Lawn Fertilizer Rate Calculator

The calculator defaults to 1 lb of nitrogen per 1,000 sq ft as a common planning rate. The correct amount depends on grass species, season, soil test, product release profile, irrigation, and local restrictions. Never exceed the product label or local extension recommendation.

A US fertilizer bag prints three numbers like 24-0-11 right on the label. The first number is the percent of nitrogen (N) by weight, the second is phosphorus reported as P2O5 (phosphate), and the third is potassium reported as K2O (potash). A 50 lb bag of 24-0-11 contains 12 lb of nitrogen (50 x 24 percent), 0 lb of phosphate, and 5.5 lb of potash. The calculator uses the same math: enter the three label numbers, the lawn area, and your target N rate, and it returns the pounds of bagged product you need.

There is no universal four-application schedule. Timing and annual nitrogen depend on grass species, region, soil, product, and local nutrient rules. Use a soil test and your local extension service's current calendar, then enter the recommended rate in the calculator.

Excess phosphorus can run off into waterways and contribute to algae growth. Some states and localities restrict phosphorus fertilizer on established turf, with exceptions that vary by jurisdiction. The calculator does not apply those rules. Check your current local requirements, the product label, and a soil test before choosing a product.

Calibrate any new spreader before the first real pass. Measure off a 100 sq ft test patch (10 ft x 10 ft). Compute the fertilizer that patch should receive from the calculator (for example, 5,000 sq ft at 1 lb N divided by 24 percent N gives 20.8 lb of product, or 0.42 lb per 100 sq ft). Weigh out exactly that amount, load it in the spreader at the manufacturer's suggested setting, and walk the test patch at a steady pace. If product is left in the hopper, the setting is too low; if you ran out before finishing, the setting is too high. Adjust one notch at a time and retest. Most lawn fertilizer bags also print a starting setting for major brand spreaders (Scotts, Earthway, Spyker, Lesco) on the back of the bag, which is a good first guess.