Conductivity Unit Converter: µS/cm, mS/cm, S/m, MΩ·cm and ppm TDS
Enter a value and see all conductivity, resistivity and TDS equivalents. A grade limit table for pure water and a ready conversion table are on the page and readable with JavaScript turned off.
Enter One Value, See Every Unit
Select the input unit; µS/cm, mS/cm, S/m, MΩ·cm and ppm TDS equivalents are calculated instantly.
Equivalents
| µS/cm | – |
|---|---|
| mS/cm | – |
| S/m | – |
| MΩ·cm | – |
| ppm TDS (0.5) | – |
Conversions: 1 mS/cm = 1000 µS/cm · 1 S/m = 10,000 µS/cm · MΩ·cm = 1 ÷ µS/cm · TDS ≈ factor × µS/cm.
Ready Conversion Table
Example values in µS/cm with their equivalents in the other units; three TDS factors side by side.
| µS/cm | mS/cm | S/m | MΩ·cm | TDS ×0.5 | TDS ×0.64 | TDS ×0.7 |
|---|---|---|---|---|---|---|
| 0.055 | 0.000055 | 0.0000055 | 18.182 | 0.0275 | 0.0352 | 0.0385 |
| 0.1 | 0.0001 | 0.00001 | 10 | 0.05 | 0.064 | 0.07 |
| 1 | 0.001 | 0.0001 | 1 | 0.5 | 0.64 | 0.7 |
| 1.3 | 0.0013 | 0.00013 | 0.76923 | 0.65 | 0.832 | 0.91 |
| 5 | 0.005 | 0.0005 | 0.2 | 2.5 | 3.2 | 3.5 |
| 10 | 0.01 | 0.001 | 0.1 | 5 | 6.4 | 7 |
| 50 | 0.05 | 0.005 | 0.02 | 25 | 32 | 35 |
| 100 | 0.1 | 0.01 | 0.01 | 50 | 64 | 70 |
| 500 | 0.5 | 0.05 | 0.002 | 250 | 320 | 350 |
| 1000 | 1 | 0.1 | 0.001 | 500 | 640 | 700 |
| 5000 | 5 | 0.5 | 0.0002 | 2500 | 3200 | 3500 |
| 50000 | 50 | 5 | 0.00002 | 25000 | 32000 | 35000 |
TDS columns are in ppm and are approximate values that depend on the factor.
Pure Water Grades: Conductivity and Resistivity Limits
Limits defined by the standards, at 25 °C. Columns marked calculated are unit conversions, not values stated by the standard itself.
| Water grade | Conductivity (µS/cm) | Resistivity (MΩ·cm) | Source |
|---|---|---|---|
| Theoretical pure water (H₂O ions only) | 0.055 | 18.18 | Calculated value, not a grade limit |
| ASTM D1193 Type I | ≤ 0.056 | ≥ 18.0 | ASTM D1193, 25 °C |
| ASTM D1193 Type II | ≤ 1.0 | ≥ 1.0 | ASTM D1193, 25 °C |
| ASTM D1193 Type III | ≤ 0.25 | ≥ 4.0 | ASTM D1193, 25 °C |
| ASTM D1193 Type IV | ≤ 5.0 | ≥ 0.2 | ASTM D1193, 25 °C |
| USP purified water (PW) and water for injection (WFI), in-line | ≤ 1.3 | ≥ 0.77 (calculated) | USP <645> Stage 1, 25 °C |
| ISO 3696 Grade 1 | ≤ 0.1 (0.01 mS/m) | ≥ 10 (calculated) | ISO 3696, 25 °C |
| ISO 3696 Grade 2 | ≤ 1.0 (0.1 mS/m) | ≥ 1.0 (calculated) | ISO 3696, 25 °C |
| ISO 3696 Grade 3 | ≤ 5.0 (0.5 mS/m) | ≥ 0.2 (calculated) | ISO 3696, 25 °C |
The USP purified water and WFI limit belongs to the USP <645> Stage 1 in-line measurement and is 1.3 µS/cm at 25 °C; Stage 1 is in practice evaluated against the table limit at the measured temperature. See the standard itself for Stage 2 and Stage 3 conditions. The ASTM D1193 and ISO 3696 values are for laboratory reagent water. For pharmaceutical water systems see purified water systems, WFI systems and pure water conductivity measurement.
To Measure the Value in the Line
Conductivity measurement
Hamilton, Walchem and Pyxis conductivity sensors and controllers, from low-conductivity water to boiler water.
Pure water and WFI
In-line conductivity in PW and WFI systems, the USP <645> method and verification.
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Conductivity Units: Frequently Asked Questions
Unit conversion, resistivity, TDS factor and the USP limit.
How do I convert between µS/cm, mS/cm and S/m?
1 mS/cm = 1000 µS/cm. 1 S/m = 10 mS/cm = 10,000 µS/cm; divide a µS/cm value by 10,000 for S/m and by 1000 for mS/cm. 1 mS/m = 10 µS/cm. The conversions come from the unit definitions and do not depend on temperature; conductivity itself changes with temperature, so values are normally stated at 25 °C.
How is resistivity (MΩ·cm) calculated from conductivity?
Resistivity is the inverse of conductivity: resistivity (MΩ·cm) = 1 ÷ conductivity (µS/cm). For example 0.055 µS/cm is about 18.18 MΩ·cm and 1.3 µS/cm is about 0.77 MΩ·cm. In pure water measurement resistivity and conductivity carry the same information; work in the unit your instrument displays.
How do I get ppm TDS from µS/cm and which factor should I use?
TDS (ppm) ≈ factor × conductivity (µS/cm). The factors 0.5, 0.64 and 0.7 are commonly used. The actual factor depends on the composition of the dissolved salts and on temperature, and can range from roughly 0.5 to 0.9, so a TDS conversion is an estimate, not a measurement. When reporting a result, state the factor used or give the conductivity value directly.
What is the USP conductivity limit for purified water and WFI?
Under USP <645> Stage 1, in-line conductivity of purified water and water for injection must be no more than 1.3 µS/cm at 25 °C. Stage 1 is evaluated against the table limit at the measured temperature, which requires a measurement with temperature compensation turned off. See the standard for Stage 2 and Stage 3 conditions. More detail: pure water conductivity measurement.
Why does pure water conductivity not fall below 0.055 µS/cm?
The self-ionization of water (H⁺ and OH⁻) gives a conductivity of about 0.055 µS/cm at 25 °C, equal to a resistivity of 18.18 MΩ·cm. This is the theoretical floor; dissolved gases such as CO₂ raise the conductivity of pure water quickly on contact with air, which is why low conductivity is measured in-line.
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