Fertilizer Calculator
Pounds of fertilizer product from your lawn size and nitrogen rate.
Input sheet
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Fertilizer rates are written in pounds of nitrogen per 1,000 sq ft, but bags are sold by total weight at some N-P-K percentage — and converting between the two is where lawns get burned. This calculator does the conversion.
How it works
Nitrogen needed = lawn area ÷ 1000 × the rate. Product weight = nitrogen needed ÷ the bag's N percentage — a 10% bag delivers 1 lb of N per 10 lb of product.
The three numbers on a bag are the N-P-K analysis: percent nitrogen, phosphate, and potash by weight. A 10-10-10 bag is 10% nitrogen, so delivering 1 lb of actual nitrogen takes 10 lb of that product; a 24-0-4 bag needs only about 4.2 lb.
The standard application for an established lawn is 1 lb of nitrogen per 1,000 sq ft, and staying at or below that per application is the burn threshold that matters. The calculator multiplies your lawn's thousands of square feet by the rate, then divides by the bag's N fraction.
Nitrogen needed = lawn area ÷ 1000 × rate (lb N per 1,000 sq ft). Product to apply = nitrogen needed ÷ (bag's first number ÷ 100).
Worked examples
A 50 × 40 ft lawn at 1 lb N/1,000 sq ft with a 10-10-10 bag. → 20 lb of product.
2,000 sq ft needs 2 lb of N; ÷ 0.10 = 20 lb of 10-10-10.
5,000 sq ft with a 24-0-4 turf fertilizer. → About 20.8 lb of product.
5 × 1 = 5 lb N; ÷ 0.24 ≈ 20.8 lb — a stronger bag stretches much further.
Tips & gotchas
- Set the spreader to half rate and make two perpendicular passes — striped lawns come from single-pass misses, burned ones from overlaps.
- Water in after applying (about ¼ inch) unless the label says otherwise; granules sitting on dry blades is how burn happens.
- Match the analysis to a soil test when you can — many lawns need no phosphorus at all, and some states restrict it.
FAQ
What rate should I use?
1 lb of nitrogen per 1000 sq ft per application is the standard for established lawns; never exceed that in one pass or you risk burning the grass.
What do the three numbers mean?
Percent by weight of nitrogen (leaf growth), phosphate (roots and establishment), and potash (stress tolerance). A 50 lb bag of 24-0-4 contains 12 lb of actual nitrogen.
How many times a year should I fertilize?
Cool-season lawns: heaviest in fall, lightly in spring — 2–4 applications totaling 2–4 lb N per 1,000 sq ft per year. Warm-season grasses feed through late spring and summer instead.
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