How much nutrient per gallon?
Enough to raise the water from what it is to what you want. A nutrient concentrate adds conductivity in proportion to how much you put in — a fixed number of millisiemens per milliliter per gallon — so the dose is the EC you need to add, times the gallons, divided by that strength. Everything on this page is that line; the only hard part is the strength, which the label almost never states directly. Two ways to get it: a typical value for the format, or — better — one measured mix of your own bottle.
A 3-part like FloraSeries runs about 0.1 mS/cm per ml per gallon, all parts combined. To take 20 gallons of 0.3 tap water to 1.9 on the meter — 1.6 of nutrient — is 1.6 × 20 ÷ 0.1 = 320 ml, split by the label's ratio: 122 Micro, 113 Gro, 85 Bloom. The tap water is subtracted because its 0.3 is calcium and bicarbonate the meter counts and the plants can't use.
ml of each part, by reservoir and target
| Reservoir | 0.8 mS/cm net | 1.2 mS/cm net | 1.6 mS/cm net | 2 mS/cm net |
|---|---|---|---|---|
| 5 gal | 40 ml 15 / 14 / 11 | 60 ml 23 / 21 / 16 | 80 ml 30 / 28 / 21 | 100 ml 38 / 35 / 27 |
| 10 gal | 80 ml 30 / 28 / 21 | 120 ml 46 / 43 / 32 | 160 ml 61 / 57 / 43 | 200 ml 76 / 71 / 53 |
| 20 gal | 160 ml 61 / 57 / 43 | 240 ml 91 / 85 / 64 | 320 ml 122 / 113 / 85 | 400 ml 152 / 142 / 106 |
| 40 gal | 320 ml 122 / 113 / 85 | 480 ml 182 / 170 / 128 | 640 ml 243 / 227 / 170 | 800 ml 304 / 284 / 213 |
3-part liquid at 0.1 mS/cm per ml/gal, split Micro / Gro / Bloom in the late-growth ratio (6 : 5.6 : 4.2). Net of tap water — add your tap's EC to the target to get the meter reading. A different stage changes the ratio, not the total.
What "full strength" actually is
Feed charts read like recipes, but they're one number in disguise. Add up the ml per gallon on any row of General Hydroponics' own 3-part chart and divide the row's EC by it:
| Chart row | Micro / Gro / Bloom, ml/gal | Total | Chart EC | mS/cm per ml/gal |
|---|---|---|---|---|
| Seedling / clone | 1.8 / 1.8 / 1.8 | 5.4 | 0.4–0.5 | 0.083 |
| Early growth | 4.9 / 4.6 / 3.4 | 12.9 | 1.2–1.4 | 0.101 |
| Late growth | 6 / 5.6 / 4.2 | 15.8 | 1.4–1.7 | 0.098 |
| Early bloom | 7.6 / 6.6 / 8.5 | 22.7 | 2–2.4 | 0.097 |
| Mid bloom | 4.6 / 4.6 / 6.6 | 15.8 | 1.4–1.7 | 0.098 |
| Late bloom | 3.3 / 3.3 / 4 | 10.6 | 0.9–1.1 | 0.094 |
GH FloraSeries 3-part feed program (500 ppm scale). The last column is the same to within rounding on every row: the chart is EC × 10 ml per gallon, and the stage only decides how the ml split between bottles. That constant is what the calculator's "typical" strength for a 3-part is.
Typical is fine; measured is exact, and it takes one bucket. Add a known amount — say 20 ml total — to 5 gallons of water that read 0.3, stir, and read the meter: 0.72. The strength is (0.72 − 0.3) ÷ (20 ÷ 5) = 0.105 mS/cm per ml/gal. Now the 20-gallon, 1.6-net mix above is 305 ml instead of 320 — a 5% difference you'd otherwise chase with a second round of "add a bit more". The calculator does the arithmetic, and saves the result to your grow space so every later dose uses it. Recalibrate when you change brands or bottles; the concentrate in a new jug is the same, but "a capful" isn't.
Dry salts: grams, not milliliters
Jack's 321 and the MasterBlend recipe are the same idea in powder: three bags in a fixed ratio, about 0.31 mS/cm per gram per gallon combined. Jack's is 3.6 : 1.2 : 1.2 (Part A, calcium nitrate, Epsom salt) for 1.7–2.0 mS/cm; MasterBlend is 2 : 2 : 1. A $15 gram scale is the whole tool.
| Reservoir | Part A (5-12-26) | Calcium nitrate | Epsom salt | Total |
|---|---|---|---|---|
| 5 gal | 17.9 g | 6.0 g | 6.0 g | 29.8 g |
| 10 gal | 35.8 g | 11.9 g | 11.9 g | 59.7 g |
| 20 gal | 71.6 g | 23.9 g | 23.9 g | 119.4 g |
For 1.85 mS/cm net at 0.31 per g/gal. Dissolve the calcium nitrate separately and add it last, or it precipitates with the sulfate as gypsum — a white cloud that is your calcium leaving the solution.
Topping up: it's already at 1.1
Most dosing isn't a fresh reservoir. Plants drink, the EC drifts, and the question is how much to add to what's there. Same formula with the current reading as the starting point:
| Reservoir reads | To reach 1.6 in 20 gal, add |
|---|---|
| 0.8 mS/cm | 160 ml, split by ratio |
| 1 mS/cm | 120 ml, split by ratio |
| 1.2 mS/cm | 80 ml, split by ratio |
| 1.4 mS/cm | 40 ml, split by ratio |
3-part at 0.1. If the reservoir reads above the target, the answer is water, not nutrient — the calculator says so and how much is the top-off calculator's job.
Just following the label
If you'd rather dose by the label's rate than by EC, the math is multiplication — with one trap: European labels are in ml per liter and a gallon is 3.785 liters, so "2 ml/L" is 7.6 ml per gallon, not 2. Per part:
| Label rate, ml/gal | Same in ml/L | 5 gal | 10 gal | 20 gal |
|---|---|---|---|---|
| 2 | 0.53 | 10 ml | 20 ml | 40 ml |
| 4 | 1.06 | 20 ml | 40 ml | 80 ml |
| 5 | 1.32 | 25 ml | 50 ml | 100 ml |
| 8 | 2.11 | 40 ml | 80 ml | 160 ml |
| 10 | 2.64 | 50 ml | 100 ml | 200 ml |
Each part separately. Then read the meter: the label rate was written for the maker's water and a "typical" plant, and the EC it lands on is the number that matters — which is what the calculator's target mode and the calibration mix are for.
Why did the meter read higher than the calculator said?
Three usual reasons, in order. The tap water wasn't subtracted — 0.3–0.6 of every reading is the water itself; enter it. The bottle is stronger than "typical" — brands vary by half or double; one calibration mix ends the guessing. The meter isn't compensating for a warm or cold reservoir — the EC page handles that. If it read lower, add the same list plus one: the nutrient wasn't stirred in, and the probe sat in a clean layer. Stir, wait a minute, read again — then, whichever way it went, use the reading to calibrate and the next mix lands.
Mixing order, and the two ways to get it wrong
Water first, then each part separately, stirring between. For a 3-part, Micro goes in first — it carries the calcium — then Gro, then Bloom; for salts, calcium nitrate dissolves on its own and joins last. Never pour concentrates into each other or into a nearly empty reservoir: calcium meets phosphate or sulfate at full strength and drops out as a white precipitate that the plants never see and the meter never counts. Measure, don't cap. A bottle cap is 5–15 ml depending on the bottle; a $3 syringe is exact, and exactness is the entire point of calibrating. Then read the meter after mixing — the calculator's number is a prediction, the meter is the fact, and the difference between them is the next calibration.
Two limits the calculator watches: a net target over 3 mS/cm (salt stress for everything but tomatoes), and a total dose over the label's maximum per gallon, which you can enter — labels state the ceiling even when they don't state the strength.
Sources: General Hydroponics, FloraSeries 3-Part Feed Program (ml/gal and EC by week, 500 ppm scale, mixing order); Jack's 321 program (3.6 / 1.2 / 1.2 g per gallon, 1.7–2.0 mS/cm); MasterBlend 4-18-38 recipe (12 / 12 / 6 g per 5 gallons); 1 US gallon = 3.785 L, 1 tsp = 4.93 ml, 1 tbsp = 14.79 ml. Stage EC bands as on the EC ↔ PPM page.