Garden Fertilizer Dosage Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/6/2026
The total fertilizer dose is calculated with area (m²) × recommended dosage (g/m²) ÷ 1000, giving a result in kilograms. For a 100 m² lawn with a dosage of 30 g/m², you need 3 kg of fertilizer, or 0.6 of a 5 kg bag (so one full bag in practice).
Explanation
A fertilizer's dosage is always expressed by the manufacturer in grams per square meter (g/m²), a value that varies enormously depending on the type of fertilizer (nitrogen-based, complete NPK, organic), the target crop (lawn, vegetable garden, fruit trees), and the season of application — there is no universal value, only the product label is authoritative. This calculator simply converts that dosage, once known, into a usable total quantity: the area to treat multiplied by the dosage gives the total mass needed in grams, converted to kilograms, then divided by the bag size to know how many bags to buy. It's helpful to first precisely calculate the area involved with our area calculator before using this one, especially for an irregularly shaped garden broken down into several rectangles. Over-dosing isn't harmless: beyond the recommended dose, excess fertilizer can burn roots, favor leaf growth at the expense of fruit or flowers, or run off into surface water during rain — which is why it's worth calculating the dose precisely rather than spreading it "by eye".
Example: fertilizing a 100 m² lawn
Inputs
Area: 100 m². Recommended dosage: 30 g/m². Bag size: 5 kg.
Calculation
Total dose = (100 × 30) ÷ 1000 = 3 kg. Number of bags = 3 ÷ 5 = 0.6.
Result
You need 3 kg of fertilizer in total, a bit less than one 5 kg bag — a single bag is plenty, with some left over for a future application.
Frequently asked questions
Where can I find the recommended dosage in g/m²?
It's always shown on the product packaging, usually as "X g/m²" or "X kg for Y m²" (in which case, convert by dividing the kg by the m² then multiplying by 1000). This dosage was set by the manufacturer specifically for this product: never reuse another fertilizer's dosage, even for an apparently similar use.
What happens if I spread too much fertilizer?
Over-dosing can burn the roots and foliage ("fertilizer burn"), unbalance the plant's growth at the expense of flowering or fruiting, and run off into surface water during the next rain, contributing to waterway pollution. It's better to slightly under-dose and repeat the application than to over-dose in a single pass.
Should I water after spreading fertilizer?
For most solid fertilizers, light watering after spreading helps dissolve the granules and carry the nutrients down to the roots, without leaching the fertilizer too deep. See our rainwater harvesting calculator to estimate how much water is available for garden watering.