This fertilizer calculator converts a bag’s N-P-K numbers and your lawn or garden’s size into the actual pounds of product to spread — the calculation that stands between a green lawn and a striped, burnt one.
The bag’s first number is a percentage: 24-0-11 is 24% nitrogen by weight, so every pound of product carries 0.24 lb of N. Exceeding label rates doesn’t green faster — it burns, feeds runoff and algal blooms downstream, and wastes money. A soil test every 2–3 years tells you whether the P and K numbers matter at all; many soils need nitrogen only.
Product = Nitrogen Needed ÷ Bag Percentage
Lawns eat nitrogen by the pound per 1,000 square feet; bags sell it diluted into a product that’s part nutrient, part carrier. The conversion — total product = (area ÷ 1,000 × target N rate) ÷ (N% ÷ 100) — is the whole science, and it works identically for garden beds (per-bed areas) and for any analysis from 46-0-0 urea to 10-10-10. Part of our Ecology Calculators collection.
Worked Example (Verified)
- 5,000 sq ft lawn, 1 lb N/1,000, bag 24-0-11 (defaults): N needed = 5 lb → product = 5 ÷ 0.24 = 20.8 lb (about half a 40 lb bag — spreader setting ≈ 4.2 lb per 1,000 sq ft)
- Same lawn, cheap 10-10-10: 5 ÷ 0.10 = 50 lb of product — weak analysis means hauling carrier, which is why pros buy high-N formulations for lawns
- Urea 46-0-0: 10.9 lb covers it — powerful and correspondingly unforgiving of overlap stripes
- Split applications beat single dumps: two half-rates a month apart feed steadier and burn less than one heroic dose
Application Discipline
- Calibrate the spreader on a driveway sweep: weigh product in, cover a measured 1,000 sq ft, weigh the remainder — the dial numbers are aspirations.
- Half-rate in two perpendicular passes = the standard cure for striping.
- Slow-release (polymer-coated / WIN nitrogen) forgives timing sins; quick-release rewards precision and punishes the day before a heat wave.
- Never fertilize frozen, drought-stressed or about-to-be-deluged ground — the nutrient should meet roots, not the storm drain.
Related Tools
- Lawn Seed Calculator — reseeding pairs with feeding
- Compost Calculator — the slow organic nitrogen route
- Ecology Calculators — the full collection
Frequently Asked Questions
How much fertilizer do I need for 5,000 square feet?
It depends on the target rate and the bag: at 1 lb N per 1,000 sq ft you need 5 lb of actual nitrogen, which is 20.8 lb of a 24% product, 50 lb of 10-10-10, or 10.9 lb of urea (46-0-0). This calculator does that division for any bag analysis.
What do the three numbers on a fertilizer bag mean?
N-P-K: percentages by weight of nitrogen, phosphate (P₂O₅) and potash (K₂O). A 40 lb bag of 24-0-11 holds 9.6 lb of nitrogen, none of phosphate, and 4.4 lb of potash. The nitrogen number drives lawn math because grass consumes N fastest.
How much nitrogen does a lawn need per year?
Common guidance: 2–4 lb N per 1,000 sq ft per year for cool-season lawns (split into 2–4 applications), 3–5 for warm-season grasses in long seasons, less for low-input or shaded lawns. One application shouldn’t exceed ~1 lb of quick-release N per 1,000.
Can I just use 10-10-10 for everything?
You can, but soils accumulate the phosphorus they don’t need (many are already P-rich from years of balanced fertilizer), and excess P is the runoff villain. A soil test reveals what your ground actually lacks — frequently nitrogen alone, with P and K already abundant.
Organic vs synthetic fertilizer amounts?
The math is identical (percentage is percentage), but organic products run low analyses (e.g. 5-2-0) so product weights balloon, and their nitrogen releases as microbes work — temperature-dependent and slow. The calculator gives the total product weight either way; timing expectations differ.
Is this fertilizer calculator free?
Yes — free, instant, no sign-up. Always follow the bag’s label and local rules (some regions restrict phosphorus and winter applications).