Lawn Fertilizer Calculator
Calculate how much fertilizer a lawn needs to deliver a target amount of nitrogen.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Convert a user-entered nitrogen target and the fertilizer label’s nitrogen percentage into product mass and whole-bag cost. This is not an application recommendation; use a soil test, local guidance and the product label.
Worked example
calculated with the default values below
Lawn area 5,000 sq ft · User-entered nutrient target 0.75 lb · Nitrogen (first bag number) 24 % · Bag weight 40 lb · Price per bag 45 $ Nitrogen required 3.75 lb · Fertilizer product needed 15.6 lb · Bags needed 1 · Cost 45.00 $ · Application rate 3.13 lb per 1,000 sq ft
General background references
Disclaimer
These are general gardening estimates. Planting windows, growth, water and nutrient needs vary by place and plant. Check local extension guidance and product labels before acting on an estimate.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
N=A÷1000×r; P=N÷(n÷100); bags=ceil(P÷w); cost=bags×price
- A
- lawn area (sq ft)
- r
- user-entered N target (lb/1,000 sq ft)
- n
- label N percentage
- N
- nitrogen needed (lb)
- P
- product needed (lb)
- w
- bag weight (lb)
The estimate charges for whole bags, not a prorated fraction. Use a soil test, local guidance and the fertilizer label to choose an application rate.
Frequently asked questions
Why do I divide nitrogen need by the bag's first number?
Fertiliser bags list nutrients as N-P-K percentages, and only the first number is nitrogen. To deliver 3.75 lb of nitrogen on a 5,000 sq ft lawn at 0.75 lb per 1,000 sq ft, divide by the 24% N figure, giving about 15.6 lb of product, under one 40 lb bag costing 45. The application rate works out to roughly 3.1 lb per 1,000 sq ft.