About the Fertilizer Calculator
This fertilizer calculator converts a recommendation such as "apply 1 lb of nitrogen per 1,000 sq ft" into the actual amount of product to put in your spreader. Enter your lawn or garden area, the three N-P-K numbers printed on the bag, the nutrient you are targeting and the rate, and it returns the pounds of fertilizer needed, the number of bags and the cost.
The three numbers on a fertilizer label are the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 29-0-4 contains 14.5 lb of nitrogen. Lawn programs and soil-test reports are almost always written in pounds of nutrient per 1,000 sq ft, so this conversion is the step most people get wrong.
Typical cool-season lawns receive about 0.5–1 lb of nitrogen per 1,000 sq ft per application. Always follow your soil test, local regulations on phosphorus and the label, and avoid applying before heavy rain.
With the default inputs, the fertilizer to apply is 17.2 lb. Change any value above to recalculate instantly.
How to use the fertilizer calculator
- 1Measure the area you want to fertilize in square feet.
- 2Enter the three N-P-K numbers from the fertilizer bag.
- 3Choose the nutrient your soil test or lawn program targets and the rate per 1,000 sq ft.
- 4Enter the bag weight and price.
- 5Calibrate your spreader to the pounds-per-1,000 sq ft figure and apply.
Formula and method
The grade on a fertilizer bag gives the percentage by weight of each nutrient. To deliver a target rate of nutrient per 1,000 sq ft, divide the rate by the nutrient fraction to get pounds of product per 1,000 sq ft, then scale by your area. For example, 1 lb of N from a 29% nitrogen product needs 1 ÷ 0.29 = 3.45 lb of product per 1,000 sq ft.
Once the product weight is known, the pounds of every other nutrient applied follow from the same percentages. Bags are rounded up; the cost of product used is the fraction of bags actually spread times the bag price.
- Rate
- Target pounds of nutrient per 1,000 sq ft
- Nutrient %
- N, P₂O₅ or K₂O percentage from the bag grade
- Area
- Area to fertilize in square feet
Worked examples
5,000 sq ft lawn, 29-0-4, 1 lb N
To apply 1 lb of nitrogen per 1,000 sq ft from a 29% N product you need 1 ÷ 0.29 = 3.45 lb of product per 1,000 sq ft. Over 5,000 sq ft that is about 17.2 lb, delivering 5 lb of nitrogen and 0.69 lb of potash — well under one 50 lb bag.
Balanced 10-10-10 on a 3,000 sq ft garden
At 10% nitrogen you need 10 lb of product per 1,000 sq ft to supply 1 lb of N, so 3,000 sq ft needs 30 lb. Because the grade is balanced it also supplies 3 lb each of phosphate and potash.
Phosphate from triple superphosphate (0-46-0)
A soil test calls for 1 lb of P₂O₅ per 1,000 sq ft. With a 46% phosphate product that is 1 ÷ 0.46 = 2.17 lb per 1,000 sq ft, or about 4.35 lb for 2,000 sq ft, supplying 2 lb of phosphate.
Frequently asked questions
What do the three numbers on fertilizer mean?+
They are the N-P-K grade: the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 10-10-10 contains 5 lb of each nutrient; the rest is filler and other elements.
How much nitrogen does a lawn need?+
Many extension services recommend about 0.5–1 lb of actual nitrogen per 1,000 sq ft per application, and roughly 2–4 lb per year for cool-season grasses depending on grass type and whether clippings are returned.
How many pounds of fertilizer per 1,000 square feet?+
Divide your target nutrient rate by the nutrient percentage. For 1 lb of nitrogen: a 20% N product needs 5 lb, a 29% product needs 3.45 lb and a 46% product (urea) needs 2.17 lb per 1,000 sq ft.
Can I apply too much fertilizer?+
Yes. Excess quick-release nitrogen can burn grass, and surplus nitrogen and phosphorus wash into waterways. Apply no more than the label rate, sweep granules off paved surfaces and skip applications before heavy rain.
Why does my soil test give P₂O₅ and K₂O instead of P and K?+
Fertilizer grades and soil-test recommendations use the oxide forms by convention. As long as both are in P₂O₅ and K₂O, you can plug the numbers straight into this calculator.