Is a TDS meter the same as an EC meter?

Q: Is a TDS meter the same as an EC meter?

Yes, essentially. A TDS or ppm meter measures electrical conductivity (EC) and multiplies it by an assumed factor: 0.5 on the 500 scale, 0.7 on the 700 scale, sometimes 0.64. So 1.0 mS/cm reads 500 ppm on one meter and 700 on another. The ppm figure is an estimate, not your nutrients' real concentration. Read and talk in EC, or always state the scale.

Short answer

Essentially, yes. A TDS or "ppm" meter measures electrical conductivity and multiplies it by an assumed factor: 0.5 on the 500 scale, 0.7 on the 700 scale, sometimes 0.64 ^[1]. The same 1.0 mS/cm solution reads 500 ppm on one meter and 700 on another. Work in EC (mS/cm), or always state the scale. Established.

Why

Dissolved salts carry current; more ions, more conductivity. That is what the probe measures, corrected to 25 °C ^[1].

Converting conductivity to dissolved solids needs a factor, and no single factor is right. Different salts conduct differently per gram, and for natural waters the true factor rises from about 0.65 at 1,000 µS/cm to about 0.82 at 10,000 µS/cm ^[1]. Meter makers just pick one.

EC also says nothing about which ions are present ^[2]. Sodium chloride has EC too, so a reading tells you strength, not balance.

The numbers

	EC meter	TDS / ppm meter
What the probe measures	Conductivity	Conductivity
Displays	mS/cm or µS/cm	EC × 0.5, 0.64 or 0.7
1.0 mS/cm reads	1.0	500, 640 or 700 ppm
1.413 mS/cm standard reads	1.413	~707 (500) or ~990 (700)
Comparable across meters	Yes	Only on the same scale
Tells you which nutrients	No	No

Verdict: same sensor, different label; choose a meter that can show EC and use that.

What to do

Check your meter's scale in the manual or settings.
Switch the display to EC if it allows.
Calibrate against a 1.413 mS/cm standard.
Convert any ppm feed chart to EC using its stated scale.
Measure your water first; feed charts usually mean total EC.

Common mistakes

Following a 700-scale chart with a 500-scale meter and overfeeding by 40%.
Comparing ppm numbers between growers on different meters.
Reading TDS ppm as ppm of nitrogen.
Using a meter with no temperature compensation.

Full article: EC vs PPM: the 500 and 700 scales
Articles: meter calibration
Answers: full-strength feeding chart? · meters you need
Terms: TDS · EC · ppm500 · ppm700

Follow-up questions

Is my meter 500 or 700 scale?

To tell whether your meter is 500 or 700 scale, check the manual or settings menu, or test it in a 1.413 mS/cm calibration standard: a 500-scale meter reads about 707 ppm and a 700-scale meter about 990 ppm. If the meter can display EC directly, switch it to EC and stop using ppm altogether.

How do I convert ppm to EC?

To convert ppm to EC, divide by your meter's factor: ppm divided by 500 on the 500 scale, or by 700 on the 700 scale, gives EC in mS/cm. So 800 ppm500 is 1.6 mS/cm, while 800 ppm700 is about 1.14 mS/cm. To switch scales, multiply 500-scale ppm by 1.4 to get 700-scale ppm.

Is TDS ppm the same as ppm of nitrogen on a recipe?

No. A TDS meter's ppm is an estimate of all dissolved salts from conductivity, while a recipe's ppm of nitrogen or calcium is the actual mass of that element per litre, also written mg/L. They are different numbers. A feed with 180 mg/L nitrogen might read anywhere from about 800 to 1,300 on a TDS meter, depending on scale and recipe.

Sources

Prasad L, Meehan M, 2025. Using electrical conductivity and total dissolved solids meters to field test water quality. NDSU Extension WQ1923. ndsu.edu/agriculture/extension/publications/using-electrical

Sonneveld C, Voogt W, 2009. Plant Nutrition of Greenhouse Crops. Springer. doi.org/10.1007/978-90-481-2532-6