Which EC in which week, which pH in which medium — and how much RO water your tap water needs to get there. Free, no sign-up.
RO water is the low-salt water you cut with — normally EC 0.0. Soft tap water works too; just enter its EC in the same field. The EC of your tap water is in your supplier's water report, or you measure it once. It is the starting point your nutrients build on — water EC plus nutrient EC gives the total.
The calculator gives you a starting dose for your feed water. What decides it is not the pH on its own but the water hardness — that is what determines how hard your water pushes back against the acid.
Your hardness band comes from your water supplier — it is on the bill or on their website. That is enough: the carbonate hardness follows from it, and that is what decides how much acid you need. The pH alone does not tell you — two waters with the same pH can differ by a factor of five.
| Stage | EC (mS/cm) |
|---|---|
| Germination | 0.2–0.4 |
| Seedling | 0.4–0.8 |
| Vegetative | 0.8–1.4 |
| Early flower | 1.4–1.6 |
| Mid flower | 1.6–1.8 |
| Late flower | 1.2–1.6 |
| Flush | below 0.4 |
| pH | 6.0–7.0 ideal 6.3–6.8 |
| Stage | EC (mS/cm) |
|---|---|
| Germination | 0.4–0.6 |
| Seedling | 0.6–1.0 |
| Vegetative | 1.2–1.6 |
| Early flower | 1.4–1.8 |
| Mid flower | 1.8–2.2 |
| Late flower | 1.4–1.8 |
| Flush | below 0.4 |
| pH | 5.5–6.5 ideal 5.8–6.2 |
| Stage | EC (mS/cm) |
|---|---|
| Germination | 0.4–0.6 |
| Seedling | 0.6–1.0 |
| Vegetative | 1.2–1.6 |
| Early flower | 1.4–1.8 |
| Mid flower | 1.6–2.0 |
| Late flower | 1.2–1.6 |
| Flush | below 0.4 |
| pH | 5.5–6.2 ideal 5.8–6.0 |
Reference values, not fixed rules — what your plant shows you is what counts.
These calculators give you reference values for your own judgement. They replace neither a measurement on site nor expert advice — all figures without guarantee.
The EC value (electrical conductivity, in mS/cm) tells you how much salt is dissolved in the water in total — in other words how strong your feed is. The pH value tells you whether the plant can take that offer up at all.
That is why the two always belong together: a perfectly dosed feed is worth nothing if the pH is off. At pH 7.5 iron and manganese are effectively unavailable, even though they are sitting right there in the water. The plant then shows a deficiency that gets worse with every extra dose of nutrients.
Which is why the most common cause of sudden deficiency symptoms is not too little feed but the wrong pH. Reaching for the bottle when leaves yellow, instead of the meter, usually makes it worse.
Most charts online give a single EC range for every kind of grow. That is the most common mistake on this subject — between soil and hydro there are around 0.4 to 0.6 mS/cm, and with pH the gap is wider still.
Soil is a buffer. It holds organic matter, binds nutrients and releases them slowly. That is why it tolerates a higher pH (6.0–7.0) and needs less EC: part of the supply comes from the medium itself.
Coco is inert — it barely buffers, and it binds calcium and magnesium at the same time. The pH sits lower (5.5–6.5) and cal-mag is effectively mandatory. EC targets run slightly above soil, because nothing is topping the plant up.
Hydro has no medium taking part at all. Everything the plant gets is in the solution, and the roots hang directly in it. The pH band is the narrowest (5.5–6.2) precisely because there is no buffer to absorb swings. For deep water culture there is a page of its own: the DWC calculator explains why EC and pH climb in a reservoir by themselves.
So use the switch above the table. A value that is perfectly normal in hydro can already mean overfeeding in soil.
If your tap water already carries EC 0.6, that is 0.6 you cannot use for nutrients. In a seedling week with a target of 0.6 there is nothing left at all — the water alone fills the budget.
The way out is cutting it with low-salt water, usually from an RO filter. The calculator above works out how much you need: RO water = volume × (tap EC − target) ÷ (target − RO EC). With 10 litres of tap water at EC 0.8 and a target of 0.4, you add 10 litres of RO water.
The mixing itself is linear and therefore exact — EC values mix in direct proportion to volume. That is what makes this calculation trustworthy, and it is precisely what does not hold for pH (see below).
It looks like the same kind of sum, and it is not. Mix water at pH 7.0 with water at pH 5.0 in equal parts and you do not get 6.0 — you get whatever the buffering of the two waters allows.
The reason is carbonate hardness. Tap water carries dissolved carbonate that catches acid and holds the pH; RO water carries almost none. So the harder water dominates the result far beyond its share of the volume. Measured, a 1:1 cut of tap and RO water drops the pH by around 0.3 — while the arithmetic that ignores buffering predicts 0.93. In a range where 0.3 already means locked-out nutrients, that is not a rounding error.
There is a second reason not to trust a pH reading here: pure RO water has almost no ions, and a pH meter needs ions to measure. The value it shows drifts and reacts to the air. It is not a number worth putting into a calculation.
That is why this page has no pH mixing calculator, and it is deliberate. Mix first, then measure, then correct with the calculator above.
The sequence matters more than most people expect, and getting it wrong is the reason a lot of feeds end up at the wrong pH.
1. Cut the water first. If you are mixing tap and RO water, do that before anything else — the EC of the base decides everything that follows.
2. Then the nutrients, in the order the manufacturer states. Cal-mag before base nutrients where it applies; never pour two concentrates into each other undiluted, or you get precipitation that no amount of stirring undoes.
3. pH last. Nutrients shift the pH on their own, often by half a point or more. Setting the pH before feeding means doing it twice.
4. Stir, wait, measure again. A minute is enough. Acid needs a moment to work through the volume, and the first reading after pouring is almost always too low.
Calibrate. A pH meter drifts — with use, with age, with temperature. Monthly with the two buffer solutions (4.0 and 7.0) is the sensible interval; before an important measurement is better. An uncalibrated meter is worse than none, because it produces confident wrong numbers.
Do not let the probe dry out. The glass membrane needs to stay wet. Storage solution, not distilled water — distilled water pulls ions out of the probe and ruins it over time.
Temperature affects both readings. Most decent meters compensate automatically; cheap ones do not. Measure at the temperature you actually feed at, not with water fresh from the tap.
Runoff tells you more than the feed. What goes in is your plan; what comes out is what the medium is actually doing. If runoff EC climbs across several waterings, salt is building up — that is the moment to flush, not when the leaves already show it.
Burnt leaf tips are the classic sign of EC being too high. They start at the very tip and work inwards. Cutting back the feed helps; more nutrients make it worse, which is unfortunately the instinctive reaction.
Pale, uniformly light green leaves point the other way — too little EC, or a pH so far off that nitrogen is not getting in. Measure before you feed: both look identical on the plant.
Yellowing between the veins while the veins stay green is the signature of a lock-out, usually iron or magnesium. Almost always a pH problem, almost never a shortage of the element itself.
EC rising in the reservoir without anyone adding anything is normal in hydro, not a fault — the plant takes up more water than salt. That is a case of its own and covered on the DWC page.
It depends on the medium: soil 6.0 to 7.0, coco 5.5 to 6.5, hydro 5.5 to 6.2. The narrower the buffer, the tighter the band — hydro has none at all, which is why it is the most demanding. The table above splits it by stage as well.
Roughly: seedlings 0.6 to 1.0, vegetative 1.2 to 1.6, flower 1.4 to 1.8, and down again for the flush. Those are hydro figures; soil sits around 0.4 to 0.6 mS/cm lower because the medium supplies part of it. Use the medium switch above the table.
Yes, entirely. If your tap water reads EC 0.6 and you want 1.2, your nutrients may only add 0.6. This is the most common reason for accidental overfeeding: people dose to the target as if the water were at zero.
RO water = volume × (tap EC − target) ÷ (target − RO EC). With 10 litres at EC 0.8 and a target of 0.4, that is 10 litres of RO water. The calculator above does it, in both directions.
No, and that is why this page does not offer it. pH does not mix in proportion — carbonate hardness buffers the result. Measured, a 1:1 cut of tap and RO water drops the pH by about 0.3, while the naive arithmetic predicts 0.93. Mix, measure, then correct.
Because there is almost nothing in it to measure. A pH probe needs ions, and RO water has hardly any — so the reading drifts and reacts to the air. Measure after adding nutrients, not before.
Nutrients first, pH last. Feeds shift the pH on their own, often by half a point or more. Setting the pH beforehand just means doing it twice.
They measure the same thing on different scales. The catch: there are two conversion factors in circulation — 500 (NaCl, common in the US) and 700 (KCl, common in Europe). EC 1.0 is either 500 or 700 ppm depending on which your meter uses. That is why EC is the safer figure to work in.
Yes. Both calculators use the same formulas as the ones in the CRIS app — the EC mixing and the pH-down dose by water hardness. In the app the values also attach to each journal entry, so you can follow how they develop across the grow.
pH, EC, VPD, DLI, running costs and more — in the CRIS app, right next to your grow journal, so measured values land on the right entry. Offline, no account.