Fertilizer Calculator
Calculate how much fertilizer to apply to hit a target nutrient rate. Enter your area, the lb of nutrient per 1,000 sq ft, and the bag's N-P-K percentage.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Find how much fertilizer product to apply to deliver a target amount of nutrient (usually nitrogen) per 1,000 sq ft.
Show the formula & steps
How the Fertilizer Calculator Works
Fertilizer bags list a target dose as pounds of nutrient per 1,000 square feet, but you spread the product, not pure nutrient. This calculator converts a nutrient target into the actual weight of fertilizer to apply, using the N-P-K percentage printed on the bag.
Enter the area you are treating, the target nutrient rate, and the relevant nutrient percentage. The tool returns the total nutrient needed, the fertilizer product to apply, and the rate per 1,000 sq ft so you can calibrate your spreader.
The Formula
Nutrient needed (lb) = target rate × area ÷ 1,000 Fertilizer (lb) = nutrient needed ÷ (nutrient % ÷ 100)
The first step scales the per-1,000 rate up to your whole area. The second step accounts for the fact that only part of the bag is the nutrient you want.
Worked Example
You want to apply 1 lb of nitrogen per 1,000 sq ft to a 5,000 sq ft lawn using a 20-10-10 fertilizer (20 percent nitrogen):
- Nutrient needed = 1 × 5,000 ÷ 1,000 = 5 lb of nitrogen
- Fertilizer = 5 ÷ 0.20 = 25 lb of product
So you would spread 25 lb of the 20-10-10 bag across the 5,000 sq ft lawn, which works out to 5 lb of product per 1,000 sq ft.
Product Needed per 1 lb of Nitrogen
The lower the nutrient percentage, the more product you spread to deliver the same nutrient:
| Nitrogen % in bag | Product per 1 lb N (per 1,000 sq ft) |
|---|---|
| 10% | 10.0 lb |
| 16% | 6.25 lb |
| 20% | 5.0 lb |
| 25% | 4.0 lb |
| 30% | 3.33 lb |
| 46% (urea) | 2.17 lb |
Tips for Accurate Application
- Match the nutrient to the number. Use the first N-P-K figure for nitrogen, the second for phosphate, the third for potash.
- Calibrate your spreader. Convert the total product to the rate per 1,000 sq ft and set the spreader to deliver that amount evenly.
- Split heavy feedings. Applying the full season’s nitrogen at once can burn plants; spread it across several applications.
- Round to a sensible amount. Buy slightly more than calculated to cover edges and refills, and store any excess dry and sealed.
Frequently asked questions
How much fertilizer do I need per 1,000 square feet?+
Divide the target nutrient rate by the nutrient fraction in the bag. To apply 1 lb of nitrogen per 1,000 sq ft using a 20 percent nitrogen product, you need 1 ÷ 0.20 = 5 lb of product per 1,000 sq ft. Multiply by the number of thousands of square feet to get the total.
What do the three N-P-K numbers mean?+
They are the percentages by weight of nitrogen (N), available phosphate (P2O5), and soluble potash (K2O). A 20-10-10 bag is 20 percent nitrogen, 10 percent phosphate, and 10 percent potash. Use whichever percentage matches the nutrient you are targeting.
How do I convert pounds of nutrient to pounds of product?+
Divide the pounds of nutrient by the nutrient percentage written as a decimal. For 2 lb of nitrogen from a 25 percent nitrogen fertilizer, that is 2 ÷ 0.25 = 8 lb of product. The lower the percentage, the more product you must spread.
What is a typical nitrogen application rate for lawns?+
Most cool-season and warm-season lawns want about 0.5 to 1 lb of nitrogen per 1,000 sq ft per application, and roughly 2 to 4 lb total per year split across several feedings. Always follow the product label and local guidance, since over-application can burn turf and pollute runoff.
Can I use this calculator for phosphate or potassium?+
Yes. Enter the target rate for that nutrient and use the matching percentage from the N-P-K analysis: the second number for phosphate and the third for potash. The math is identical no matter which of the three nutrients you are dosing.