Calculate SimplyHow to use
Start with the worked example to understand the inputs, then replace every relevant value with your own figures. Use one currency and consistent units throughout. Add or remove rows where offered; irrelevant conditional fields are disabled and excluded. Select Calculate after editing. Review negative values and any “not determined” result rather than assuming every scenario must be profitable. Download a text summary or print the page for your records.
Formula and assumptions
Use a water or solution test for the baseline of one selected element. Enter a dry fertilizer’s guaranteed N–P2O5–K2O analysis, not a product’s vague “strength” label. Phosphate converts to elemental P using 0.4364, and potash to elemental K using 0.8301. For dilute water, ppm is treated as mg/L. Required grams equal the positive ppm deficit times liters divided by 1000 and the selected elemental mass fraction. The result also shows other NPK contributions, which may oversupply another element. This is one-blend arithmetic, not a complete nutrient recipe. EC and pH cannot be predicted from NPK alone; measure them and follow product instructions.
grams = max(target ppm − baseline ppm,0) × liters / (1000 × elemental mass fraction); P = P2O5 × 0.4364; K = K2O × 0.8301
Worked example
Example only: replace rates with your own figures.
Inputs
- Reservoir volume unit: L
- Reservoir water volume: 100
- Target nutrient element: N
- Target elemental concentration: 150 ppm
- Measured baseline for selected element: 20 ppm
- Label nitrogen N: 15.5 %
- Label phosphate P2O5: 0 %
- Label potash K2O: 0 %
- Dry fertilizer mass: 83.871 g
- Added nitrogen N: 130 ppm
- Added phosphorus P: 0 ppm
- Added potassium K: 0 ppm
- Final selected-element concentration: 150 ppm
Frequently asked questions
Are the example rates current market prices?
No. Example inputs demonstrate arithmetic, not live prices, tax rules, salary surveys or sponsor benchmarks. Enter a dated quote, bill, manufacturer specification or verified fee schedule that applies to your situation. Compare a conservative and an optimistic scenario separately.
Why is a result not determined?
Some calculations have no meaningful value when a denominator is zero, savings are non-positive, a target cannot be reached or your data do not bracket an intersection. The tool leaves that result undetermined instead of displaying infinity or inventing a recommendation. Check the method and your inputs.
Sources
Sources explain the underlying concepts, not an endorsement of this website or its results. Method review: 20 October 2026.