Fertilizer Calculator (N-P-K)

How the math works

Fertilizer grades are percent by weight. A bag labeled 10-10-10 is 10% nitrogen (N), 10% phosphate (P₂O₅), and 10% potash (K₂O). To hit a nitrogen target of 2 lb N per 1,000 square feet with that product, you need 2 ÷ 0.10 = 20 lb of product per 1,000 sq ft. Scale by area: on 1,000 sq ft that is 20 lb; on 500 sq ft it is 10 lb. This calculator uses that extension-style formula and then multiplies the same product weight by the P and K fractions so you see what else you are applying — important when soil tests already show high phosphorus.

Worked example locked in our tests: 1,000 sq ft, 2 lb N target, 10-10-10 → 20 lb of product, delivering 2 lb N, 2 lb P₂O₅, and 2 lb K₂O. Change the middle or third number and the product weight for nitrogen stays the same, but the P and K delivered lines update.

Reading the three numbers

Nitrogen drives leafy growth and is the usual seasonal planning nutrient for mixed vegetable gardens. Phosphate supports roots and flowering; potash supports fruit quality and stress tolerance. Organic products use the same three-number labeling — blood meal might read 12-0-0; bone meal is low N and high P. Always divide the percent by 100 to get the fraction used in the formula. If N is zero on the bag, this tool cannot size a nitrogen target from that product; choose a different analysis.

Soil tests beat defaults

The default 2 lb N / 1,000 sq ft is a maintenance-style vegetable baseline, not a prescription for every bed. A county extension soil test often recommends lower rates once organic matter and residual fertility are decent. Phosphorus and potassium persist in soil longer than nitrogen — blind annual applications of balanced complete fertilizers can build P to levels that lock out micronutrients and threaten waterways. Prefer a complete fertilizer when all three nutrients are low; otherwise pair a nitrogen-forward product with compost and skip redundant P.

Application safety

Keep granules off leaves and stems, water in after spreading, and split large nitrogen doses rather than dumping a season’s worth on tender seedlings. Side-dress heavy feeders (corn, tomatoes) mid-season with part of the total rather than everything at planting. Organic sources release more slowly but can still burn if piled against stems. Never treat fertilizer as a substitute for compost structure and biology — this calculator sizes bags, not soil health.

Common mistakes

Applying by “a handful per plant” without area math, repeating 10-10-10 every year without a soil test, sizing product from the P number when the goal was nitrogen, and forgetting that a 50-lb bag of 10-10-10 holds only 5 lb of actual N. Another: confusing percent on the label with pounds of nutrient needed — percent is concentration; the target is absolute pounds of nutrient per area. Print the product weight before you leave for the garden center so you buy one bag size that covers the bed without leftover mystery half-bags from three seasons ago.

Frequently asked questions