In short: a 650 µS/cm conductivity reading, converted with the default 0.64 correlation factor, comes to 416 mg/L total dissolved solids — about 17% below the EPA's 500 mg/L Secondary Drinking Water Standard for TDS. Enter your own EC reading and pick your meter's scale above for a figure specific to your water.
TDS from conductivity is an approximation, not a direct measurement. For exact TDS, evaporate and weigh a sample per Standard Methods 2540 C.
How the conversion is calculated
Total dissolved solids is estimated by multiplying conductivity by a correlation factor, k, that reflects the average conductance of the dissolved ions present:
TDS (mg/L) = k × EC (µS/cm)
k typically ranges from about 0.55 to 0.8 depending on the water's ion composition; 0.64 is a commonly used default for general-purpose water. Divide by 17.1 for grains per gallon.
Worked example. An EC reading of 650 µS/cm with the default k = 0.64: 650 × 0.64 = 416 mg/L, equal to 416 ppm or 24.3 grains per gallon.
Common correlation factors by meter scale
Check your meter's manual to confirm which scale it uses before comparing readings between devices.
| Scale | Reference salt | Typical k |
|---|---|---|
| 500 scale | NaCl (sodium chloride) | 0.5 |
| Default / mixed | General-purpose average | 0.64 |
| 700 scale | KCl (potassium chloride) | 0.7 |
Why the correlation factor isn't one fixed number
Conductivity measures how easily electricity flows through water, which depends on both the concentration and the type of dissolved ions. Some ions, like chloride, conduct more efficiently per unit of mass than others, like sulfate or bicarbonate, so two waters with identical TDS can register different EC readings, and vice versa. The correlation factor is really a stand-in for the water's specific ion mix.
That's also why the 500 and 700 scales exist: meter manufacturers pick a single reference salt to calibrate against, and NaCl and KCl happen to conduct differently enough that the same physical water reads as two different "TDS" numbers depending on which reference the meter assumes.
Worked scenarios
RO permeate, low TDS. A 45 µS/cm reading on the 500 (NaCl) scale, k = 0.5: 45 × 0.5 = 22.5 mg/L, about 1.3 grains per gallon — well within typical reverse osmosis output.
Hard well water, high TDS. A 1,450 µS/cm reading on the 700 (KCl) scale, k = 0.7: 1,450 × 0.7 = 1,015 mg/L, about 59.4 grains per gallon — roughly double the EPA's 500 mg/L Secondary Standard, worth considering treatment for taste and scale.
What else affects the result
Temperature compensation. Conductivity rises with temperature; most meters auto-compensate to 25°C. An uncompensated reading from cold or hot water will convert to an inflated or understated TDS.
Very high or very low conductivity. The linear k-factor approach works best for typical dilute freshwater. At very low EC (ultrapure water) or very high EC (brine, seawater), the true relationship curves away from a simple straight-line factor.
What TDS doesn't tell you. A single TDS number can't distinguish between, say, dissolved calcium and dissolved nitrate. For specific contaminant concerns, test for that substance directly rather than relying on TDS as a proxy.
Frequently asked questions
What's the difference between the 500 (NaCl) and 700 (KCl) TDS scales?
They're two different reference salts used to calibrate conductivity meters that display TDS directly. A meter on the 500 (NaCl) scale uses a correlation factor around 0.5, while a 700 (KCl) scale meter uses around 0.7, so the same water can show two different TDS numbers depending on which scale the meter uses.
Why isn't there one exact conversion factor from conductivity to TDS?
Because the true ratio depends on which ions are actually dissolved in the water, and different ions conduct electricity differently for the same mass. A factor of 0.64 is a commonly used average for typical mixed-ion water, but it's always an approximation rather than an exact conversion.
Is my water safe to drink based on this TDS number?
TDS alone doesn't determine safety. The EPA's 500 mg/L TDS guideline is a Secondary Standard for taste and aesthetics, not a health-based limit, and it doesn't identify which specific substances are dissolved. If you have health concerns, test for the specific contaminants you're worried about rather than relying on TDS alone.
How is this different from the EC to PPM calculator?
The EC to PPM calculator applies a single fixed conversion for a quick estimate. This tool lets you choose or enter the correlation factor for your specific meter's scale, which matters when comparing readings across different meters or checking which scale yours actually uses.
Does temperature affect my conductivity reading?
Yes, conductivity rises with temperature, roughly 2% per degree Celsius. Most modern meters automatically compensate readings to a standard 25°C; if yours doesn't, your reading won't be directly comparable to standard values or to other meters.
Can I use this for seawater or very concentrated brine?
Not reliably. The linear correlation factor used here is intended for dilute freshwater, roughly up to a few thousand microsiemens per centimeter. At seawater-level conductivity, the relationship between EC and TDS is no longer linear, and a simple correlation factor overstates or understates the true value.
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)