Fertiliser & Nutrient Calculator
Work out how much fertiliser to apply, and the nitrogen, phosphorus and potassium it delivers.
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total fertiliser needed
| Nitrogen (N) applied | – |
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| Phosphorus (P) applied | – |
| Potassium (K) applied | – |
Fertiliser and nutrient calculator
This calculator tells you how much fertiliser to apply over an area, and how much actual nitrogen (N), phosphorus (P), and potassium (K) that delivers based on the product’s NPK ratio. Enter your area, the recommended application rate, and the NPK percentages from the packaging to feed your lawn, beds, or crops correctly.
Understanding NPK
The three numbers on fertiliser packaging — for example 12-5-8 — are the percentages of nitrogen, phosphorus, and potassium by weight. Nitrogen drives leafy green growth, phosphorus supports roots and flowering, and potassium improves overall health and disease resistance. Knowing the actual amount of each nutrient applied helps you avoid over- or under-feeding.
Applying fertiliser correctly
Always follow the application rate on the product, as too much fertiliser can scorch plants and pollute waterways. Spread it evenly and water it in unless told otherwise. To measure your area accurately, use our area calculator, and to prepare beds, the garden soil calculator.
Frequently asked questions
What does a 12-5-8 fertiliser mean?
It contains 12% nitrogen, 5% phosphorus, and 8% potassium by weight. The rest is filler and other nutrients.
Disclaimer: General guidance only. Follow the manufacturer’s instructions for your specific product and plants.
How to calculate the nutrients you are applying
The three numbers on a fertiliser pack show the percentage by weight of nitrogen (N), phosphate (P) and potash (K). To find how much of each nutrient reaches the soil, multiply the application rate by the percentage. The formula is: nutrient (g/m²) = rate (g/m²) × percentage ÷ 100.
Example: a balanced 7-7-7 granular feed applied at 70 g/m² supplies 70 × 7 ÷ 100 = 4.9 g of nitrogen, 4.9 g of phosphate and 4.9 g of potash per square metre. For a 25 m² vegetable plot, you need 25 × 70 = 1,750 g, or 1.75 kg of product.
Working backwards from a nutrient target
Sometimes you know how much nitrogen you want rather than how much product to spread. Rearranging the formula gives: rate (g/m²) = target nutrient ÷ (percentage ÷ 100). To deliver 4 g of nitrogen per m² from a 20% nitrogen lawn feed, apply 4 ÷ 0.20 = 20 g/m². Always stay within the maximum rate printed on the pack, because over-feeding can scorch roots and grass.
Converting rates for US labels
US products give rates in pounds per 1,000 square feet. One gram per square metre is about 0.205 lb per 1,000 ft², so 70 g/m² is roughly 14.3 lb per 1,000 ft². US and UK labels both express phosphorus as P₂O₅ and potassium as K₂O. To convert to the pure elements, multiply P₂O₅ by 0.436 and K₂O by 0.830.
Typical NPK values of common feeds
| Product type | Approximate NPK | Main use |
|---|---|---|
| Balanced granular (Growmore type) | 7-7-7 | General base dressing |
| Blood, fish and bone | Around 5-5-5 | Slow-release organic feed |
| Pelleted chicken manure | Around 4-2-1 | Organic nitrogen boost |
| Sulphate of potash | 0-0-50 | Fruit and flowering |
| Liquid tomato feed | High K, e.g. 4-3-8 | Fruiting crops |
Organic products vary from batch to batch, so check the analysis on your own pack. Apply granular feeds to moist soil, rake or water them in, avoid spreading just before heavy rain, and follow the label’s guidance on keeping pets and children off treated areas.