Lawn Fertilizer Calculator
Work out how many pounds of fertilizer product to spread to hit a target nitrogen rate per 1000 sq ft.
Never exceed about 1 lb of nitrogen per 1000 sq ft in a single application, more than that can burn the lawn. Water it in after spreading, and sweep any granules off driveways and sidewalks so they don't wash into storm drains.
How it works
Fertilizer labels list three numbers, the N-P-K, and the first one is the percentage of nitrogen by weight in the bag. Agronomists generally recommend applying around 1 lb of actual nitrogen per 1,000 sq ft per feeding (sometimes less, depending on season and grass type). Since the bag isn't pure nitrogen, you need more of the product itself to deliver that 1 lb, and how much more depends on the N percentage on the label.
Worked example: a 24-0-8 fertilizer, 5,000 sq ft of lawn, targeting the standard 1 lb of nitrogen per 1,000 sq ft. Since the bag is 24% nitrogen, it takes 1 / 0.24, or about 4.17 lbs of product, to deliver 1 lb of nitrogen per 1,000 sq ft. Scaled up to 5,000 sq ft, that's 20.8 lbs of product to spread across the whole lawn.
FAQ
Why not just apply more to be safe?
More nitrogen than the lawn can use doesn't make the grass greener, it burns the blades, encourages disease and thatch, and a chunk of it runs off into storm drains rather than the roots. Staying at or under 1 lb of nitrogen per 1,000 sq ft per application is the standard agronomy target for a reason.
How do I read the N-P-K numbers on a bag?
The three numbers are nitrogen, phosphorus, and potassium, in that order, as a percentage of the bag's total weight. A 24-0-8 bag is 24% nitrogen, 0% phosphorus, and 8% potassium. Only the first number, the N%, feeds into this calculator.
Should I split the application into a smaller amount over more visits?
Splitting a season's total nitrogen into two or three lighter feedings, each still under 1 lb per 1,000 sq ft, generally gives steadier growth and less risk of burn than one heavy application. Use this calculator per visit with whatever target you've settled on for that feeding.
Does the calculator account for slow-release vs. fast-release nitrogen?
No, it only converts your target nitrogen rate into a product weight. Slow-release products feed the lawn over several weeks and are more forgiving if you're slightly over, while fast-release (water-soluble) nitrogen acts quickly and burns more easily, so stick closer to label rates with those.
For the full feeding picture, see understanding lawn fertilizer numbers, a simple seasonal fertilizing schedule, and organic vs. synthetic fertilizer.