrews’s Opinion
According to Dr. Maya Andrews, a water quality specialist from the U.S. Environmental Hydration Institute:
“pH and TDS are two different but complementary metrics. You can’t judge water quality using only one. For example, a neutral pH doesn’t guarantee purity if TDS is high. Similarly, low TDS doesn’t mean balanced water if the pH is unstable.”
Her advice aligns with international standards: both values should be tested periodically, especially if your home uses an RO or UV filtration system.
How to Measure pH and TDS
You can easily measure both with handheld digital meters.
- pH Meter: Insert probe into a glass of water. Read the value (ideal 6.5–8.5).
- TDS Meter: Measures conductivity to estimate total dissolved solids (ideal 80–150 ppm).
For convenience, combo meters are available that display both pH and TDS readings simultaneously.
If you’re interested in testing accuracy, explore TDS water tester accurate portable digital meter for drinking water quality.
Relationship Between pH and TDS
While pH and TDS are distinct, they sometimes influence each other. High TDS often corresponds with more alkaline water, as dissolved minerals like calcium and magnesium raise pH. Conversely, extremely low TDS (as in RO water) can make pH slightly acidic due to lack of buffering minerals.
In other words:
- More minerals → Higher TDS → Slightly alkaline pH
- Fewer minerals → Lower TDS → Slightly acidic pH
Maintaining both within the recommended range ensures optimal hydration and appliance protection.
How RO Systems Manage pH and TDS
RO systems use multiple filters to balance these two factors:
- Sediment Filter: Removes large particles.
- Carbon Filter: Removes chlorine and chemicals that affect pH.
- RO Membrane: Reduces TDS and purifies water.
- Remineralization Filter: Adds essential minerals to restore TDS and neutralize pH.
For more on filter functions, read reverse osmosis membrane filter in RO system.
Effects of Incorrect pH or TDS
- Low pH (<6.5): Corrosion in metal pipes, sour taste, potential for leaching lead or copper.
- High pH (>8.5): Scaling in appliances, bitter taste.
- Low TDS (<50 ppm): Flat taste, reduced mineral intake.
- High TDS (>500 ppm): Salty taste, potential health issues with certain contaminants.
FAQs
1. What is the ideal pH and TDS for drinking water?
The ideal pH is between 6.5 and 8.5, and TDS should range from 80 to 150 ppm for balanced taste and hydration.
2. Does higher TDS mean bad water?
Not always. Some natural mineral waters have high TDS but are safe. It depends on what minerals make up the TDS.
3. Why does RO water have low pH?
Because it lacks minerals that buffer acidity. Remineralization filters can fix this easily.
4. Can pH and TDS meters be used together?
Yes. Many digital testers can measure both simultaneously to ensure complete water analysis.
5. What affects pH more — temperature or minerals?
Mineral composition affects pH more than temperature. Heating water slightly lowers pH but doesn’t change safety.
Final Words
Both pH and TDS are fundamental indicators of water quality, each revealing a different story. While pH defines the chemical balance of your water, TDS quantifies the total minerals and dissolved content. Maintaining both within optimal limits ensures safe, pleasant-tasting, and long-lasting drinking water.
For households using RO systems, regularly monitoring both pH and TDS is key to balanced hydration and system efficiency. If your water feels flat or metallic, check these two parameters — they’re the first clues to what’s really happening in your water.
Hasan Al Sarker is a Reverse Osmosis Specialist. He has worked for many years to ensure safe drinking water for all. His research paper has been published in several journals, including Issue, Medium, and Slideshare. He is recognized as a water doctor among specialists though he did not attend medical college.
Besides working as a researcher of reverse osmosis technology, he is also very fancy with the kitchen and cooking. His guides are reading thousands of people every day. As a head of content, he is responsible for all the published articles at RO System Reviews.