Gardening
Fertilizer Calculator
Enter lawn or bed area, application rate and bag size to see product needed and bags to buy.
How much fertilizer do I need?
Fertilizer labels quote an application rate — often grams per square metre or kilograms per 100 m². Enter your lawn or bed area and the rate in g/m² (kg/100 m² ÷ 100 = kg/m², or multiply kg/100 m² by 10 to get g/m²). The calculator multiplies area by rate, adds optional waste for overlap at edges, and rounds up to whole bags from the bag mass. Follow product NPK instructions and local rules near watercourses. After beds are filled, soil volume is handled by raised bed soil or garden soil; planting counts use plant population.
Worked example
100 m² at 25 g/m² = 2.5 kg; +5% ≈ 2.63 kg. A 10 kg bag covers that in a single bag with surplus.
Limits and assumptions
Does not convert liquid fertilizers by dilution. Always obey the maximum rate on the label — more is not better.
Frequently asked questions
What does NPK mean on the bag?
Nitrogen (N), phosphorus (P) and potassium (K) — the three main plant nutrients. This tool only works out how much product to buy from the application rate; it does not balance NPK ratios.
How do I enter kg per 100 m²?
Multiply by 10 for g/m² (e.g. 2.5 kg/100 m² → 25 g/m²) and use that rate.
Lawn vs beds?
Use the area you will actually treat. Paths do not get product.
Organic vs synthetic?
Quantity maths is the same; nutrient release and timing differ — read the pack.