Hydroponic Nutrient Calculator
Work out how much concentrated nutrient stock to add to a reservoir to hit a target in ppm.
What it does
Hydroponic growers keep nutrients in a concentrated stock and add a measured amount to the reservoir. The arithmetic behind that is one balance of mass, and it is the same calculation Penn State Extension walks through in its guide to nutrient solution concentrations: the final concentration times the final volume equals the concentration of the stock times the volume of stock, plus whatever the water brings with it.
In dilute water, parts per million and milligrams per liter are the same number, which is why every recipe in the literature can be written either way. The important detail is that the same element has to appear on both sides of the equation. A bag of fertilizer labelled 10-5-10 gives you nitrogen, phosphorus and potassium in oxide form, so the phosphorus content is not the number 5 on the bag.
The target numbers themselves come from a crop. The University of Florida extension publication on nutrient solution formulation for hydroponic tomatoes gives elemental targets by growth stage, and the United States Department of Agriculture publishes the modified Hoagland recipe used in plant research, where the same elements appear as molarities. Those two documents are the reference points for the table on this page.
One more thing matters more than people expect: the water. Well water, rainwater and treated water all carry calcium, magnesium, sodium and sometimes chloride, and a target of 150 ppm of calcium with 30 ppm already in the water means you add less fertilizer. The calculator takes that background level into account.
The tool works on one element at a time, which is how the mixing actually happens: each concentrated stock is measured separately and added to the reservoir in turn. Stocks should not all be mixed together in one jug, because calcium and the phosphate and sulfate salts react and drop out of solution.
How to use
- Pick the element you are mixing and the units you work in.
- Enter the target concentration for that element from your recipe or the reference table.
- Enter the concentration of the stock you are adding from.
- Enter the final volume of the reservoir.
- Add the level your water already supplies, if you have a test result.
- Measure out the stock, add it to the reservoir, and top up with water to the final volume.
Formula
- Cf
- Target concentration in the finished solution, in mg/L.
- Cs
- Concentration of the concentrated stock, in mg/L.
- Cw
- Concentration the water already carries, in mg/L.
- Vf
- Final volume of the solution.
- Vs
- Volume of stock to add.
The background term only matters when the water already supplies a meaningful share of the target, which is common for calcium and magnesium in hard water and for sodium in coastal water. The calculation is a mass balance, so it assumes the water and the stock mix completely and nothing precipitates out, and it says nothing about the pH, the temperature or the balance between the elements, all of which still have to be right for the plants.
Worked example
Example: 150 ppm of nitrogen in 20 L from a 10,000 ppm stock
Defaults: metric units, nitrogen, a target of 150 ppm, a stock of 10,000 ppm, 20 L of final volume and no background in the water.
- Volume of stock: 150 x 20 / 10,000 = 0.300 L.
- So measure 300 mL of the stock.
- Mass of nitrogen added: 10,000 mg/L x 0.300 L = 3,000 mg, which is 3.00 g.
- Check: 3,000 mg / 20 L = 150 ppm in the finished solution.
- Top up with water until the total is 20 L, not 19.7 L.
- The tool shows 300.0 mL, 150.0 ppm, 3.00 g and a dilution factor of 66.7x.
Reference table
| Element | Transplant to first cluster | Fifth cluster to termination |
|---|---|---|
| Nitrogen | 70 ppm | 150 ppm |
| Phosphorus | 50 ppm | 50 ppm |
| Potassium | 120 ppm | 200 ppm |
| Calcium | 150 ppm | 150 ppm |
| Magnesium | 40 ppm | 50 ppm |
Limits
- Handles one element at a time, so each stock has to be measured separately.
- Ignores precipitation, pH shifts and the balance between the elements.
- Assumes the water contributes only the background you entered, with no other ions.
- Ignores uptake by the plants, so the reservoir drifts and has to be topped up and monitored.
Common mistakes
- Adding water to 20 litres instead of to a final volume of 20 litres, which leaves the solution too dilute.
- Mixing all the concentrated stocks together in one jug, where calcium meets phosphate and sulfate.
- Using the numbers on the fertilizer bag, which are oxides, instead of the elemental targets.
- Forgetting the calcium and magnesium already in the water, and then overfeeding.
- Assuming a 100x stock is 100 times a specific target, when a 100x stock is 100 times the final target you intend to use.
Try it yourself
Quick checks. Answers sit tucked under each question.
A true 100x stock of 15,000 ppm nitrogen is used to reach 150 ppm in 20 L. How much stock?
150 x 20 / 15,000 = 0.200 L, so 200.0 mL of stock and water to a final volume of 20 L.
How much of a 10,000 ppm calcium stock do you need for 200 ppm in 20 L?
200 x 20 / 10,000 = 0.400 L, which is 400.0 mL and 4.00 g of calcium.
Your water supplies 30 ppm of calcium and you want 150 ppm in 20 L. How much of a 10,000 ppm stock?
The water already covers part of the target, so the stock volume is 20 x (150 - 30) / (10,000 - 30) = 0.2407 L, which is 240.7 mL.
Questions
What does ppm mean in a nutrient solution?
Parts per million, which for dilute water is the same as milligrams of that element per liter of solution. So 150 ppm of nitrogen is 150 mg of nitrogen in every liter.
What is a 100x stock?
A concentrate that is 100 times the strength of the final solution, so 1 liter of it makes 100 liters of working solution. A 15,000 ppm nitrogen stock is a 100x stock for a 150 ppm target.
Should I use the numbers on the fertilizer bag?
No. A bag labelled 10-5-10 gives nitrogen, phosphorus and potassium in oxide form, and the elemental amounts are different. Use the elemental targets from a crop guide and work out the fertilizer amounts separately.
How much does the water in my well matter?
A lot for calcium, magnesium and sodium. A well with 30 ppm of calcium already covers a fifth of a 150 ppm calcium target, and if you add the full amount anyway the solution ends up over fed and the pH drifts.
Can I mix the stocks together?
Not in one concentrated jug. Calcium and phosphate or sulfate react and fall out of solution, which is why the extension recipes keep separate stocks and add them to the reservoir one at a time.
Does the tool handle the balance between nutrients?
No. It calculates the amount of one element to reach one target. The balance between nitrogen, potassium, calcium and magnesium, and the pH that goes with it, still comes from a recipe and a pH meter.
Sources
- University of Florida IFAS Extension: nutrient solution formulation for hydroponic tomatoes
- USDA Agricultural Research Service: modified Hoagland standard nutrient recipe
- Penn State Extension: calculating nutrient solution concentrations