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Fertilizer Calculator
Calculate how much fertilizer your lawn or garden needs by N-P-K rate. Pounds per area, bags required, and approximate cost.
Area, fertilizer, and application
Fertilizer per application
25 lb
Covers 5,000 sq ft
- Nitrogen (N)
- 5 lb
- Phosphorus (P)
- 1.25 lb
- Potassium (K)
- 2.5 lb
- Bags needed
- 1 × 40 lb
Yearly total
- Fertilizer per year
- 100 lb
- Bags per year
- 3
Suggested schedule (US zone 7)
- Early Spring (March-April)
- Late Spring (May)
- Late Summer (Aug-Sept)
- Late Fall (Oct-Nov)
Frequently Asked Questions about the Fertilizer Calculator
What do the numbers on a fertilizer bag mean?
Those are the N-P-K ratio: percent nitrogen (N), phosphate (P2O5), and potash (K2O) by weight. A 50 lb bag of 10-10-10 contains 5 lb of N, 5 lb of P2O5, and 5 lb of K2O. The rest is filler and minor nutrients.
How much fertilizer do I need for my lawn?
Most US lawns want 1 lb of actual nitrogen per 1,000 sq ft per application. For a 5,000 sq ft lawn with 25-0-10 fertilizer (25% N): 5 lb of N divided by 25% = 20 lb of product. The calculator handles the math for any N-P-K and any rate.
What is the right N-P-K for my lawn type?
Cool-season grass in fall: high N (30-0-10 or 28-0-3). Warm-season grass in summer: balanced (15-0-15) or starter blend. New lawns or sod: high P starter (10-20-10). Test your soil if you can; over-applying phosphorus runs off and pollutes waterways.
How often should I fertilize?
Cool-season: 2-4 applications between September and May; skip mid-summer. Warm-season: 3-5 applications May through September. Avoid late-fall N on warm-season grass (encourages disease). Slow-release blends spread out the feeding.
Can I over-fertilize?
Yes. Too much nitrogen burns leaves, causes thatch buildup, and pollutes runoff. Stay below 1 lb of N per 1,000 sq ft per application, and never apply right before heavy rain. Water-in granular fertilizer immediately to prevent burn.