Lawn Fertilizer Calculator
Work out how much fertilizer your lawn needs from its size and the target nitrogen rate — bags to buy and what it costs.
How it works
Nitrogen needed = (area ÷ 1000) × rate · Product = nitrogen ÷ (N% ÷ 100) · Bags = ⌈product ÷ bag weight⌉ · Cost = bags × price
Fertilizer is dosed by actual nitrogen, not bag weight, because the same 1 lb-per-1,000-sq-ft target takes very different amounts of product depending on the analysis. The bag’s first N-P-K number is the percent nitrogen, so a 26-0-3 bag is only 26% nitrogen by weight — you need roughly four pounds of it to deliver one pound of N. Dividing the nitrogen your lawn needs by that fraction gives the product weight, then rounding up to whole bags tells you what to actually buy.
Worked example
For a 5,000 sq ft lawn at 1 lb N per 1,000 sq ft you need 5 lb of nitrogen. With 26% fertilizer that is 5 ÷ 0.26 ≈ 19.2 lb of product. A 40 lb bag covers it, so buy 1 bag at $45.
Frequently asked questions
How do I read the N-P-K numbers?
The three numbers are the percentages by weight of nitrogen (N), phosphorus (P) and potassium (K). Only the first — nitrogen — drives this calculation, since lawn feeding rates are specified in pounds of N per 1,000 square feet.
How much nitrogen per application is safe?
A common guideline is no more than about 1 lb of quick-release nitrogen per 1,000 sq ft in a single application, with a season total of roughly 2–4 lb depending on grass type. Overapplying can burn turf and run off into waterways.
Why round up to whole bags?
You can only buy full bags, so the calculator rounds the number of bags up to make sure you have enough. Any surplus stores well and is useful for the next feeding.
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This calculator is for educational purposes. Double-check important results before acting on them.