How much fertilizer should I spread?
Type the lawn area, the three numbers on the fertilizer bag, and the rate of nitrogen you want. The fertilizer calculator gives the pounds of product to spread, the amount per 1,000 sq ft, and the phosphate and potash that come with it.
- Fertilizer to spread
- 31.25 lb
Spread 31.25 lb of fertilizer: 6.25 lb on each 1,000 sq ft.
- Fertilizer per 1,000 sq ft
- 6.25
- Fertilizer per m² (grams)
- 30.5
- Nitrogen (N) applied
- 5 lb
- Phosphate (P₂O₅) applied
- 1.25 lb
- Potash (K₂O) applied
- 2.5 lb
Fertilizer to spread: 31.25 lb. Spread 31.25 lb of fertilizer: 6.25 lb on each 1,000 sq ft.
How to calculate
Computes how much lawn fertilizer to spread from its N-P-K grade, the area and the rate of nitrogen you want, with the phosphate and potash it adds and the bags to buy.
Example with the default inputs (Lawn area 5,000 ft², Nitrogen (N), first number 16%, Phosphate (P₂O₅), second number 4%, Potash (K₂O), third number 8%, Nitrogen to apply 1, Rate unit lb per 1,000 sq ft): Spread 31.25 lb of fertilizer: 6.25 lb on each 1,000 sq ft.
Method: Fertilizer to spread = nitrogen rate × area ÷ (N% ÷ 100); phosphate = fertilizer × P% ÷ 100; potash = fertilizer × K% ÷ 100; bags = fertilizer ÷ bag weight, rounded up.
- The rate is of actual nitrogen. The grade (N-P-K) gives the percent by weight of N, P₂O₅ and K₂O.
- NC State’s rule of thumb is for 1 lb of nitrogen per 1,000 sq ft; follow your soil test and the bag label.
- 1 lb per 1,000 sq ft is about 4.88 g per m².
Worked examples
Each example is checked against the calculator on every build.
- Lawn area 1,000 ft², Nitrogen (N), first number 16%, Phosphate (P₂O₅), second number 4%, Potash (K₂O), third number 8%, Nitrogen to apply 1, Rate unit lb per 1,000 sq ft gives Fertilizer to spread 6.25 lb, Fertilizer per 1,000 sq ft 6.25, Nitrogen (N) applied 1 lb, Phosphate (P₂O₅) applied 0.25 lb, Potash (K₂O) applied 0.5 lb.Source: NC State Extension, Extension Gardener Handbook, chapter 9 Lawns (divide 100 by the first number on the fertilizer bag for the pounds of product per 1,000 sq ft that give 1 lb of nitrogen; 16-4-8 needs 100 ÷ 16 = 6.25 lb), https://content.ces.ncsu.edu/extension-gardener-handbook/9-lawns (retrieved 2026-10-03)
- Lawn area 5,000 ft², Nitrogen (N), first number 20%, Phosphate (P₂O₅), second number 5%, Potash (K₂O), third number 10%, Nitrogen to apply 0.75, Rate unit lb per 1,000 sq ft, Bag weight 50 lb gives Fertilizer per 1,000 sq ft 3.75, Fertilizer to spread 18.75 lb, Bags 1.Source: Iowa State University Extension and Outreach, How much fertilizer should I apply to my lawn? (June 2025: divide the rate of nitrogen per 1,000 sq ft by the nitrogen in the analysis; 0.75 ÷ 0.20 = 3.75 lb of 20-5-10 per 1,000 sq ft), https://yardandgarden.extension.iastate.edu/faq/how-much-fertilizer-should-i-apply-my-lawn (retrieved 2026-10-03)
- Lawn area 1,076 ft², Nitrogen (N), first number 24%, Phosphate (P₂O₅), second number 0%, Potash (K₂O), third number 11%, Nitrogen to apply 5, Rate unit g per m² gives Fertilizer per m² (grams) 20.833333, Fertilizer to spread 4.59 lb, Nitrogen (N) applied 1.1 lb, Phosphate (P₂O₅) applied 0 lb.Source: Iowa State University Extension and Outreach, How much fertilizer should I apply to my lawn? (June 2025: divide the rate of nitrogen per 1,000 sq ft by the nitrogen in the analysis; 0.75 ÷ 0.20 = 3.75 lb of 20-5-10 per 1,000 sq ft), https://yardandgarden.extension.iastate.edu/faq/how-much-fertilizer-should-i-apply-my-lawn (retrieved 2026-10-03)
How it works
With the area A, the grade N-P-K (percent), and the nitrogen rate r (in lb per 1,000 sq ft, or g per m²):
- Nitrogen applied = r × A (r turned into mass per area: 1 lb per 1,000 sq ft = 0.45359237 kg ÷ 92.90304 m²; 1 g per m² = 0.001 kg per m²).
- Fertilizer to spread = nitrogen applied ÷ (N ÷ 100).
- Fertilizer per 1,000 sq ft (lb) = r in lb per 1,000 sq ft ÷ (N ÷ 100); per m² (g) = r in g per m² ÷ (N ÷ 100).
- Phosphate (P₂O₅) applied = fertilizer to spread × P ÷ 100; potash (K₂O) applied = fertilizer to spread × K ÷ 100.
- Bags = fertilizer to spread ÷ bag weight, rounded up; shown only when a bag weight is typed.
Rules. The area is above 0 and at most 10,000,000 m². N is from 0.1% to 100%; P and K from 0 to 100%; N + P + K may not pass 100% (then there is no answer). The rate is above 0 and at most 1,000. A bag weighs from 0.01 kg to 10,000 kg.
Exact arithmetic. Typed values are exact decimals (1 lb = 0.45359237 kg and 1 ft = 0.3048 m exactly), so 1 ÷ 0.16 gives exactly 6.25. Results round once, for display.
Output format. Masses show 2 decimals in pounds (kilograms in Metric); per 1,000 sq ft shows 2 decimals; per m² shows 1 decimal in grams; bags are whole numbers. The raw masses are in kilograms.
Worked examples by hand
NC State: 1 lb of nitrogen from 16-4-8 on 1,000 sq ft. 1 ÷ 0.16 = 6.25 lb of product. Phosphate = 6.25 × 0.04 = 0.25 lb; potash = 6.25 × 0.08 = 0.5 lb.
Iowa State: 0.75 lb of nitrogen from 20-5-10, on 5,000 sq ft, 50 lb bags. 0.75 ÷ 0.20 = 3.75 lb per 1,000 sq ft; × 5 = 18.75 lb in all, 1 bag.
5 g of nitrogen per m² from 24-0-11 on 100 m². 5 ÷ 0.24 = 20.8 g per m²; × 100 m² = 2.08 kg. Nitrogen = 0.5 kg; phosphate 0.
Other questions people ask
How do I work out how much fertilizer to use?
Divide the nitrogen you want per 1,000 sq ft by the nitrogen in the fertilizer as a decimal. For 1 lb of nitrogen from 16-4-8, 1 ÷ 0.16 = 6.25 lb of product per 1,000 sq ft. Multiply by the area in thousands of square feet for the whole lawn.
What do the three numbers on a fertilizer bag mean?
The grade: the percent by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 20-5-10 holds 10 lb of nitrogen, 2.5 lb of phosphate and 5 lb of potash.
How much nitrogen should a lawn get?
It depends on the grass, the season and your soil. NC State’s rule of thumb is built on 1 lb of nitrogen per 1,000 sq ft, and Iowa State’s example uses 0.75 lb. Follow your soil test and the bag label.
Why is the rate based on nitrogen?
Both NC State and Iowa State size lawn feedings by nitrogen, the first number on the bag. The phosphate and potash then come along in the ratio of the grade. A soil test shows whether you need more of them.
Can I use grams per square metre?
Yes. Pick g per m² as the rate unit. 1 lb per 1,000 sq ft is about 4.88 g per m², so 5 g of nitrogen per m² from 24-0-11 is 5 ÷ 0.24 = 20.8 g of product per m².
Why does the calculator refuse my grade?
The first number must be at least 0.1, because a fertilizer with no nitrogen cannot meet a nitrogen rate. The three numbers together cannot pass 100%.