Plant Nutrition and Growth Enhancers

Nutrient Strength: EC, PPMs and TDS Explained

When we feed a nutrient solution to our plants, one of the most critical factors to get right is the nutrient strength. But there seems to be a whole lot of confusion lingering around how to measure the strength of your nutrient solution-especially among beginners.

The two major measurements in use today are:

EC  - Electrical Conductivity

TDS - Total Dissolved Solids


Let's start with the obvious: the more mineral-based nutrients you add to some water, the more concentrated the solution becomes. Pure water does not conduct electricity, but the more mineral ions we add, the more readily it will conduct. Therefore, the electrical conductivity (EC) of your nutrient solution is a fairly reliable measure of how much nutrient is dissolved in it overall.

To measure conductivity we can use an EC meter, also known as a conductivity meter. It has two electrodes that, when dipped in the solution, measure its electrical charge by passing a small charge between them.

It's important to measure the conductivity of your source water (before you have added any nutrients or other additive products) - this not only gives you the "baseline" measurement so you have an idea of the purity of your source water, but it also gives you an idea of what "room" there is left for additional nutrients.

If EC is Electrical Conductivity, what units is this measured in?

You may have heard growers say things like "When my plants are in full flower I feed them up to EC 2.2" - but 2.2 what?

The answer is Siemens, or more accurately, millisiemens. (One millisiemen is one thousandth of a Siemen.)

There's no need to get your head twisted over this. Siemens are to "electrical conductivity" what feet, yards, meters or inches are to "length" - it's simply the unit of electrical conductance.

The important thing to get straight is that EC refers to the scale (also known as the 'parameter') and siemens are the units on this scale. EC is the most widely accepted measurement for the strength of nutrient solutions, and is the standard in Europe and many other parts of the world. The one notable exception is North America which, for some reason, prefers to use the rather cumbersome and vague alternative: TDS.


TDS stands for Total Dissolved Solids-it quantifies the concentration of dissolved solids contained in a solution. Proponents of TDS argue that it's a more suitable parameter than EC for measuring nutrient concentration, since it measures by quantity or weight rather through the implication of electrical conductivity. 

The problem with TDS measurements are they are great in theory, but fairly absurd in practice. The only way of accurately measuring the TDS of a nutrient solution is to evaporate all the liquid and measure the residue-this would kind of defeat the point!

What is TDS measured in?

Remember, like "EC" - TDS is a scale, or a parameter, just like length, temperature and volume. The unit of TDS is ppm (parts per million.) A TDS reading of 60 ppm means there are 60 milligrams of dissolved solids in each liter of water, or 60 mg/l.

So do TDS Meters work in a different way to EC meters?

TDS meters work in actually the same was as EC meters! Both measure the electrical conductivity of the nutrient solution they are dipped in. The difference lies in how the information is displayed to you.
A TDS meter  measures the electrical conductivity in exactly the same was as an EC meter, but it simply uses an in-built conversion factor to display the strength of the nutrient solution in ppms.

It's these "conversion factors" that form the proverbial can of worms. They can vary significantly from meter to meter.

Conversion Factors


NaCl is a conversion factor based on Sodium Chloride (regular table salt.)
The conversion factor range is 0.47 to 0.5.
Non-linear meters based on NaCl typically use: 0.5 x the EC level (if converting from µS to ppm or mS to ppt) or 500 x the EC level, if converting from mS to ppm.
TDS 442™

442™ or Natural Water™ is a proprietary scale based on properties of naturally occurring fresh water.  The 442™ part is an abbreviation of 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride.
The conversion factor range is 0.65 to 0.85.
Non-linear meters based on 442™ typically use: 0.7 x the EC level (if converting from µS to ppm or mS to ppt) or 700 x the EC level, if converting from mS to ppm.


KCl is a conversion factor based on Potassium Chloride.
The conversion factor range is 0.5 to 0.57.
Non-linear meters based on KCl typically use: 0.55 x the EC level if converting from µS to ppm or mS to ppt) or 700 x the EC level, if converting from mS to ppm.

TDS 640

A less popular conversion factor.
The conversion factor range is 0.64 to 0.67.
Non-linear meters based on 640 typically use: 0.64 x the EC level if converting from µS to ppm or mS to ppt) or 640 x the EC level, if converting from mS to ppm.

Yes, four different possible conversion factors means that four different meters that give measurements in ppm may all give different readings from the same solution! However, all EC meters should give the same reading in the same solution as there's no conversion factor necessary.
I know, I know … TDS sounds like a confusing thing - but it's really just a measure of the total ions in solution. For every gallon of water you have X mg's of stuff in it. If one of your friends starts talking about their nutrient solution in terms of TDS, be sure to find out what scale they are using. Many growers, especially in Europe, in an effort to avoid confusion, use EC. If you are still confused, contact the manufacturer of your nutrients and find out what they recommend. Remember to ask them what TDS scale they use if they give you dosages in terms of ppm.
Likewise, if you are working with a TDS meter that only has a ppm display, remember you need to be sure of the conversion factor being used. TDS comes into its own when you need to measure individual elements in applications such as nutrient and water quality, tissue analysis results and soil analysis. Results from these laboratory tests will give individual elemental readings in ppm or mg/l. Remember, a TDS meter will only give you an approximation of the overall nutrient concentration, based on the conversation factor used.
Below is a table to show the relationship between the various methods of displaying the strength of a nutrient solution.

EC (mS)

EC (µS)


(EC µS x 0.5)


(EC µS x 0.55)

TDS 640

(EC µS x 0.64)

Natural Water™ 442

(EC µS x 0.7)































Jargon Buster

EC= Electrical Conductivity

TDS= Total Dissolved Solids

PPM= Parts Per Million
PPT = Parts Per Thousand

µS (or µS/cm)= micro-Siemens (one millionth of a siemen.)

mS (or mS/cm)= milli-Siemens (one thousandth of a siemen.)

NaCl= Sodium Chloride (EC-to-TDS conversion - EC x 0.5)

KCl= Potassium Chloride (EC-to-TDS conversion EC x 0.55)

442= 442 Natural Water™ (EC-to-TDS EC x 0.7)  (The "442" is an abbreviation for 40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride.)

Making Sense of your Meter

Here are some popular TDS meters along with their conversion factors, where applicable.






Displays EC and both NaCL (0.5) and 442™ (0.7)


Displays EC and both NaCL (0.5) and 442™ (0.7)



HI 98300


HI 98301 / 98302

NaCL (0.5)



HI 981404N / HI 981405N

442™ (0.7)

HI 983301N

442™ (0.7)

HI 983301N/5

NaCL (0.5)




NaCL (0.5)


None - just measures EC


Displays EC and TDS (user can select NaCl, 442 or KCl factors)



EcoTestr, TDSTestr11, PTTestr35

User Adjustable TDS Factor between 0.4 and 1.0


Default setting: 0.71



MW401, MW402

NaCL (0.5)

T75, T76

NaCL (0.5)

C65, C66, MW301, MW302

None - just measures EC


NaCL (0.5)





Tri-Meter 202

NaCL (0.5)

Tri-Meter 203

442™ (0.7)


442™ (0.7)