Fertilizer Cost per Pound of Nitrogen Calculator

How it works

A bag's price says little on its own: a cheaper bag may hold less nitrogen because it is smaller or has a lower analysis. Multiplying the bag weight by the nitrogen percentage gives the pounds of nitrogen in the bag, and dividing the price by that gives the cost per pound of nitrogen, which compares any two products directly.

Multiplying the cost per pound of nitrogen by your nitrogen rate gives the cost per 1,000 sq ft. The pounds of nitrogen in a bag divided by the rate, times 1,000, is the area one bag covers, the figure Penn State Extension and UNH Extension work out in their fertilizer examples.

For a particular lawn you buy whole bags, so the calculator also counts the bags each product needs and what they cost. A product that is cheaper per pound of nitrogen can still cost more for one lawn if it comes in a bag much larger than the lawn needs; the two verdicts are worked out separately.

Formula

lb N per bag      = bag weight × N% ÷ 100
cost per lb N     = bag price ÷ lb N per bag
cost per 1,000 sq ft = cost per lb N × N rate
bag coverage      = lb N per bag ÷ N rate × 1,000 sq ft
bags for the lawn = (N rate × area ÷ 1,000) ÷ lb N per bag, rounded up (at least 1)

Example

A 50 lb bag of 26-5-10 holds 50 × 0.26 = 13 lb of nitrogen; at $45 that is $3.46 per lb N, and at 1 lb N per 1,000 sq ft one bag covers 13,000 sq ft. A 45 lb bag of 20-0-10 holds 9 lb; at $30 that is $3.33 per lb N and covers 9,000 sq ft. Product B costs $0.13 less per pound of nitrogen.

A 5,000 sq ft lawn at 1 lb N per 1,000 sq ft needs 5 lb of nitrogen: one bag of either. The bags cost $45 and $30, so Product B costs $15.00 less for this lawn.

Assumptions and limitations

• Only nitrogen is valued. Phosphate, potash, iron, slow-release coatings and other ingredients that add to a product's price are not counted; two products with the same cost per pound of nitrogen are not necessarily equivalent.
• Prices are illustrative inputs. Sales tax, delivery and the value of leftover product are not included.
• The nitrogen rate is an input, not a recommendation. The product label is the legal instruction for applying a fertilizer, and some states limit lawn fertilizer rates by law.
• The nitrogen percentage is taken as printed on the label's guaranteed analysis.

Frequently asked questions