Lawn and Garden Fertilizer Calculator

How it works

The three numbers on a fertilizer bag are its guaranteed analysis: the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 24-0-11 holds 12 lb of nitrogen, no phosphate and 5.5 lb of potash.

Lawn and garden rates are usually written as pounds of actual nitrogen per unit of area, not pounds of product. Dividing the nitrogen rate by the nitrogen fraction gives the product needed for that area: 1 lb of N per 1,000 sq ft from a 24% N fertilizer takes 1 ÷ 0.24 = 4.17 lb of product. Scaling by your area gives the total, and dividing by the bag weight gives whole bags.

The same weight of product also carries phosphate and potash in the proportions on the label, so the calculator reports those too. Rutgers FS839, Penn State Extension and Virginia Cooperative Extension 430-011 all use this method.

Formula

product per basis = N rate ÷ (N% ÷ 100)
product for area  = product per basis × area (sq ft) ÷ basis (1,000, 100 or 43,560 sq ft)
bags              = product for area ÷ bag weight, rounded up (at least 1)
P₂O₅ applied      = product × P₂O₅% ÷ 100;  K₂O applied = product × K₂O% ÷ 100

Example

1 lb of nitrogen per 1,000 sq ft from a 24-0-11 fertilizer takes 1 ÷ 0.24 = 4.17 lb of product per 1,000 sq ft. On 5,000 sq ft that is 20.83 lb, one 50 lb bag, and it also delivers 20.83 × 0.11 = 2.29 lb of potash (0.458 lb K₂O per 1,000 sq ft) and no phosphate.

Rutgers FS839's worked example: 2 lb N per 1,000 sq ft from a 25% N fertilizer is 8 lb of product per 1,000 sq ft, so a 25,000 sq ft lawn takes 200 lb.

Assumptions and limitations

• The product label is the legal instruction for applying a fertilizer, and some states limit lawn fertilizer rates and phosphate use by law. This calculator converts a rate you enter into pounds of product; it does not recommend a rate.
• The analysis is taken as printed on the bag. Phosphate and potash are reported as P₂O₅ and K₂O, the forms labels use, not as elemental P and K.
• The rate is spread evenly over the whole area. Spreader settings on the bag assume a calibrated spreader; actual delivery varies.
• Nitrogen form is not considered. Slow-release and water-soluble nitrogen are counted the same, although extension guides allow different amounts per application for each.
• Bags are rounded up to whole bags. The result is an estimate, not a substitute for the label, a soil test or local extension guidance.

Frequently asked questions