A conductivity meter does not measure dissolved solids directly, it estimates them from how well the water carries current. This guide shows the formula, the right k factor for your water and how temperature changes the answer.
Conductivity to TDS conversion turns a quick EC reading into an estimate of dissolved solids in mg/L. The result is only as good as the k factor and the temperature correction you apply.
What Is Conductivity to TDS Conversion?
Conductivity to TDS conversion is the method of estimating total dissolved solids in mg/L by multiplying the electrical conductivity at 25 °C by an empirical factor k. For most natural and process waters k lies between 0.5 and 0.8, so 1000 µS/cm usually means roughly 500 to 800 mg/L of dissolved solids.
- Formula: TDS (mg/L) = k × EC25 (µS/cm)
- Typical k: 0.5 for NaCl type water, 0.65 for natural water, 0.7 for RO permeate meters
- Correct EC to 25 °C first, about 2 % per °C for drinking water
- 1 mS/cm = 1000 µS/cm, and mg/L is practically the same as ppm in dilute water
Conductivity is the property, while TDS is a mass of material left after evaporation, so the two are linked only through the type of ions present. If you want the physics behind the reading, start with our article on electrical conductivity and then see how a conductivity sensor works in a pipe or tank.
The picture shows typical process sensors used for conductivity to TDS work. Every one of them measures conductance between electrodes, and the transmitter or meter then converts that value into TDS using a factor you can usually edit.
How to Convert Conductivity to TDS
The working rule is simple: correct the conductivity reading to 25 °C, choose a k factor that matches the water, and multiply. Hanna Instruments gives the same relation as TDS = factor × EC25 and quotes 100 µS/cm × 0.5 = 50 ppm as its basic example.
EC25 in µS/cm
k = 0.5 to 0.8
Example: 100 µS/cm × 0.5 = 50 mg/L
When you send conductivity to TDS values to a lab or client, always write the k factor beside the number. A result of 650 mg/L with k = 0.65 and 500 mg/L with k = 0.5 come from the same water.
In a plant the reading often travels as a 4 to 20 mA signal to the DCS, where the conductivity to TDS step is just a multiplier block. Our 4 to 20 mA signal conversion calculator helps check that the scaled EC value is right before the factor is applied.
What Is the TDS Factor?
The TDS factor k is the ratio of dissolved solids in mg/L to conductivity in µS/cm, and it changes with the ions in the water. The METTLER TOLEDO conductivity guide lists NaCl at 0.49, Na2SO4 at 0.67, CaSO4 at 0.74 and NaHCO3 at 0.91, because heavier and less mobile ions carry less current per milligram.
| Water type | Typical k | Comment |
|---|---|---|
| RO permeate, DM water | 0.5 to 0.7 | Fixed scale meters often use 0.7 |
| Sodium chloride solutions | 0.49 to 0.56 | Rises with concentration |
| Natural river and lake water | 0.55 to 0.70 | 0.64 to 0.65 is a common middle value |
| Groundwater, hard water | 0.65 to 0.75 | Calcium and sulphate push k up |
| Bicarbonate rich water | up to 0.9 | Weakly conducting ions |
| Seawater, brine | about 0.6 to 0.7 | Use a salinity scale for accuracy |
The table combines figures from METTLER TOLEDO, Apera Instruments and Hill Laboratories, so treat each row as a starting point rather than a rule. Apera notes 0.49 to 0.56 for sodium chloride and 0.6 to 0.7 for streams and lakes, with 0.64 considered typical.
Hill Laboratories, a New Zealand testing lab, warns that the popular factor of 0.7 is only an average of extremes between 0.5 and 0.9. Using one factor for every water can give errors of up to 30 % in the estimated TDS.
For a site where accuracy matters, run a gravimetric TDS test to APHA 2540C on a few samples and divide the result by the measured EC25. That site specific k makes every later conductivity to TDS reading far more meaningful.
Temperature Compensation to 25 °C
Conductivity rises as water warms because ions move faster, so a reading taken at 32 °C looks higher than the same water at 25 °C. METTLER TOLEDO gives a coefficient of about 2.0 % per °C for drinking water, 2.2 to 3.0 % per °C for salts and 1.0 to 1.6 % per °C for acids.
Most meters apply linear compensation automatically, but you must still check the coefficient set in the menu. Our note on temperature and pressure compensation explains why every analytical reading is referred to a standard condition.
Solved example:
ECT = 1200 µS/cm at 32 °C, α = 2 %/°C
EC25 = 1200 ÷ 1.14 = 1053 µS/cm
TDS = 0.65 × 1053 = 684 mg/L
Notice that skipping compensation in this conductivity to TDS example would have given 0.65 × 1200 = 780 mg/L, which is about 14 % too high. That single step matters more than arguing over the second decimal of k.
TDS vs Conductivity: Which One to Measure?
Conductivity is fast, continuous and easy to put on a transmitter, while true TDS needs a filter, an oven and a balance. In practice plants measure conductivity online and use the conversion only for reporting or rough limits.
- You need a live 4 to 20 mA value for control
- Water is high purity, such as boiler feed or DM water
- You trend changes rather than absolute mass
- Limits are already written in µS/cm
- Contracts or permits state limits in mg/L
- Water has non ionic dissolved matter
- You need to find the site k factor
- Results go into a legal or audit report
Hill Laboratories points out that non ionic material such as sugars, proteins and some organics adds to TDS but not to conductivity. That is why a TOC analyzer or lab test is still needed for organic loads.
How to Convert µS/cm to ppm
People search for µS/cm to ppm because many handheld meters show both, but ppm here simply means mg/L of estimated dissolved solids. The number of ppm per µS/cm is the same k factor, so a meter set to 0.5 shows 500 ppm for 1000 µS/cm and a meter set to 0.7 shows 700 ppm.
| Conductivity | In µS/cm | TDS at k = 0.5 | TDS at k = 0.65 | TDS at k = 0.7 |
|---|---|---|---|---|
| 0.055 µS/cm (pure water) | 0.055 | 0.03 mg/L | 0.04 mg/L | 0.04 mg/L |
| 100 µS/cm | 100 | 50 mg/L | 65 mg/L | 70 mg/L |
| 500 µS/cm | 500 | 250 mg/L | 325 mg/L | 350 mg/L |
| 1 mS/cm | 1000 | 500 mg/L | 650 mg/L | 700 mg/L |
| 5 mS/cm | 5000 | 2500 mg/L | 3250 mg/L | 3500 mg/L |
| 10 mS/cm | 10000 | 5000 mg/L | 6500 mg/L | 7000 mg/L |
Ultrapure water at 25 °C has a conductivity of only 0.055 µS/cm, which equals 18.18 MΩ·cm resistivity, according to the METTLER TOLEDO guide. At that level a TDS number is meaningless, so power plants and pharma users report conductivity or resistivity instead.
Indian drinking water limits are written as TDS, and the India Water Portal quotes IS 10500 with 500 mg/L as the desirable limit and 2000 mg/L as the permissible limit when no better source exists. With k = 0.65, 500 mg/L corresponds to roughly 770 µS/cm at 25 °C.
Field Tips and Troubleshooting
Most bad conductivity to TDS numbers come from the sensor or the settings, not the formula. Check the cell constant first using our guide on conductivity cell constant calibration, then confirm the factor and coefficient in the menu.
| Symptom | Likely cause | What to check |
|---|---|---|
| TDS differs between two meters | Different k factor or compensation | Compare factor and α in both menus |
| Reading drifts upward slowly | Fouled or scaled electrodes | Clean cell, verify with 1413 µS/cm standard |
| Low reading in fast flow | Air bubbles on electrodes | Mount sensor with flow upward, add vent |
| Jumps when pump starts | Ground loop or electrical noise | Check shield and isolation of the signal |
| Lab TDS far above estimate | Non ionic solids present | Find site k by gravimetric test |
| Error in cold samples | Wrong α or no compensation | Set α to 2 %/°C for drinking water |
Calibrate with a standard close to your working range. METTLER TOLEDO recommends 84 µS/cm for accurate low range work and treats 10 µS/cm and lower standards as check solutions only.
A good sample point matters as much as a good sensor. Our article on the sample conditioning system covers cooling, pressure reduction and flow control for online water analyzers.
Where Plants Use Conductivity to TDS Readings
You will meet conductivity to TDS conversion wherever water quality is reported in mg/L but measured online. The common places are listed below, often next to pH and turbidity measurement.
- Sensor cell constant verified with a standard
- Temperature coefficient matches the water type
- k factor written on the datasheet or tag
- Sensor mounted where no air collects
- Signal scaling checked from transmitter to DCS
- Site k factor confirmed by lab test once a year
After a cleaning or calibration, use the calibration error calculator to record the as found error of your meter. Power plant users should also read about the silica analyzer and the sodium analyzer, which watch the impurities conductivity cannot isolate.
The NDSU Extension video shows field use of EC and TDS meters, including why the factor setting changes the displayed value. Watch it once and then repeat the solved example above with your own meter.
Conductivity to TDS FAQ
First correct the conductivity reading to 25 °C, either in the meter or by dividing by one plus alpha times the temperature difference. Then multiply EC25 in µS/cm by a k factor between 0.5 and 0.8 to get TDS in milligrams per litre.
For example, 1000 µS/cm with a factor of 0.65 gives about 650 mg/L of dissolved solids. Always record which factor you used so readings can be compared later.
There is no single factor, because it depends on which ions are dissolved in the water. METTLER TOLEDO quotes 0.55 to 0.70 for natural water, with 0.65 often used as a middle value.
Sodium chloride solutions sit near 0.5, while bicarbonate rich water can reach 0.9. For accurate work, find your own factor from a lab evaporation test on a few samples.
No, they are different quantities, since µS/cm is conductivity and ppm is a concentration of dissolved solids. A meter shows ppm only after it multiplies conductivity by its stored factor.
So 1000 µS/cm can appear as 500 ppm on one meter and 700 ppm on another. In dilute water one ppm is practically equal to one milligram per litre, which is why both units appear on reports.
The India Water Portal quotes IS 10500 with a desirable TDS limit of 500 mg/L for drinking water. When no better source is available, the permissible limit extends to 2000 milligrams per litre.
Using a factor of 0.65, the 500 mg/L limit is roughly 770 µS/cm at the 25 degree reference. Remember that low TDS alone does not prove water is safe, because microbes and some toxic metals add little conductivity.
Conductivity rises by about two percent for every degree of temperature rise in drinking water, so readings must share a reference point to be compared. Most standards and meters use 25 °C as that reference, although some older work uses 20 degrees.
The meter measures sample temperature and corrects the reading using a coefficient called alpha. If alpha is wrong for your water, the converted TDS will carry the same error.
Yes, every handheld TDS meter is really a conductivity meter with a built in factor and temperature compensation. Many models can switch the display between EC and ppm with one button.
Basic pens often fix the factor at 0.5 or 0.7, so check the manual before trusting the number. For process work, a transmitter that outputs conductivity directly is usually the better choice.
Related Articles
- Electrical Conductivity Explained
- Conductivity Sensor Working Principle
- Conductivity Sensor Cell Constant Calibration
- TOC Analyzer Working Principle
- Turbidity Measurement Guide
External References
- METTLER TOLEDO Conductivity Guide (PDF)
- Hanna Instruments: Relationship Between TDS and EC
- Total Dissolved Solids, Wikipedia
What We Learn Today
- Conductivity to TDS conversion multiplies conductivity at 25 °C by a k factor, usually between 0.5 and 0.8, to estimate dissolved solids in mg/L.
- The k factor depends on the ions present, so sodium chloride water sits near 0.5 while natural and hard waters run closer to 0.65 or higher.
- Temperature compensation of about two percent per degree is essential, and a site specific factor from a lab test gives the most reliable results.
