Fertilizer Calculator
Convert a target nitrogen rate into how much fertilizer to spread, using the first N-P-K number on the bag, for any lawn or bed area. Calculates in your browser.
Product to apply
-
Show the math
The first N-P-K number is the percent nitrogen by weight, so:
product = (N rate × area ÷ 1,000) ÷ (N% ÷ 100)
The 1 lb of nitrogen per 1,000 sq ft default is only a starting value: use the rate on your product label or your local extension office’s recommendation for your grass and season. Don’t exceed label rates; over-application burns plants and runs off. All math runs locally in your browser.
What this does
A fertilizer calculator converts a target nitrogen rate into how much fertilizer to spread, using the first N-P-K number on the bag, for any lawn or bed area.
How to use it
- Enter the area to feed.
- Enter the target nitrogen rate.
- Enter the first N-P-K number.
- Read the pounds of fertilizer.
How it works
The first N-P-K number is the percent nitrogen by weight. Working from actual nitrogen lets you hit a consistent rate whatever the analysis.
fertilizer = N needed ÷ (N% ÷ 100)
Understanding your result
Working from actual nitrogen, rather than the bag weight, lets you hit a consistent rate whatever the analysis. Spreading more than the target risks burning foliage and polluting runoff, so weigh the amount rather than guessing, and split heavy rates into two lighter applications.
Example
For 1 lb of N per 1,000 sq ft from a 20-5-10 bag, spread 5 lb per 1,000 sq ft.
Last updated .
Frequently asked questions
What do the N-P-K numbers mean?
They are the percent by weight of nitrogen, phosphorus, and potassium. This tool uses the first number, nitrogen, to size the application.
What rate should I apply?
Use the rate on the product label or your local extension office’s recommendation for your grass and season. The tool’s 1 lb of nitrogen per 1,000 sq ft is only a starting value to replace.
Why not just use the bag rate?
Bag rates assume one product. Working from actual nitrogen lets you compare fertilizers and hit a consistent rate whatever the analysis.
Is anything stored?
No. Everything is calculated locally in your browser.
Related tools
Garden Soil Calculator
Work out how much soil a raised bed or several beds need from their size and fill depth, split into compost and topsoil, with bags and cubic yards. Calculates in your browser.
Grass Seed Calculator
Find how much grass seed a lawn needs at your seeding rate, or how many sod rolls and pallets cover the same area, from the square footage. Calculates in your browser.
Plant Spacing Calculator
Work out how many plants a bed needs from its size and your plant and row spacing, for a square grid or a denser triangular layout. Calculates in your browser.
Frost Date Calculator
Turn your local last spring and first fall frost dates into a growing-season length and a planting schedule for starting seeds indoors and transplanting out. Calculates in your browser.