In short: multiply your EC reading in µS/cm by 0.64 for a fast PPM estimate. A 650 µS/cm reading comes to about 416 ppm, roughly 17% below the EPA's 500 mg/L Secondary Standard for TDS. Switch the scale buttons above if you know your meter uses NaCl (0.5) or KCl (0.7) calibration.
This is a quick estimate using a single average factor. For an adjustable factor and more detail, see the conductivity to TDS calculator linked below.
How the quick conversion works
PPM is estimated by multiplying your EC reading by a single correlation factor:
PPM = EC (µS/cm) × 0.64
0.64 is a commonly used general-purpose factor. PPM and mg/L are numerically the same for dilute water.
Worked example. 650 µS/cm × 0.64 = 416 ppm.
Quick lookup at the default factor
Round EC values converted at the default 0.64 factor, for a fast eyeball check.
| EC (µS/cm) | PPM |
|---|---|
| 100 | 64 |
| 250 | 160 |
| 500 | 320 |
| 1,000 | 640 |
| 2,000 | 1,280 |
Why this is a shortcut, not a measurement
A single correlation factor stands in for the true relationship between conductivity and dissolved solids, which actually depends on which ions are present. Chloride-heavy water and sulfate-heavy water with identical TDS can register noticeably different EC readings. 0.64 is a reasonable average across typical water, which is exactly why it's fast: you trade a bit of precision for a number you can get from a single reading and no lookup.
If you need to match a specific meter's scale, compare readings between devices, or want the result in grains per gallon as well, the conductivity to TDS calculator gives you those controls.
Worked scenarios
500 (NaCl) scale. A 1,200 µS/cm reading on the 500 scale: 1,200 × 0.5 = 600 ppm, about 20% above the 500 mg/L Secondary Standard.
700 (KCl) scale. A 300 µS/cm reading on the 700 scale: 300 × 0.7 = 210 ppm, well under the Secondary Standard.
Frequently asked questions
What is EC to PPM conversion?
It's a quick way to estimate total dissolved solids in parts per million from a conductivity (EC) meter reading, using a single average correlation factor rather than a full lab measurement.
Is PPM the same as mg/L?
Yes, for dilute aqueous solutions like drinking water, parts per million and milligrams per liter are numerically equivalent, since a liter of dilute water weighs almost exactly one kilogram.
Which factor should I use, 0.5, 0.64 or 0.7?
Match it to your meter's scale if you know it, printed in the manual or on the device. If you're not sure, 0.64 is a reasonable general-purpose default for typical water.
How accurate is this quick conversion?
It's an approximation. The true correlation factor varies roughly from 0.55 to 0.8 or higher depending on which ions are actually dissolved, so this is good enough for a fast estimate but not a substitute for a calibrated TDS or gravimetric measurement.
What EC reading indicates good drinking water?
There's no single answer, but as a rough guide, staying under about 780 µS/cm keeps you under the EPA's 500 mg/L Secondary Standard for TDS at the default 0.64 factor. That standard is about taste and aesthetics, not a health-based limit.
Where can I get a more precise TDS conversion?
See the conductivity to TDS calculator, which lets you enter your own correlation factor, switch between microsiemens and millisiemens, and also see the result in grains per gallon.
Sources & references
- APHA, AWWA, WEF. Standard Methods for the Examination of Water and Wastewater, Method 2510 (Conductivity).
- U.S. Environmental Protection Agency (EPA). Secondary Drinking Water Standards (Total Dissolved Solids).
Sarah Bennett, MS
Water Quality & Sustainability Specialist · Portland, Oregon
Environmental scientist (MS, Water Resources) with 9+ years in residential water quality, treatment, and usage efficiency, and 150+ completed assessments. Sarah writes and reviews the treatment, usage, and cost calculators on watercalculator.us. Read full bio
MS Environmental Science, Water Resources (2018) · BS Environmental Engineering (2016)