NPK Fertilizer Calculator – how much and which fertilizer to use
Too much nitrogen and your tomatoes grow leaves instead of fruit. Too little potassium and the fruit stays small and sour. Guessing at fertilizer is the most common garden mistake. This calculator works out how much of a specific fertilizer to apply to your bed, and which fertilizer fixes the deficiency your soil test found.
What the fertilizer calculator does
It answers two practical questions: how much of a given fertilizer to spread on a bed of a certain size, and which fertilizer to pick when a soil test shows a shortage of a particular nutrient. The database covers organic fertilizers (compost, manure, worm castings, wood ash) and mineral ones, each with real nutrient content and recommended rates.
Every fertilizer lists nitrogen (N), phosphorus (also converted to P₂O₅) and potassium (K and K₂O), plus calcium, magnesium and organic matter. So you see not just a label like "NPK 5-3-8" but the actual amount of pure nutrient that reaches the soil.
Converting a fertilizer rate to your bed area
Fertilizer rates are usually given per 100 m² or per m². The calculator multiplies the recommended rate by your bed area and shows the amount to weigh out, along with how much pure N, P and K that delivers.
| Fertilizer | Type | N | Recommended rate |
|---|---|---|---|
| Mature compost (>12 months) | organic | ~1.8% | 3–5 kg/m² |
| Farmyard manure | organic | ~0.5% | 3–4 kg/m² |
| Superphosphate | mineral | — | per P deficiency |
| Potassium sulfate | mineral | — | per K deficiency |
Phosphorus: P or P₂O₅?
Fertilizer bags usually state phosphorus and potassium as oxides (P₂O₅, K₂O), not as the pure element. 1% P₂O₅ is only ~0.44% actual phosphorus, and 1% K₂O is ~0.83% potassium. The calculator converts this for you so comparisons between products are fair.
Matching a fertilizer to a soil-test deficiency
If you have a soil analysis result, the calculator connects to it: for each deficiency (low potassium, acidic pH, low organic matter…) it suggests fertilizers that fix it and prioritises the ones that address several shortages at once. For acidic pH it points to carbonate or magnesium lime; for low organic matter, to compost, manure or worm castings.
- critical deficiency (very low) — top priority
- low level — fertilizing recommended this season
- optimal level — no extra fertilizing
- high level — reduce that nutrient
Organic or mineral fertilizer?
Organic fertilizers (compost, manure, worm castings) act slowly, improve soil structure and build organic matter, but their content varies. Mineral fertilizers act fast and are precise, but overused they salt the soil and rebuild no organic matter. In practice the best results come from combining the two: organic matter as the base of fertility, plus a mineral top-up for a specific seasonal deficiency.
Frequently asked questions
How much fertilizer do I need per m²?
It depends on the fertilizer. Mature compost is about 3–5 kg/m², manure 3–4 kg/m². Mineral fertilizers are dosed far more sparingly — enter your bed area and pick a fertilizer and the calculator gives the exact amount.
What does NPK on the bag mean?
The three numbers are the percentage of nitrogen (N), phosphorus as P₂O₅ and potassium as K₂O. So 5-10-5 means 5% N, 10% P₂O₅ and 5% K₂O. Nitrogen drives leafy growth, phosphorus roots and flowering, potassium fruiting and resilience.
Can I over-fertilize?
Yes. Excess nitrogen gives lush leaves at the expense of yield and invites disease, and excess mineral salts salt the soil and scorch roots. That is why the calculator shows the real amount of pure nutrient, not just the mass of product.
Is the calculator free?
Yes, completely. It runs in your browser and in the mobile app, with no sign-up, and works offline.
NPK Fertilizer Calculator – how much and which fertilizer to use
Every tool runs in your browser and in the mobile app — free, no sign-up, works offline too.
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