Fertilizer Calculator
Match your fertilizer to your soil and crop needs. Calculate NPK application rates, total fertilizer needed, and estimated costs.
Fertilizer Application Calculator
Enter the total area you need to fertilize. 1 acre = 43,560 sq ft.
Soil Test Levels
Set levels based on your soil test results. If unknown, leave at "Medium" (no adjustment).
Low
Medium
High
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High
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Medium
High
Fertilizer NPK Values (from bag label)
Enter the three numbers from your fertilizer bag (e.g., 10-10-10 or 5-10-5).
You selected: 10-10-10
Common sizes: 5 lb, 10 lb, 40 lb, 50 lb.
Application Results
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Total Fertilizer (lbs)
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Bags Needed
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Estimated Cost
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Actual N Applied (lbs)
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Actual P2O5 Applied (lbs)
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Actual K2O Applied (lbs)
N Application Rate
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How It Works
Our fertilizer calculator uses research-based nitrogen requirements from university extension services. N-P-K stands for Nitrogen, Phosphorus (as P2O5), and Potassium (as K2O) — the three primary nutrients plants need. The three numbers on a fertilizer bag (e.g., 10-10-10) represent the percentage by weight of each nutrient.
Understanding NPK Ratios
- Nitrogen (N) — Leaf and stem growth. Crucial for lawns and leafy greens.
- Phosphorus (P) — Root development, flowering, and fruiting. Important for tomatoes and peppers.
- Potassium (K) — Overall plant health, disease resistance, and water regulation.
Formula
N Needed (lbs) = Base Rate × (Area ÷ 1000) × Soil Adjustment
Fertilizer Needed (lbs) = N Needed ÷ (N% ÷ 100)
P₂O₅ Applied (lbs) = Fertilizer Needed × (P% ÷ 100)
K₂O Applied (lbs) = Fertilizer Needed × (K% ÷ 100)
Bags = Fertilizer Needed ÷ Bag Weight (round up)
Cost = Bags × Price per Bag
Pro Tip: Always soil test before fertilizing. Over-fertilization wastes money, harms beneficial soil organisms, and can pollute groundwater. Most home gardens need less fertilizer than you think — compost alone may be sufficient for many vegetable gardens.