Calculator
Lawn seed and fertilizer calculator
This tool turns the rates printed on a seed bag and a fertilizer bag into the amount you need for your lawn. It does not choose a rate for you. You enter the numbers from your own labels, and it does the arithmetic. The result updates as you type, and nothing leaves your device.
20 lb of seed and 31.3 lb of fertilizer
Worked example (shown when scripts are off): a 5,000 square foot lawn is five units of 1,000 square feet. At 4 lb of seed per unit that is 20 lb. To put down 1 lb of nitrogen per unit with a 16 percent nitrogen product, each unit needs 1 divided by 0.16, or 6.25 lb of product, so 31.25 lb in total.
Before you spread
The label always wins. Seed rates differ by grass type and by whether you are starting a new lawn or overseeding, and fertilizer rates and timing depend on your grass, soil and region. A soil test and your state extension service give advice for your yard.
How the calculator works
Lawn products are labeled per 1,000 square feet, so the tool divides your area by 1,000 and multiplies by the rate. For seed, pounds needed equals area divided by 1,000, times the rate per 1,000 square feet.
Fertilizer needs one extra step. The percentage on a fertilizer bag is the share of the bag weight that is nitrogen. A 16 percent product has 0.16 lb of nitrogen per pound of product. To deliver your target nitrogen, the product needed is the target divided by the nitrogen fraction. The tool then multiplies by the number of thousands of square feet. It also shows how far one bag goes: bag weight times the nitrogen fraction, divided by the target, times 1,000.
Worked example
A 3,200 square foot lawn uses 3.2 units. Overseeding with a mix labeled at 3 lb per 1,000 square feet takes 9.6 lb of seed, so two 5 lb bags are enough. Fertilizing at 0.75 lb of nitrogen per 1,000 square feet with a 20 percent product takes 3.2 times 0.75 divided by 0.20, or 12 lb of product. A 15 lb bag would cover about 4,000 square feet at that rate, so one bag is enough with some left over.
Picking the numbers to enter
Read the seed bag. It lists a seeding rate for new lawns and often a lower one for overseeding. Cool-season and warm-season grasses differ, and so do the species within a mix. For nitrogen, state extension guidance for lawns is usually in the range of about 0.5 to 1 pound of nitrogen per 1,000 square feet in a single application, with several applications a year at most for some grasses and fewer for others. Many programs also advise slow-release forms and avoiding fertilizer before heavy rain. Use the amount recommended for your grass and region, not a general number.
Spreading well
Set the spreader as the product label specifies, since the correct setting depends on the machine. A common method is to apply half the amount in one direction and half at right angles to avoid stripes. Sweep any product off hard surfaces so it does not wash into storm drains. Some places restrict fertilizer use near water or at certain times, so check local rules.
Limits of the estimate
The tool assumes an evenly shaped lawn and exact coverage. Real lawns have beds, trees and edges, so measure the grass itself. It does not account for seed losses, wind, or slopes, and it cannot tell you whether your soil needs fertilizer at all. A soil test shows what is missing, and adding nutrients the soil already has costs money and can harm waterways.
Where this comes from
The method follows how seed and fertilizer labels are written, rates per 1,000 square feet and a nitrogen percentage by weight. The general nitrogen range reflects lawn care guidance from US university extension services and the Environmental Protection Agency advice on fertilizer runoff. Specific rates for your grass, soil and state may differ. This page is general information, not a fertilizer prescription.