Fertilizer Calculator

Fertilizer is rated by the nitrogen rate, pounds of nitrogen (N) per 1,000 square feet, because nitrogen is what drives lawn colour and growth. The bag tells you how concentrated it is with the N-P-K label, where the first number is the percentage of nitrogen by weight. A 20-5-10 bag is 20% nitrogen, so 50 lb of it carries 10 lb of actual nitrogen.

The two-step formula is:

Nitrogen needed (lb) = (area ÷ 1,000) × rate Product needed (lb) = nitrogen needed ÷ (N% ÷ 100)

Worked example

A 5,000 sq ft lawn fed at 1 lb N per 1,000 sq ft using a 20-0-5 fertilizer:

• Nitrogen needed = (5,000 ÷ 1,000) × 1 = 5 lb of N
• Product needed = 5 ÷ (20 ÷ 100) = 5 ÷ 0.20 = 25 lb of fertilizer
• Bags = 25 ÷ 50 = 0.5 → rounded up to 1 bag (50 lb)

So one 50 lb bag covers this lawn with some to spare. Notice the bag % is the lever: the same 5 lb of N from a 10% product would need 50 lb, twice as much.

Common nitrogen rates by goal

Pitfalls to avoid

• Never exceed ~1 lb of quick-release N per 1,000 sq ft in one application. More than that and you risk fertilizer burn, brown streaks where the salt index spiked.
• Spread the annual total across several feeds, not one heavy dump. Grass uses nitrogen steadily through the season.
• Calibrate the spreader. The right pounds means nothing if the spreader rate is off; do a test pass on a tarp and weigh it.
• Read the actual bag, not a generic label. A “lawn food” can be anything from 5% to 30% N, and the % is the whole calculation.
• Water it in after applying granular fertilizer to move the nutrients into the soil and off the blades.