Water-Soluble Fertilizer Calculator
Enter your fertilizer label numbers (N, P2O5, K2O), choose which nutrient you want to target, set the desired concentration in ppm or mmol/L, and specify the total solution volume. The calculator shows the exact dry weight to dissolve and the resulting concentration of every major nutrient in that solution. Switch between metric (grams, litres) and imperial (ounces, gallons) at any time.
How to use this calculator
Select your fertilizer formula from the preset list or choose Custom and type in the N, P2O5, and K2O percentages from your bag label. Then pick which nutrient you want to target (usually nitrogen for vegetative growth), enter your desired concentration in ppm or mmol/L, and type in how many litres or gallons of solution you need to prepare. The calculator gives you the exact weight of dry fertilizer to dissolve and shows the resulting concentration of every major nutrient in that solution. The dose-rate field (g/L or oz/gal) lets you scale the recipe to any batch size without re-entering everything.
The formula behind the calculation
Water-soluble fertilizer labels state nutrient content as percentages by weight. One gram of fertilizer dissolved in one litre of water creates a 1,000 mg/L solution of the fertilizer, so the percentage of a given element in that solution is (% on label) / 100 x 1,000 = (% on label) x 10 ppm. To hit a target concentration in ppm, you need this many grams of fertilizer per litre: dose (g/L) = target ppm / (element % x 10). Multiply by the total volume in litres to get the batch weight. Phosphorus and potassium on fertilizer labels are expressed as oxides (P2O5, K2O) rather than elemental form - this calculator converts them automatically using P = P2O5 x 0.4364 and K = K2O x 0.8301.
ppm, mmol/L, and electrical conductivity
Parts per million (ppm) means milligrams of the element per litre of water, which is identical to mg/L. It is the most common unit for greenhouse and hydroponic fertilizer programs because it maps directly to the weight of nutrient ions in solution. The molar equivalent, mmol/L, is used more often in scientific literature and in European growing guides. To convert: ppm = mmol/L x molar mass of the element. For example, 14.29 mmol/L of nitrogen (molar mass 14.007 g/mol) equals 200 ppm N. Electrical conductivity (EC) is a faster field measurement: a 200 ppm N solution from a balanced 20-20-20 fertilizer typically reads around 1.2-1.5 mS/cm, though the exact relationship varies by fertilizer composition and water mineral content.
Injector and stock-solution mixing
Many commercial greenhouse and nursery operations use injector systems that dilute a concentrated stock solution into the water line at a set ratio (e.g. 1:100, 1:200). To prepare a stock solution for a 1:100 injector at a target of 200 ppm N with a 20-20-20 fertilizer, multiply the calculated dose rate by the injector ratio: 0.1 g/L x 100 = 10 g/L in the stock tank. When preparing stock solutions, always use clean water and dissolve completely before adjusting the volume. Calcium and sulfate fertilizers should be kept in separate stock tanks because they can precipitate (form gypsum) if combined in concentrated form.
Typical target concentrations by crop type and growth stage
| Crop / Stage | N target (ppm) | Feed frequency | Notes |
|---|---|---|---|
| Seedlings / propagation | 50-75 | Every watering | Start dilute to prevent root burn |
| Leafy vegetables (lettuce) | 125-175 | Every watering | Consistent feeding, keep EC 1.0-1.8 |
| Herbs (basil, mint) | 100-150 | Every watering | Moderate feeder, prefers steady low dose |
| Tomatoes - vegetative | 150-200 | Every watering | High nitrogen phase before flowering |
| Tomatoes - fruiting | 150-250 | Every watering | Increase K to 200+ ppm for fruit quality |
| Cannabis - vegetative | 150-200 | Every watering | Moderate N, balanced P and K |
| Cannabis - flowering | 100-150 | Every watering | Reduce N, raise K and P |
| Ornamentals / houseplants | 100-150 | Weekly | 20-20-20 or balanced formula works well |
| Orchids | 50-100 | Weekly | Weakly, weekly - dilute feeding prevents burn |
| Succulents / cacti | 25-50 | Monthly | Very low feeders, avoid high N |
General guidelines for water-soluble fertilizer programs. Adjust to your crop, media, and water quality.
Frequently asked questions
How do I calculate ppm from fertilizer percentages?
Divide the target ppm by the product of the elemental percentage and 10. For example, if your fertilizer contains 20% nitrogen and you want 200 ppm N, the dose is 200 / (20 x 10) = 1.0 g/L. Dissolve 1 gram per litre and you will have 200 ppm N. This calculator does all the arithmetic automatically.
Why does the calculator use P2O5 and K2O instead of elemental P and K?
Fertilizer labeling regulations in the United States and many other countries require that phosphorus and potassium be declared as oxides (P2O5 and K2O). The elemental forms are lower: elemental P is about 43.6% of P2O5, and elemental K is about 83.0% of K2O. This calculator reads the label values directly and converts them for you.
What is a safe nitrogen ppm range for most plants?
Most vegetable and ornamental crops grow well at 100-200 ppm nitrogen. Seedlings do better at 50-75 ppm, and heavy feeders like tomatoes during fruiting can handle up to 250 ppm. Above 500 ppm, the risk of osmotic stress and leaf burn increases significantly. Always check crop-specific guidelines and adjust for your water quality and growing media.
Can I use this calculator for hydroponic systems?
Yes. The ppm-based calculation is the same whether you are feeding in soil, coco coir, rockwool, or deep water culture. For hydroponics, set your target nutrient based on a published feed chart for your crop and growth stage, then dissolve the calculated weight into your reservoir. Monitor EC after mixing and top up the reservoir with plain water between feedings to prevent salt accumulation.
How do I prepare a stock solution for an injector?
First find the dose rate in g/L from this calculator, then multiply it by the injector ratio. For a 1:100 injector and a dose of 0.1 g/L, the stock concentration is 10 g/L. Prepare that concentration in your stock tank and the injector will automatically dilute it to 0.1 g/L (200 ppm N for a 20% N fertilizer) in the water line. Keep calcium-containing fertilizers in a separate stock tank from sulfate or phosphate products.
What does the dose-rate (g/L or oz/gal) output mean?
This is the ratio of fertilizer to water that achieves the target concentration for one unit of volume. Once you know it, you can scale the recipe to any batch size without recalculating: multiply the dose rate by however many litres or gallons you need that day.