How much water is missing from the tank, what EC does the top-up need, and how do you bring an EC that has run too high back down? Works for DWC, RDWC and any other reservoir. Free, no sign-up.
Preview means: you know what you are topping up with and want to see where the EC ends up. “What do I need?” works the other way round — you set your target EC and it tells you what EC the top-up water must have. In DWC you normally top up with clear, pH-adjusted water without nutrients (EC 0.0 for RO water, otherwise the EC of your tap water), because the EC in a reservoir climbs by itself. If it has dropped too low instead, the top-up water has to sit above your target — which is exactly what the second direction works out.
Two routes get you there, and the calculator gives both: add water if there is still room in the tank — or swap out part of the solution if it is already full. The second case is the more common one in DWC, and almost nobody explains it.
Work with the actual amount in the tank, not the tank volume — otherwise you add too much water. Come down in steps: a big jump in EC stresses the plant more than a slightly high reading does.
| 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.
It is the most underrated number in DWC. Warm water holds less oxygen — and oxygen at the roots is the one thing this whole method rests on.
| Temperature | What it means |
|---|---|
| 18–20 °C | Ideal. Maximum dissolved oxygen, roots stay white and firm. |
| 20–22 °C | Still acceptable, but the oxygen level is already measurably lower. |
| from 23 °C | Critical. Root rot (Pythium) spreads fast, and water at 24 °C holds around 20 % less oxygen than at 18 °C. |
These calculators give you reference values for your own judgement. They replace neither a measurement on site nor expert advice — all figures without guarantee.
Most people expect the opposite: the plant eats nutrients, so the EC ought to drop. In a reservoir the reverse happens all the time — the EC climbs day by day, even though nobody added any feed.
The reason is simple: the plant takes up more water than nutrient salt. Transpiration through the leaves and evaporation from the tank add to it. What stays behind is a smaller volume of water carrying almost the same load of salt — in other words a stronger solution than the day before.
From that follows the most important rule of daily DWC life: top up with plain water, not with nutrient solution. Pour feed in every day and you ratchet the EC upwards until the plant shows salt stress — burnt leaf tips that look like a deficiency and are often answered with yet more feed.
How strong the effect is depends on the ratio of plant to tank. A small tank with a big plant in flower can climb several percent in EC on a warm day. That is why level and EC belong to the daily checks in DWC, not the weekly ones.
Only when the EC falls was the solution genuinely too weak — then it is time to feed. Which makes EC the most honest feedback a plant can give you: it tells you whether it currently wants more water or more nutrient.
Both have their place, and it is not a matter of taste. Topping up happens continuously, as soon as the level has visibly dropped — with clear, pH-adjusted water. The full solution gets changed every 7 to 14 days.
The reason for the full change is that a nutrient solution does not empty evenly. The plant takes up nitrogen and potassium faster than calcium and sulphate. After two weeks the tank may still read the same EC — but the composition behind that number has shifted. An EC value cannot show the difference: it measures the sum of all salts, never their ratio.
That is the limit of every EC reading, and it is worth knowing. A perfect EC is no proof of a healthy solution — it only rules out that the solution is too strong or too weak.
Which calculator when: if there is room in the tank, you add water. If it is full and the EC is still too high, you swap out — drain part of the solution and replace it with the same amount of fresh water. The second calculator above works out both amounts at once.
Almost every DWC grower knows it: you set the pH to 5.8 in the evening and the next morning it reads 6.3. That is not a faulty meter and not a mistake — it is the plant itself, and there is a very clean explanation behind it.
Plants have to stay electrically neutral. If a root takes up a negatively charged particle, it has to release another negatively charged one. By far the biggest item on the menu is nitrate (NO₃⁻) — the main form nitrogen takes in every mineral feed. For every nitrate taken up, the root gives a hydroxide ion (OH⁻) back to the water. And OH⁻ is exactly what pushes pH up.
A second effect applies in DWC that soil does not have: the air stone drives dissolved CO₂ out. Carbon dioxide forms carbonic acid in water and holds the pH down. Blow it out and that acid is missing — the pH climbs further. The stronger the aeration, the more pronounced it gets.
In practice: a slowly rising pH is a good sign. It shows the plant is taking up nitrogen and actively growing. A pH that does not move for days is the more suspicious reading — it can mean the roots have largely stopped working.
You still have to act, because above pH 6.2 iron and manganese become poorly available. Set the value to the lower half of the target band, around 5.7, and let it drift upwards. That way the solution passes through the whole usable range over the day — different nutrients are taken up best at slightly different pH values. How much acid you need for that depends mostly on your water hardness; the pH and EC calculator works it out.
Your water has an EC before the first drop of nutrient goes in. Depending on where you live it sits somewhere between 0.2 and 0.9 mS/cm — dissolved salts from the mains, mostly calcium and magnesium.
That makes every target figure ambiguous as soon as the starting point is missing. An example, same target of 1.6 mS/cm:
With tap water at EC 0.4 you add nutrients worth 1.2 mS/cm. With RO water at EC 0.0 you add nutrients worth the full 1.6. That is a third more feed for an identical target — and nobody notices, because both end up reading the same number on the meter.
So: measure your water EC once and count it in every time. The targets in the table above are totals — water plus nutrients, exactly as your meter shows them. They assume ordinary tap water at 0.3 to 0.4 mS/cm.
If you work with pure RO water your starting point is 0.0 — and then something important is missing: calcium and magnesium, which tap water supplies anyway. Bring the water up to about 0.3 to 0.4 mS/cm with cal-mag first, before the actual feed goes in. Otherwise you trade one problem for another.
If your tap water is very hard, there is little headroom left: starting at 0.9 and aiming for 2.0 in mid flower leaves only 1.1 for the nutrients. Then it is worth cutting with RO water — the pH and EC calculator works out how much you need.
Compare EC charts online and you will find flowering figures anywhere between 1.3 and 2.4 mS/cm. That spread looks like disagreement but usually has a plain reason: the charts mean different media.
In DWC the roots hang permanently in the solution. Everything is available all the time, uptake runs with almost no losses, and there is no dry phase in which salts concentrate in a medium. So less is needed here than in soil or coco.
Coco sits at the top end: it binds calcium and magnesium and releases them only slowly, and part of every feed washes straight through. Soil sits in between — it is often pre-charged and supplies part of the diet itself.
Be careful with the high figures some shop pages quote for flowering. They mostly come from commercial feed schedules built for facilities with CO₂ enrichment, strong light and controlled climate. Under living-room conditions the same concentration produces salt stress.
In DWC an overdose also shows within hours rather than within days: there is no medium to buffer the mistake. Which is exactly why the cautious number is the right one here — a slightly weak solution costs you a little growth, a strong one costs you roots.
Yes. The maths needs only three things: how much solution is in the tank, its measured EC, and your target. It does not care what is attached — it holds for DWC and RDWC just as it does for ebb and flow, drip systems, aeroponics or a gravity-fed tank of the kind Autopot uses. What does not carry over is the water temperature table further down: that one only counts where the roots sit in water permanently.
With clear, pH-adjusted water — as long as the EC is climbing or holding steady. That is the normal case in DWC, because the plant takes up more water than salt. Only if the EC falls over several days was the solution too weak, and then you add nutrients.
Every 7 to 14 days. The reason is not the EC value but the composition: nitrogen and potassium are taken up faster than calcium and sulphate. After two weeks the EC can still be right while the ratio behind it has long since shifted — and an EC meter cannot show that.
Because the plant takes up nitrate and releases hydroxide ions (OH⁻) into the water in exchange — it has to stay electrically neutral. On top of that the air stone drives out dissolved CO₂, which would otherwise hold the pH down. A slowly rising pH is a sign of active uptake, not a fault.
5.5 to 6.2, ideally 5.8 to 6.0. The commonly quoted 5.5 to 6.5 is the general cannabis range including soil. In DWC no medium buffers anything, and above 6.2 iron and manganese become poorly available. Set the value towards the lower end and let it drift up.
18 to 20 °C is ideal, up to 22 °C is acceptable. From about 23 °C it gets critical: warm water holds less oxygen — at 24 °C around 20 % less than at 18 °C — and in oxygen-poor water Pythium, the organism behind root rot, multiplies very quickly.
Then you cannot dilute, so you swap out instead: drain part of the solution and replace it with the same amount of fresh water. The second calculator above gives you both amounts at once — how much to add, and how much to swap.
Yes. The values in the table are totals, exactly as your meter shows them — water plus nutrients. They assume ordinary tap water at 0.3 to 0.4 mS/cm. With pure RO water (EC 0.0) you first bring it up to that starting point with cal-mag.
Usually for one of two reasons: they mean a different medium — coco takes more than DWC — or they reproduce commercial feed schedules built for CO₂ enrichment and strong light. In DWC there is no medium to buffer an overdose, and it takes effect within hours.
CRIS stores EC, pH and water temperature per day and shows you the trend — so you can see whether your EC is rising or falling instead of guessing.