Calculating actual nutrients from a fertiliser percentage
Quick answer
Multiply the weight of product by the declared percentage:
Actual nutrient = fertiliser quantity × nutrient percentage
Then, for phosphorus and potassium only, convert the oxide figure to the element: P₂O₅ → P × 0.436 and K₂O → K × 0.83. That second step is the one almost every consumer guide leaves out.
Why the percentage is a weight
FAO states the basis directly: for solid fertilizers, the percentage generally refers to the weight basis — 20 percent N means 20 kg of N in 100 kg of product. Everything else follows from that one sentence.
Liquids are the exception to watch. FAO notes that for liquid fertilizers both weight and volume percentages are used, and gives the example that 20 percent N by weight in a solution of specific gravity 1.3 corresponds to 26 percent by volume, that is 260 g of nitrogen per litre. If you treat a volume percentage as a weight percentage, your figure is wrong by about a third.
The method, step by step
- Read the analysis on the label. Note each nutrient and its percentage, and whether the product is solid or liquid.
- Fix the quantity you are actually handling — in grams or kilograms for a solid, litres for a liquid.
- Multiply. Quantity × percentage ÷ 100 = the amount of that nutrient as the label declares it.
- Convert the oxides. Multiply the P₂O₅ figure by 0.436 for phosphorus, and the K₂O figure by 0.83 for potassium. Nitrogen needs no conversion.
- Compare like with like. If you are checking a figure from research, remember research usually speaks in elements and labels speak in oxides.
Worked example 1 — a balanced product
500 g of 20-20-20:
- N: 500 × 20% = 100 g
- P₂O₅: 500 × 20% = 100 g → P = 100 × 0.436 = 43.6 g
- K₂O: 500 × 20% = 100 g → K = 100 × 0.83 = 83 g
Worked example 2 — a high-potassium compound
500 g of 12-12-17:
- N: 500 × 12% = 60 g
- P₂O₅: 500 × 12% = 60 g → P = 60 × 0.436 = 26.16 g
- K₂O: 500 × 17% = 85 g → K = 85 × 0.83 = 70.55 g
Both examples describe what is inside a given weight of product. Neither is a recommendation for any crop.
Reading a research figure against a label
FAO gives the conversion in the other direction too: an application rate reported in research as 26 kg P per hectare translates into 60 kg P₂O₅ per hectare. Same quantity, two conventions. Treat this as a warning about arithmetic, not as a rate for your garden — a hectare figure from a field-crop study has no bearing on a domestic planting.
| From | To | Multiply by |
|---|---|---|
| P₂O₅ | P | 0.436 |
| P | P₂O₅ | 2.29 |
| K₂O | K | 0.83 |
| K | K₂O | 1.20 |
| CaO | Ca | 0.715 |
| MgO | Mg | 0.603 |
Why this matters more on sandy soil
FAO lists low storage capacity for mineral nutrients in sandy soils among the problems of arid-region soils. On a soil with low cation exchange capacity, a soluble nutrient that is over-applied is not banked for later — much of it leaches past the root zone with the next irrigation. Knowing exactly how much nutrient a scoop contains is therefore not pedantry here; it is the difference between feeding the plant and feeding the water table.
General arid-region guidance, not a Kuwait measurement.
Common mistakes
- Skipping the oxide conversion and assuming the label figure is the element.
- Applying a liquid's volume percentage as if it were a weight percentage.
- Scaling a per-hectare research rate down to a pot by simple division.
- Calculating a nutrient amount and treating it as a recommended dose.
- Measuring by an unmarked scoop and assuming it holds a known weight.
This is arithmetic, not a dose
Everything on this page tells you what is in a quantity of product. None of it tells you what any plant should receive. That decision needs the crop and its growth stage, whether it is in ground or container, the area or number of plants, the frequency, the irrigation context, and a soil and water analysis — and the product label is the binding authority.
We found no Kuwait institutional source stating fertiliser rates, and we do not invent one.
Related glossary terms
Sources
- FAO — Fertilizer and Plant Nutrition Bulletin 16: Plant nutrition for food security. Roy, Finck, Blair & Tandon. FAO, Rome, 2006. ISBN 92-5-105490-8. Chapter 5 and the units-and-conversion-factors table. fao.org — accessed 26 August 2026.
Last reviewed: 27 August 2026.