The conversion factor is the whole problem
A conductivity meter measures one thing: how readily the solution passes an electric current. That is reported as electrical conductivity, in millisiemens per centimetre or the equivalent microsiemens. Every ppm or TDS figure you have ever seen is that same measurement multiplied by a conversion factor chosen by whoever built the meter, and there are three factors in common use.
| Factor | Also called | EC 1.0 reads as | EC 2.0 reads as |
|---|---|---|---|
| 0.5 | 500 scale, NaCl scale | 500 ppm | 1,000 ppm |
| 0.64 | 640 scale | 640 ppm | 1,280 ppm |
| 0.7 | 700 scale, KCl or 442 scale | 700 ppm | 1,400 ppm |
The gap between the extremes is forty percent. If a feed chart is written on the 700 scale and your meter runs on the 500 scale, matching the printed number puts you forty percent over strength, and nothing in the reading warns you. This is the single most common way that a carefully written feed schedule goes wrong, and it produces symptoms that look like every other problem in the room.
The fix is trivial once you see it: work in EC. EC is the measurement. Every meter can display it, every chart can be translated into it, and it carries no scale ambiguity. If your meter only shows ppm, find out which factor it uses from the manual, and convert once at the start rather than guessing every week.
What EC does not tell you
Conductivity responds to dissolved ions in general. It cannot distinguish which ions, and it weights them by how well they conduct rather than by how much the plant needs them. Two reservoirs at exactly the same EC can hold quite different mineral profiles, and a solution can drift badly out of balance while the meter sits reassuringly still, because as one element is taken up faster than another the total conductivity barely moves.
This matters most where the source water carries a substantial load. Water at EC 0.6 has used a third of an EC 1.8 target before a drop of feed goes in, and the minerals occupying that third are whatever the supply happens to carry rather than anything chosen. Hard water is typically heavy in calcium and magnesium, and a feed designed for clean water on top of that becomes a different formulation. A water analysis from the supplier costs nothing and settles it.
Temperature also moves the reading. Conductivity rises as the solution warms, by roughly two percent per degree Celsius. Nearly all modern meters compensate automatically to a 25 degree reference, but they do it by assuming a compensation coefficient, and a meter left in the sun or reading a reservoir at 15 degrees is working harder than its calibration assumes. Take readings at a consistent temperature and calibrate the meter regularly with fresh standard solution, because a drifted meter reads confidently wrong.
Working out a dose without trusting the label
Label dose rates are written for a reference water and a reference crop, and yours is neither. The reliable route is to establish your own reference point once: measure the source water, mix a known volume of concentrate into a known volume of that water, measure again, and subtract. That gives you the EC rise per millilitre per gallon for your concentrate in your water, and it scales linearly over the range anyone actually uses.
That is exactly what the reference dose and reference rise fields here are for. Once you have them, any target EC converts straight into a dose. Repeat the measurement when you open a new container or change the concentrate, and expect the number to move slightly.
Multi-part concentrates need one further rule that has nothing to do with arithmetic. The parts are formulated to be kept apart until they are dilute, because some of the compounds they contain will react with each other and drop out of solution as a solid if they meet at full strength. That precipitate takes the elements with it and it does not come back. Each part goes into the full volume of water, stirred in, before the next one is opened.
All of these products are corrosive. Adjusters for pH more so than the feeds themselves. Add concentrate to water and never the reverse, wear eye protection when handling anything undiluted, and keep the containers closed and out of reach of children and animals.
What a target strength is and is not
This calculator will convert any target you give it and will never suggest one. Appropriate solution strength depends on the species, the cultivar, the growth stage, the light level, the temperature, the substrate and how the solution is delivered, and a figure that is right for one combination can be actively harmful in another. It also interacts with light: a plant under high light transpires hard and takes up water quickly, which changes how a given strength behaves in the root zone entirely.
Get the target from a feed chart written for what you are growing, or from your own records of what worked, and use this page to hit it accurately and to stop the scale confusion from undoing the effort. For how that solution then behaves over the days that follow, the reservoir top-up and drift calculator takes over. For the transpiration side that drives uptake in the first place, see the VPD calculator, and for dry granular feeding of soil and lawns, which works on a completely different basis, the fertilizer application calculator is the right tool.
Questions people ask
Which ppm scale should I use?
Whichever one your meter uses, and then stop using ppm for anything that matters. The scale is a property of the instrument, not of the water, so there is no correct choice and no way to convert a ppm number between people without knowing both their meters. Find the factor in your meter manual, note it somewhere near the reservoir, and record everything in EC from then on. EC is the actual measurement all three scales are derived from, it is unambiguous, and every chart and every forum post can be translated into it once. The forty percent spread between the 500 and 700 scales is a large enough error to matter and it is entirely avoidable.
What EC should my solution be?
That is not a question this page can answer, and any source that gives you a single number without asking what you are growing is guessing. Appropriate strength varies by species, by cultivar within a species, by growth stage, by how much light the plant is receiving, by temperature and by the substrate. The same number that suits a mature fruiting plant under strong light can be far too much for a seedling or a low-light foliage crop. Take the target from a feed chart written for your crop, or from your own records, and treat the calculator as the thing that hits it accurately.
Why is my measured EC lower than the calculator predicted?
The most likely cause is that the reference dose and reference rise you entered came from a label rather than from your own measurement, and the label assumed water unlike yours. The second is precipitation: if two concentrate parts met before they were diluted, some of the minerals left the solution as a solid and are no longer conducting anything. The third is meter calibration drift, which happens quietly over weeks and is fixed with fresh calibration solution. The fourth is temperature, if the solution is well away from the 25 degree reference the meter compensates to. Re-measure your own reference point in your own water and the prediction usually falls into line.
Can I convert EC to ppm of a specific nutrient?
No. Conductivity is a bulk property of everything dissolved in the water, weighted by how well each ion conducts. It gives no information about which elements are present or in what ratio, so it cannot be broken down into nitrogen, potassium or anything else. If you need elemental concentrations, they come from the formulation you mixed plus a laboratory analysis of the source water, or from a solution analysis. The ppm shown on a conductivity meter is a scaled EC reading and shares none of the meaning that ppm carries in a laboratory report, which is a genuinely confusing collision of terminology.
Does the order I add concentrates in matter?
Yes, and it is the one procedural rule worth stating. Multi-part concentrates are separated because some of their compounds will react and precipitate out if they meet at full strength, taking nutrients out of solution permanently. Each part goes into the full volume of water and is stirred in before the next container is opened, so they only ever meet dilute. Beyond that, all of these products are corrosive, adjusters for pH especially so: add them to water rather than the other way round, keep them off skin and eyes, and store them closed and out of reach. This calculator gives quantities and no further procedure.