Calculate fertilizer amount based on area and NPK ratios.
The fertilizer calculator turns an application target like "1 lb of nitrogen per 1,000 ft²" into the actual weight of product to spread, using the NPK percentage printed on the bag. Granular fertilizer is never pure nutrient — a 20-5-10 bag is only 20% nitrogen by weight — so spreading the bag weight as if it were nutrient weight under-feeds, while ignoring the percentage entirely over-applies and risks burning turf. Enter your area in square feet, the nutrient target per 1,000 ft², and the first NPK number of your product to get pounds of product needed.
Two steps. First, nutrient needed (lb) = target rate per 1,000 ft² × area ÷ 1,000. Second, product weight = nutrient needed ÷ (N% ÷ 100). The N percentage is the first number in the NPK code: 20-5-10 means 20% nitrogen, 5% phosphate (P₂O₅), and 10% potash (K₂O) by weight. The same arithmetic works for phosphorus or potassium targets — just substitute the second or third bag number as the percentage.
Target 1 lb N per 1,000 ft² with a 20-5-10 turf fertilizer: nutrient needed is 5 lb of N, and 5 ÷ 0.20 = 25 lb of product. That is half of a typical 50 lb bag across the whole lawn.
A 500 ft² garden plot at 1 lb N per 1,000 ft² needs 0.5 lb of N, which is 5 lb of 10-10-10. Because the product is balanced, the same pass also delivers 0.5 lb each of phosphate and potash.
For 2,000 ft² at a reduced 0.5 lb N per 1,000 ft², you need 1 lb of N — only 2.17 lb of urea. High-analysis products like urea need careful, even spreading because so little material carries the full dose.
Percent by weight of nitrogen (N), phosphate (P₂O₅), and potash (K₂O). A 50 lb bag of 20-5-10 contains 10 lb N, 2.5 lb phosphate, and 5 lb potash; the remaining weight is carrier material.
Around 1 lb of actual N per 1,000 ft² per application is the common turf guideline, with lighter 0.5 lb doses in summer heat. The calculator accepts any rate, so you can follow a soil-test recommendation directly.
Yes. The math is identical — divide the needed nutrient weight by the bag percentage. For a 1 lb K₂O target with 20-5-10, use the third number: 1 ÷ 0.10 = 10 lb of product per 1,000 ft².
It returns both the pure nutrient weight your area needs and the product weight to spread. The product figure is always larger because the nutrient is only a fraction of the bag — that is the number to set your spreader by.