Lawn Fertilizer Calculator

Lawn Fertilizer formula

Nitrogen needed (lb) = Lawn area (sq ft) / 1,000 x rate (lb N per 1,000 sq ft)
Fertilizer (lb) = Nitrogen needed / (first bag number / 100)
Spreader rate (lb per 1,000 sq ft) = rate / (first bag number / 100)
Coverage per bag (sq ft) = Bag weight / spreader rate x 1,000

Worked example

A 120 x 60 ft lawn with an 800 sq ft side yard is 7,200 + 800 = 8,000 sq ft. For a fall feeding at 0.75 lb of nitrogen per 1,000 sq ft with a 29-0-4: 8,000 / 1,000 x 0.75 = 6 lb of nitrogen. 6 / 0.29 = 20.69 lb of product, spread at 0.75 / 0.29 = 2.59 lb per 1,000 sq ft.

A 45 lb bag holds 45 x 0.29 = 13.05 lb of nitrogen, enough for 13.05 / 0.75 = 17.4 thousand sq ft, so it covers 17,400 sq ft or about 2.2 applications on this lawn. Buy one bag and keep the rest for the next feeding.

Quick reference

How to calculate lawn fertilizer

Fertilizer recommendations are given in pounds of actual nitrogen per 1,000 square feet, not pounds of product, because every bag has a different strength. Measure the lawn, subtract the house, driveway and beds, and divide by 1,000. Multiply by the nitrogen rate you want, then divide by the first number on the bag as a decimal. A 5,000 square foot lawn at 1 lb of nitrogen with a 24-0-6 needs 5 / 0.24 = 20.8 lb of product.

Bag coverage claims assume the maker's rate, which is often 0.75 to 1 lb of nitrogen. If you are feeding lightly, at 0.5 lb, a bag goes twice as far. Weigh out the product for the whole lawn before you start so you can tell if the spreader is running heavy or light halfway through.

Rates, timing and spreader settings

Cool-season lawns (fescue, bluegrass, rye) get most of their nitrogen in fall, with a lighter feeding in spring and little or none in summer heat. Warm-season lawns (Bermuda, zoysia, St. Augustine) are fed from late spring through summer while they are growing. Stay at or under 1 lb of nitrogen per application, and 0.5 to 0.75 lb for quick-release products, which can burn at higher rates.

Spreader charts on the bag list a setting for the maker's rate on common spreaders. If your rate differs or the chart does not list your spreader, calibrate: set the spreader low, spread over a measured 1,000 square feet, and weigh what came out. Two passes at right angles with half the product each avoid stripes and doubled edges.

Key facts

• The three numbers on a fertilizer bag are the percent by weight of nitrogen, phosphate and potash, so a 50 lb bag of 10-10-10 holds 5 lb of each.
• One pound of actual nitrogen per 1,000 sq ft is the standard maximum for a single application of a quick-release fertilizer.
• Most lawns need 2 to 4 lb of nitrogen per 1,000 sq ft per year, split into 2 to 4 applications; fine fescue and shade lawns need the least.
• A 24-0-6 fertilizer goes on at 4.17 lb per 1,000 sq ft to deliver 1 lb of nitrogen, so a 40 lb bag covers 9,600 sq ft.
• Water in fertilizer with about a quarter inch of irrigation, and never apply to frozen ground or right before heavy rain.

Frequently asked questions

Sources and references

• •Calculations Used to Determine the Amount of Fertilizer Needed to Treat Turf (lb N per 1,000 sq ft divided by bag analysis) (Penn State Extension)
• •FS839: How to Calculate the Amount of Fertilizer Needed for Your Lawn (Rutgers NJAES)
• •Calculating Lawn Fertilizer Rates fact sheet (nitrogen per bag, coverage, spreader calibration) (University of New Hampshire Extension)
• •Fertilizing lawns (annual nitrogen rates and timing) (University of Minnesota Extension)