# Lawn fertilizer calculator

> Enter the numbers on the bag and your lawn size. See how many pounds of fertilizer to spread per 1,000 sq ft and in total, and how many bags you need.

Canonical: https://lucernel.com/tools/lawn-fertilizer-calculator
Last updated: 2026-09-26

Enter the three numbers on the bag, your lawn size and the rate of nitrogen you want. You get the pounds of product per 1,000 sq ft, the total, and how many bags to buy.

Free, no account. Everything runs in your browser. Nothing you type is sent or stored. Interactive version: https://lucernel.com/tools/lawn-fertilizer-calculator

## How it works

Lawn recommendations are given in pounds of nitrogen (N) per 1,000 square feet, but you spread product, not nitrogen. The first number on the bag is the percentage of nitrogen by weight, so you divide the rate you want by that percentage.

Example from Iowa State University Extension: to apply 1 lb of N per 1,000 sq ft with a 32-0-4 fertilizer, spread 1 ÷ 0.32 = 3.125 lb of product per 1,000 sq ft. On a 5,625 sq ft lawn that is 3.125 × 5,625 ÷ 1,000 = 17.58 lb.

The same product also carries phosphate (P₂O₅) and potash (K₂O) in the proportion of its second and third numbers; the tool shows how much of each you apply.

Formula: Product per 1,000 sq ft = N rate ÷ (N% ÷ 100). Total = product per 1,000 sq ft × lawn area ÷ 1,000.


## Good to know

- The right nitrogen rate depends on your grass, your region and the season. A soil test or your state extension office gives the best number.
- Read the bag: its label rate and spreader settings come first, and some states limit phosphorus on lawns.
- Keep fertilizer off driveways and sidewalks, and sweep it back onto the grass.

## Frequently asked questions

### How much fertilizer do I need for 1,000 square feet?

Divide the pounds of nitrogen you want by the nitrogen percentage on the bag. For 1 lb of N with a 24-0-10 product: 1 ÷ 0.24 = 4.17 lb of product per 1,000 sq ft.

### What do the three numbers on a fertilizer bag mean?

Nitrogen, phosphate (P₂O₅) and potash (K₂O), each as a percentage of the bag weight. A 50 lb bag of 24-0-10 holds 12 lb of nitrogen.

### How do I measure my lawn?

Split it into rectangles, multiply length by width for each one, and add them up. Leave out the house, driveway and flower beds.

## Lawnzo tells you when, not only how much.

Lawnzo builds a season plan for each lawn: when to fertilize, mow, seed and water, checked against the forecast, with a journal of what you did.

- Seasonal plans for your grass type
- Weather checks that keep, delay or skip a task
- A journal of products, quantities and photos

[Lawnzo](https://lucernel.com/apps/lawnzo)

## Sources (checked 2026-09-26)

- [Iowa State University Extension: How to calculate fertilizer rates for the lawn](https://yardandgarden.extension.iastate.edu/how-to/how-calculate-fertilizer-rates-lawn)
- [University of Missouri Extension: Calculating fertilizer rates](https://extension.missouri.edu/publications/mg10)