Fertilizer NPK Calculator
Calculate actual nitrogen, phosphorus, and potassium in fertilizer bags.
Enter NPK ratio, bag weight, and area to get application rates.
Fertilizer NPK Calculator helps you understand exactly how much nitrogen (N), phosphorus (P₂O₅), and potassium (K₂O) is in a bag of fertilizer and how to apply it correctly.
What is NPK? The three numbers on a fertilizer bag (like 10-10-10 or 24-0-6) represent the percentage by weight of:
- N — Nitrogen: promotes leaf and stem growth, green color
- P — Phosphorus (as P₂O₅): supports root development, flowering, and fruiting
- K — Potassium (as K₂O): strengthens overall plant health, disease resistance, and stress tolerance
The Formula:
Nutrient Weight = Bag Weight × (NPK Percentage / 100)
For example, a 50-pound bag of 10-10-10 contains:
- Nitrogen: 50 × 0.10 = 5 lbs (2.27 kg)
- Phosphorus: 50 × 0.10 = 5 lbs (2.27 kg)
- Potassium: 50 × 0.10 = 5 lbs (2.27 kg)
The remaining 35 lbs is filler material (carrier, coating, micronutrients, etc.).
Application Rate:
Most lawns need about 1 pound of nitrogen per 1,000 sq ft (0.45 kg per 93 sq m) per application.
Fertilizer Needed = Desired N per 1,000 sq ft / (N% / 100)
For a 24-0-6 fertilizer to deliver 1 lb N per 1,000 sq ft:
1 / 0.24 = 4.17 lbs of fertilizer per 1,000 sq ft
Common NPK Ratios and Uses:
- 10-10-10: Balanced, all-purpose fertilizer for general gardens
- 24-0-6: High nitrogen, good for established lawns
- 5-10-5: High phosphorus, good for flowering plants and new seedlings
- 0-0-60: Pure potassium (potash), for potassium-deficient soils
- 15-15-15: Balanced, higher concentration than 10-10-10
Metric Conversions: 1 pound = 0.4536 kg. 1,000 sq ft = 92.9 sq m. A 25-kg bag is approximately 55 lbs.
Practical Tips: Always do a soil test before fertilizing to know what your soil actually needs. Over-fertilizing with nitrogen causes excessive leaf growth at the expense of roots and flowers. Too much phosphorus can run off into waterways and cause algal blooms. Apply fertilizer when rain is expected within 24 hours or water in immediately after application.
Safety Note: Never apply more than 1 lb of nitrogen per 1,000 sq ft in a single application. For higher annual needs, split into multiple applications 6 to 8 weeks apart.