Fertilizer calculator by area
The three numbers on a fertilizer bag are the N, P₂O₅ and K₂O percentages; this turns a nitrogen rate into product weight.
Loading the tool…
Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter area, nitrogen rate, n in fertilizer, p₂o₅ and k₂o.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Nitrogen needed (g) = area × rate (g N/m²). Fertilizer (g) = nitrogen ÷ (N % ÷ 100). P₂O₅ and K₂O supplied = fertilizer × their % ÷ 100.
Example
Inputs: Area = 100 m², Nitrogen rate = 5 g N/m², N in fertilizer = 20 %, P₂O₅ = 5 %, K₂O = 10 %. Result: Fertilizer to spread: 2.5 kg; Per square meter: 25 g/m²; Nitrogen supplied: 500 g.
Limits and caveats
- The right nitrogen rate depends on grass, soil and season; follow the label or a soil test.
- P and K are shown as oxides, as on the label, not as pure elements.
Questions about this tool
What do numbers like 20-5-10 mean?
The bag is 20% nitrogen, 5% P₂O₅ and 10% K₂O by weight, so a 10 kg bag holds 2 kg of nitrogen.
Why start from nitrogen?
Lawn feeding advice is usually written as nitrogen per area, and the product amount follows from that.
Should I spread it all at once?
Heavy doses can scorch grass; much guidance prefers splitting it into smaller feeds and watering in each time.