Lawn Fertilizer Calculator
Calculate the exact amount of fertilizer to spread on your lawn to hit a target nitrogen rate, based on the NPK percentage printed on the bag.
m
m
g/m²
%
kg
Equivalent to 22.05 lb
- 1
Lawn area
10 × 8 = 80 - 2
Product needed (grams)
80 × 25 ÷ (20% ÷ 100) = 10,000Divide target N by the nitrogen fraction to find total fertilizer. - 3
Convert to kilograms
10,000 ÷ 1,000 = 10
How does this calculator work?
To hit a target nitrogen rate, multiply lawn area by the target N (g/m²) and divide by the nitrogen fraction (%N ÷ 100). With a 25 g/m² target on an 80 m² lawn using a 20% N bag, you need about 10 kg of product, or roughly one 10 kg bag.
Formula
How this is calculated
First the tool finds your lawn area by multiplying length by width (both in metres) to get square metres. You then set a target nitrogen rate in grams per square metre — a common maintenance rate is about 25 g/m², which is roughly 0.5 lb of nitrogen per 1000 sq ft.
Because fertilizer is only part nitrogen, the total product needed is the nitrogen you want divided by the fraction of nitrogen in the bag. The %N comes from the first number of the NPK label (for example, 20-5-10 means 20% nitrogen). So Product = Area × Target N ÷ (%N ÷ 100). The result is converted from grams to kilograms and pounds.
The number of bags is the product divided by the bag weight, rounded up to the next whole bag, since you cannot buy a partial bag. Because of that rounding, the "actual N applied" figure shows the slightly higher nitrogen rate you would get if you spread every full bag evenly. All distance inputs must be in the same unit (metres) and must be positive; a %N of zero is impossible since the bag would contain no nitrogen.
Frequently asked questions
A typical maintenance rate is around 25 g/m² per application (about 0.5 lb of nitrogen per 1000 sq ft). Cool-season grasses often get a few applications across the growing season; always follow the product label.
It is the first number in the NPK ratio on the bag. A 20-5-10 fertilizer contains 20% nitrogen, 5% phosphorus (as P₂O₅) and 10% potassium (as K₂O), so you would enter 20.
Bags are sold in whole units, so the tool rounds up to the nearest full bag. Spreading those whole bags over your lawn delivers a little more nitrogen than the exact target, which the actual-rate figure reflects.
Also known as
TG we-Calculate Editorial Team. (2026). Lawn Fertilizer Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/fertilizer-calculator
TG we-Calculate Editorial Team. "Lawn Fertilizer Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/fertilizer-calculator.
TG we-Calculate Editorial Team, "Lawn Fertilizer Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/fertilizer-calculator
@misc{wecalculate_fertilizer_calculator, title = {Lawn Fertilizer Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/fertilizer-calculator}}, year = {2026}, note = {TG we-Calculate} }
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