What a lawn fertilizer calculation is for
Bagged lawn fertilizer is sold by weight, but grass needs a specific amount of nutrient, most importantly nitrogen. Three numbers on the bag, such as 24-6-12, give the percentage by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). A bag of 24-6-12 is 24% nitrogen, so a 20-pound bag contains only 4.8 pounds of actual nitrogen. Working out how many pounds of product to spread on a given area is the calculation this page performs.
Use it before you buy or spread: enter your lawn area in square feet, choose the analysis printed on your bag, and select a season. The calculator assumes an application rate of 1 pound of actual nitrogen per 1,000 square feet, a widely used single-application target, and returns the pounds of product per application, the total for the season, and the phosphate and potash delivered. Always check the label of your product and your local extension service, since rates and timing vary by grass type and region.
Formula and variables
- N target = area ÷ 1,000 × 1 lb. This is the pounds of actual nitrogen you want to apply.
- Product per application = N target ÷ (N% ÷ 100). A higher nitrogen percentage means less product to reach the same nitrogen amount.
- P2O5 and K2O supplied = product weight × their percentages ÷ 100.
- Season total = product per application × number of applications. The calculator assumes 2 in spring, 3 in summer, 2 in fall and none in winter; adjust this to your own lawn program.
Worked example
A 5,000 sq ft lawn will be fed in spring with a 24-6-12 fertilizer. The nitrogen target is 5,000 / 1,000 × 1 = 5 lb N. Because the bag is 24% nitrogen, product per application = 5 / 0.24 = 20.83 lb, shown as 20.8 lbs. Spring uses 2 applications, so the season total is 41.7 lbs. Each application supplies 5.00 lb N, 20.83 × 0.06 = 1.25 lb P2O5, and 20.83 × 0.12 = 2.50 lb K2O. That is exactly what the calculator prints for these inputs.
Common mistakes and how to interpret the result
- Using bag weight as nutrient weight. A 50 lb bag of 10-10-10 holds only 5 lb of nitrogen. Always work from the percentage.
- Measuring the whole property. Use only the turf area, excluding house, driveway, beds and patios, or you will over-buy.
- Applying the season total at once. The total is split across several applications. Spreading it all at one time can burn grass.
- Ignoring the product label. Slow-release and quick-release products, and cool-season versus warm-season grasses, have different recommended rates and timing.