What this calculator does
Liquid feeding is specified in parts per million of nutrient, usually nitrogen. Because the fertiliser is only partly nutrient, the product concentration has to be higher than the target: a 200 ppm nitrogen target from a 20% product needs 1,000 mg/L of product.
Once the concentration is known the mass is just concentration times volume. At 100 litres that 1,000 mg/L works out to 100 grams. The whole calculation is two multiplications, and the reason it is worth doing carefully is that liquid feed is applied repeatedly, so an error compounds over a season rather than being a single event.
The formula
The product concentration in mg/L is the target ppm divided by the label percentage as a fraction. Since 1 ppm is 1 mg/L in water, that division is the whole of the nutrient-to-product conversion. Multiplying the concentration by the water volume in litres and dividing by 1,000 gives the fertiliser mass in grams.
| Term | Meaning |
|---|---|
| ppm | Parts per million, equivalent to milligrams per litre in water. |
| Constant feed | Applying dilute fertiliser at every watering rather than occasional concentrated doses. |
| EC | Electrical conductivity, a direct measure of total dissolved salts, used to check a solution rather than calculate it. |
The inputs explained
| Field | What to enter |
|---|---|
| Target nutrient concentration (ppm (mg/L)) | Target nutrient concentration in ppm. 100 to 200 ppm nitrogen is a common constant-feed range for container growing. |
| Nutrient percentage on the label (%) | The label percentage of the nutrient you are targeting, usually nitrogen, from the NPK analysis. |
| Water volume (L) | Volume of water to be mixed, in litres. |
When to use it
Mixing a constant-feed solution
Container and greenhouse growing commonly feeds at every watering with a dilute solution, and the ppm target is how that is specified.
Setting up an injector
Fertiliser injectors dose from a concentrated stock at a fixed ratio, and the stock concentration is worked out from the target multiplied by the injector ratio.
Converting between products
Two products at different analyses need different masses for the same ppm, and the calculation shows by how much.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much fertiliser does each ppm target need?
A range of ppm targets using a 20% product in 100 litres of water.
| Target ppm | Fertilizer needed | Fertilizer concentration | Water volume |
|---|---|---|---|
| 50 ppm | 25.00 g | 250.00 mg/L (g/m³) | 100.00 L |
| 100 ppm | 50.00 g | 500.00 mg/L (g/m³) | 100.00 L |
| 200 ppm | 100.00 g | 1,000.00 mg/L (g/m³) | 100.00 L |
| 400 ppm | 200.00 g | 2,000.00 mg/L (g/m³) | 100.00 L |
Questions
What ppm should I feed at?
For container growing, 100 to 200 ppm nitrogen at every watering is a common constant-feed range, with seedlings at the lower end and heavy feeders higher. Crop, growing medium and water quality all shift it, so published guidance for the specific crop is worth more than a general figure.
Is ppm the same as mg/L?
In water, yes, near enough. A litre of water weighs about a kilogram, so a milligram in a litre is one part per million by mass. The approximation holds for dilute solutions at ordinary temperature, which covers all practical fertiliser mixing.
Why is the product concentration higher than the ppm target?
Because the product is not pure nutrient. A 20% nitrogen product is four fifths other material, so five times the mass is needed to deliver the target nitrogen. The lower the analysis, the larger the gap.
Should I check the mix afterwards?
An EC meter is the practical check, measuring total dissolved salts in the finished solution. It will not tell you the nitrogen specifically, but it catches gross mixing errors and accounts for salts already present in the source water, which the calculation assumes are zero.
For granular fertiliser applied by area, see the fertilizer application rate calculator. For light levels in the same growing setup, see the daily light integral calculator.