Fertilizer Calculator
Buying fertilizer by guesswork wastes money and risks burning your lawn. The number that matters is actual nitrogen, not bag size — a 50 lb bag of 10-10-10 only contains 5 lbs of N, while a 50 lb bag of 30-0-4 contains 15 lbs. Enter your area, your target nitrogen rate, and your product's N-P-K ratio to find exactly how many bags you need per application and what it costs across the whole season.
Lawn & Product Details
The Fertilizer Math (Why Bag Size Lies)
The amount of product you must spread is driven entirely by how much actual nitrogen your grass needs, divided by the nitrogen fraction of the bag:
Product (lb) = (Area ÷ 1,000) × N-rate × (100 ÷ N%)
Two bags of identical weight can contain wildly different amounts of N. A 40 lb bag of 10-10-10 holds 4 lb of N; a 40 lb bag of 29-0-4 holds 11.6 lb. Spreading "one bag per acre" therefore feeds the second lawn almost three times as much. Always back-solve from the N number, never from the bag.
Worked Example: A 5,000 sq ft Lawn
You have 5,000 sq ft of tall fescue and want the standard 1 lb N per 1,000 sq ft, four times a year. That's 5 lb of actual nitrogen per application. With a 20-5-10 product (20% N), you need 5 ÷ 0.20 = 25 lb of product. A 50 lb bag covers two applications, so you buy 1 bag per application, or 4 bags a year at $25 each = $100/season. Switch to a 30-0-4 product and the same 5 lb N needs only 16.7 lb of product — under half a bag per app — cutting your annual spend toward $65 for the same feeding.
Don't Ignore Phosphorus and Potassium
The P and K in N-P-K matter, but most established lawns already have enough in the soil. A soil test is the only way to know. If your test shows adequate P and K, buying a "balanced" 10-10-10 just pays for nutrients your grass won't use — a straight or high-first-number product (like 30-0-4 or 46-0-0 urea) delivers the N you actually need far more cheaply. Over-applying phosphorus also fuels algal blooms in nearby water.
Real-World Case Study: Fescue Lawn Rescue
A 8,000 sq ft tall fescue lawn in Zone 7 was thinning from under-fertilizing. A soil test showed adequate P and K but low N. Using a 30-0-4 product at 1 lb N per 1,000 sq ft per application, 4 times a year: 8 × 1 = 8 lb N per app, needing 8 ÷ 0.30 = 26.7 lb of product. A 40 lb bag at $28 covered 1.5 applications — so 3 bags per season = $84/year. Compared to the old 10-10-10 approach (80 lb per application at $32, four times = $128/year), the targeted high-N product saved $44/year and the lawn thickened noticeably within two months.
Key takeaway: A soil test + right N-P-K formula eliminates waste. Straight nitrogen is almost always cheaper per pound of actual N than balanced fertilizers for established lawns.
Fertilizer Product Comparison: Same N, Different Cost
All products below deliver 5 lb of actual nitrogen per application to a 5,000 sq ft lawn. The cost per pound of N varies dramatically by product type.
| Product | N-P-K | Product Needed | Typical Price (50 lb) | Cost per App | Cost per lb N |
|---|---|---|---|---|---|
| Urea (46-0-0) | 46-0-0 | 10.9 lb | $22 | $4.83 | $0.97 |
| Ammonium Sulfate (21-0-0) | 21-0-0 | 23.8 lb | $20 | $9.52 | $1.90 |
| Balanced (20-5-10) | 20-5-10 | 25.0 lb | $25 | $12.50 | $2.50 |
| Starter (18-24-12) | 18-24-12 | 27.8 lb | $30 | $16.67 | $3.33 |
Note: Starter blends are worth the premium when establishing new seed (young roots need P), but for established lawns they waste money on nutrients the soil already has.
Frequently Asked Questions
What does the N-P-K number on fertilizer mean?
It's the percentage by weight of nitrogen, phosphorus (P₂O₅), and potassium (K₂O). Only the first number drives how much product you need to hit a target N rate.
How much nitrogen does a lawn actually need?
About 1 lb of actual N per 1,000 sq ft per application for cool-season grass, 3–4 times a year. Over 1.5 lb per pass risks burn.
How do I convert square feet to acres?
One acre = 43,560 sq ft. The calculator converts automatically when you pick "acres."
Can you apply too much fertilizer?
Yes — it burns grass, spreads disease, and runs off into waterways. A soil test every 2–3 years prevents over-applying P and K.
Is liquid or granular cheaper per pound of N?
Granular is usually cheaper per pound of actual N and spreads more evenly; liquid costs more but tank-mixes with herbicides. Compare price ÷ (bag weight × N%).