The stock label says “1:100” and the reservoir holds 20 gallons. How much goes in? About 757 mL of each part: 757 mL of A and, separately, 757 mL of B. Cornell’s lettuce recipes build this in directly: the weight of fertilizer for 1 gallon of stock equals the weight for 100 gallons of working solution. So a ×100 concentrate means 10 mL of each part per liter, or about 1.3 fl oz per gallon.
Below is a stock solution dilution calculator with three modes. “Reservoir from stock” gives milliliters for any volume. “Injector” checks whether your stock matches the injector ratio and shows the EC to expect at the outlet. “Top-up by EC” tells you how much concentrate to add to a tank that already holds solution. After the calculator, we explain where the formulas come from and where they go wrong.
Stock solution dilution calculator: reservoir, injector, top-up
- Take the strength from your stock recipe: ×100 means the stock is 100 times stronger than the working solution. If a bottled nutrient lists its dose in mL/L, the strength is 1000 divided by the dose, so 4 mL/L is ×250.
- The calculator assumes all parts share one strength, as in Cornell’s two-tank setup. If your parts have different doses, like 1.8, 1.2, and 0.6 mL/L in three-part bottled nutrients, calculate each part on its own: set “Parts” to “One” and enter that part’s strength.
- Measure the full-dose EC once: mix the stock into a few liters of your own water using the “Reservoir from stock” mode and read the meter. The injector and top-up modes need that number.
Stock solution 1:100: how many mL per liter and per tank
Bodie Pennisi and Raymond Kessler of the University of Georgia define the ratio this way: 1:100 means 1 part stock to 99 parts water, 100 parts of working solution in total. Joyce Latimer of Virginia Tech says the same thing from the other end: a 1:100 injector delivers 100 gallons of solution for every gallon of stock. That gives the calculator’s formula: milliliters of each part equal reservoir liters times 1000, divided by the strength.
| Strength | Per gallon, fl oz | Per liter, mL | Per 100 L, mL | 1 gal of stock makes |
|---|---|---|---|---|
| ×50 | 2.56 | 20 | 2000 | 50 gal |
| ×100 | 1.28 | 10 | 1000 | 100 gal |
| ×200 | 0.64 | 5 | 500 | 200 gal |
| ×250 | 0.51 | 4 | 400 | 250 gal |
The numbers in the table are per part. Neil Mattson of Cornell University and Cari Peters of J.R. Peters Laboratory prepare tanks A and B separately and dilute each one 1:100. Split those 10 mL between the two, 5 mL of A and 5 mL of B, and the reservoir gets half the dose.
A stronger stock needs fewer jugs for the same amount of solution. What a gallon of finished solution costs from bottled concentrates versus single salts is worked out in bottled nutrients vs dry salts.
Stock solution for an injector: strength equals ratio
An injector, or proportioner, pulls stock into the water line at a set ratio. Latimer puts it plainly: for a 1:100 and a 1:200 injector to both deliver 200 ppm nitrogen from the same fertilizer, the stock for the 1:200 injector has to be twice as concentrated. It works the other way too: a ×200 stock through a 1:100 injector delivers a double dose. The “Injector” mode compares the two numbers and, if the stock is too strong, tells you how much water to add to it.
Ratios of common injectors, per the University of Georgia: Dosatron from 1:50 to 1:500 (higher-flow models up to 1:1000), Anderson from 1:50 to beyond 1:1000, Smith Measuremix fixed at 1:100 or 1:200. You set an adjustable injector to match your stock. For a fixed one, you mix the stock at exactly the strength it pulls.
A and B never share one stock tank. Calcium nitrate and phosphate in the same concentrate precipitate, so UGA recommends a dual-head injector, and Cornell runs two 1:100 injectors in series. For the other pairs that cannot share a concentrate, see the wiki on fertilizer chemical compatibility.
To confirm the injector actually holds its ratio, the UGA bulletin gives two methods:
- By volume. Measure how much stock the injector took up and how much solution it delivered, then divide the second by the first. In the bulletin’s example, 370 mL of stock produced 18,925 mL of solution, a 1:51 ratio, about 1% off.
- By EC. Measure the EC of clear water, run the injector for a few minutes, collect a sample, and measure it. The difference is the fertilizer EC. Compare it with the manufacturer’s chart or, for your own recipe, with the expected EC the calculator shows.
Stock solution for top-ups: how much to add to a partly full tank
The tank has 60 L left at EC 1.5, and you want 100 L at EC 1.6. Topping up with 40 L of full-dose solution almost works here: you land at 1.62. But if the tank was at 2.2, the same top-up leaves you at 2.04 instead of 1.6. The calculator works from a salt balance. Some salts are already in the tank, the top-up water brings a little more, and the stock supplies the rest. You measure how much EC a full dose adds over your water, so the math fits your recipe and your water source. For the example above, with water at EC 0.3 and a full dose at 1.8, that comes to 387 mL of each part of a ×100 stock.
If the tank EC is already above target, the calculator tells you how far to top up with plain water. If even a full tank of water will not bring it down, it tells you to drain some first. How to account for source water EC is covered in the wiki on EC in hydroponics.
The method has a limit. EC measures total salts, not what those salts are. Plants take up elements unevenly, so after a few top-ups the ratios in the tank drift away from the recipe even while EC reads right. Topping up by EC keeps concentration steady between full solution changes. It does not replace them.
Adding concentrate to a tank with plants in it also goes through water: dilute each part separately in a jar with a few liters of tank water and pour it in with the pump running, away from the intake. The step-by-step is in the wiki on A and B stock solutions.
Stock solution A and B: mixing order
- Fill the reservoir 50-70% with clean water.
- Add the A dose and stir.
- Add the B dose and stir. Never pour A into B or B into A directly, not even a drop.
- Top up to full volume.
- Check EC and pH. If your water is alkaline, Cornell adds acid to tank B, the one without calcium nitrate, or keeps it in a separate tank. The hydroponic acid dosing calculator works out the dose.
The NPK calculator works out how many grams of salts your recipe needs. For a ×100 stock, enter a volume 100 times the size of your jug, the same way Cornell does it. This calculator picks up where the stock is already made.
Stock solution storage and labeling
An unlabeled jug turns into a puzzle within a month: what is in it, how strong, when it was made. Write the contents, date, and strength on every container, since strength is what the calculator asks for. Keep acids and liquid concentrates in acid-resistant containers in a ventilated room. Storage conditions and shelf life for dry salts and concentrates are in the wiki on fertilizer storage.
Stock solution dilution: key points
- mL of each part = reservoir liters × 1000 / strength. ×100 means 10 mL of A and 10 mL of B per liter, about 1.3 fl oz each per gallon.
- Stock strength must equal the injector ratio. A ×200 stock through a 1:100 injector doubles the dose.
- Dilute A and B separately, through water, and feed them through separate injector heads.
- Topping up by EC holds concentration between changes but does not control what the solution contains.
- Check the injector by volume, or by outlet EC minus water EC.
