Fertilizer Calculator
Lawn fertilizer is applied by a target amount of actual nitrogen per 1,000 square feet — not by product weight — because bags differ in strength. Enter your area, the nitrogen rate, and the bag's %N for the pounds of product to spread.
Fertilizer
5lb
- Actual nitrogen applied
- 1 lb N
- Lawn area
- 1,000 sq ft
How it’s calculated
Actual nitrogen needed = nitrogen rate × (area ÷ 1,000). Pounds of product = that ÷ (%N ÷ 100), where %N is the first of the three N-P-K numbers on the bag. A stronger bag (higher %N) needs fewer pounds for the same nitrogen.
Worked example
1,000 sq ft, a 1 lb N/1,000 rate, a 20-5-10 fertilizer (20% N): nitrogen = 1 × (1,000 ÷ 1,000) = 1 lb N; product = 1 ÷ 0.20 = 5 lb of fertilizer.
FAQs
- What do the three N-P-K numbers mean?
- They're the percentages by weight of nitrogen (N), phosphorus (as P₂O₅), and potassium (as K₂O). A 20-5-10 bag is 20% nitrogen, so 100 lb of it contains 20 lb of actual N. This calculator uses the first number to convert your nitrogen target into pounds of product.
- Why measure by nitrogen instead of by bag weight?
- Because the same 5 lb of two different products delivers very different amounts of nitrogen. Lawn-feeding rates are given in pounds of actual nitrogen per 1,000 sq ft so they're comparable across products — then you back out the product weight from the bag's %N.
- How much nitrogen is too much?
- Keep a single application of quick-release (water-soluble) nitrogen to about 1 lb of actual N per 1,000 sq ft — more can burn the lawn and run off into waterways. Slow- or controlled-release products feed over time without spiking the salt concentration, so they can be applied somewhat higher per feeding. Either way, spread the season's total over several feedings and follow the bag's and your local guidance.
Sources
- Product lb = (N rate × area ÷ 1,000) ÷ (%N ÷ 100). Standard turf-nutrition math; %N is read off the bag's N-P-K label (editable), not assumed.