Fertilizer Calculator
Click a preset to auto-fill NPK values, or enter a custom grade below
Recommended: 0.5–1 lb/1,000 sq ft per application for lawns
Enter Fertilizer Details
Select a fertilizer grade, enter your target nutrient rate and area, then see exactly how many pounds of product to apply, bags to buy, cost, and nutrients delivered.
How to Use the Fertilizer Calculator
Choose Your Calculator Mode
Select one of three modes at the top: 'How Much Do I Need?' calculates product quantity from a target rate; 'What Am I Applying?' shows nutrients delivered from a known product amount; 'Blend Calculator' computes the resulting NPK of a custom multi-product mix.
Enter or Select Fertilizer Grade
Click a quick-select preset button (Urea 46-0-0, DAP 18-46-0, 10-10-10, etc.) to auto-fill the NPK fields, or type your own N%, P₂O₅%, and K₂O% values directly from your fertilizer bag label.
Set Target Rate and Area
Enter your target nitrogen application rate (lbs per 1,000 sq ft, lbs per acre, or kg per hectare) from your soil test recommendation or extension guidelines. Then enter your area size and choose the matching unit (sq ft, acres, hectares).
Review Results and Export
The calculator instantly shows fertilizer pounds needed, bags required, total cost (if bag price is entered), and nutrients delivered per unit area. Check the surplus/deficit section to see if over- or under-applying any nutrient. Export results to CSV or print for your records.
Frequently Asked Questions
What do the three numbers on a fertilizer bag mean?
The three numbers (e.g., 10-5-5) represent the guaranteed percentages by weight of Nitrogen (N), Phosphate (P₂O₅), and Potash (K₂O) in that order. A 50-lb bag of 10-5-5 contains 5 lbs of nitrogen, 2.5 lbs of phosphate, and 2.5 lbs of potash — the remaining 40 lbs is carrier material and filler. Note that phosphorus is expressed as the oxide P₂O₅, not elemental P, and potassium is expressed as K₂O. To convert to elemental amounts: divide P₂O₅ by 2.29 to get elemental P; divide K₂O by 1.20 to get elemental K.
How much nitrogen should I apply per application?
For soluble, fast-release fertilizers applied to lawns, most university extension programs recommend no more than 1 lb of nitrogen per 1,000 sq ft per application. Applying more risks burning the turf and increasing nitrate runoff into waterways. Slow-release or polymer-coated fertilizers can be applied at up to 1.5 lbs N per 1,000 sq ft per application because nutrients release gradually. Organic fertilizers are even safer at higher single-application rates. Annual totals vary by grass type: cool-season grasses need 2–4 lbs N per 1,000 sq ft per year; warm-season grasses need 3–6 lbs per year.
Why does the calculator show a surplus or deficit for some nutrients?
Most fertilizer products have fixed N:P:K ratios. When you target a specific nitrogen rate, the amount of product required often delivers more or less phosphorus and potassium than your targets. For example, using Urea (46-0-0) to meet a nitrogen goal delivers no phosphorus or potassium at all — creating a deficit for those nutrients. The surplus/deficit display shows exactly how far each nutrient will deviate from your target, helping you decide whether to supplement with additional single-nutrient products or choose a different fertilizer grade that better matches your nutrient goals.
What is the Blend Calculator mode used for?
The Blend Calculator computes the resulting NPK ratio when you mix multiple fertilizer products together. For example, if you blend 100 lbs of Urea (46-0-0) with 50 lbs of Triple Superphosphate (0-46-0), the calculator tells you the exact NPK percentage of the resulting mixture. This is essential for commercial growers who blend fertilizers in bulk, for organic growers mixing different amendments, and for anyone who wants to achieve a custom nutrient ratio not available in a single bagged product. The results include the blended N%, P%, and K%, plus a donut chart showing the composition breakdown.
Do I need a soil test before using this calculator?
While this calculator works without a soil test, a soil test is strongly recommended before any fertilizer application. A soil test from your local cooperative extension service reveals your current nutrient levels, soil pH, organic matter content, and often provides crop-specific fertilizer recommendations tailored to your region. Applying fertilizer without a soil test means you could be adding nutrients that are already adequate (wasting money) or missing a deficiency in a nutrient you're not targeting. Soil tests typically cost $10–$30 and can save hundreds of dollars in unnecessary fertilizer purchases while improving plant performance.
How does Urea compare to Ammonium Nitrate as a nitrogen source?
Urea (46-0-0) has the highest nitrogen concentration of any solid fertilizer and is the most economical nitrogen source by cost per pound of N. However, surface-applied urea is prone to volatilization loss — nitrogen escapes as ammonia gas, especially on warm, moist soils or alkaline soils. Incorporating urea into the soil or applying before rain greatly reduces this loss. Ammonium Nitrate (34-0-0) is less prone to volatilization and is immediately available to plants in both ammonium and nitrate forms. It was historically popular but is now harder to purchase due to safety regulations. For most homeowners, urea-based fertilizers work well when watered in promptly after application.