# How much fertilizer should I spread?

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.

- Page: https://www.acalculator.org/construction/fertilizer-calculator
- JSON spec: https://www.acalculator.org/construction/fertilizer-calculator.json
- Version: 16fd0f17705c

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| area | Lawn area | The area to fertilize. |
| n | Nitrogen (N), first number | The first number of the grade on the bag: percent nitrogen. |
| p | Phosphate (P₂O₅), second number | The second number of the grade: percent P₂O₅. |
| k | Potash (K₂O), third number | The third number of the grade: percent K₂O. |
| rate | Nitrogen to apply | The rate of actual nitrogen you want, in the unit chosen below, from your soil test or the bag label. |
| ru | Rate unit | Whether the nitrogen rate is in pounds per 1,000 sq ft or grams per square metre. |
| bag | Bag weight | The weight of one bag of the fertilizer. Leave empty to skip the bag count. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| product | Fertilizer to spread | The nitrogen to apply ÷ (N% ÷ 100), over the whole area. |
| perThousand | Fertilizer per 1,000 sq ft | Pounds of product on each 1,000 square feet: pounds of N per 1,000 sq ft ÷ (N% ÷ 100). |
| perSquareMetre | Fertilizer per m² (grams) | Grams of product on each square metre. |
| nitrogen | Nitrogen (N) applied | The rate of nitrogen × the area. |
| phosphate | Phosphate (P₂O₅) applied | The fertilizer to spread × P% ÷ 100. |
| potash | Potash (K₂O) applied | The fertilizer to spread × K% ÷ 100. |
| bags | Bags | The fertilizer to spread ÷ the bag weight, rounded up. |

## Method

Fertilizer to spread = nitrogen rate × area ÷ (N% ÷ 100); phosphate = fertilizer × P% ÷ 100; potash = fertilizer × K% ÷ 100; bags = fertilizer ÷ bag weight, rounded up.

## Assumptions

- 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

1. area = 92.90304, n = 16%, p = 4%, k = 8%, rate = 1, ru = lb1000 gives product = 2.834952, perThousand = 6.25, nitrogen = 0.453592, phosphate = 0.113398, potash = 0.226796. 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).
2. area = 464.5152, n = 20%, p = 5%, k = 10%, rate = 0.75, ru = lb1000, bag = 22.679619 gives perThousand = 3.75, product = 8.504857, 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).
3. area = 100, n = 24%, p = 0%, k = 11%, rate = 5, ru = gm2 gives perSquareMetre = 20.833333, product = 2.083333, nitrogen = 0.5, phosphate = 0. 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).

## FAQ

### 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%.

## Sources

- NC State Extension, Extension Gardener Handbook, chapter 9 Lawns (to apply 1 lb of nitrogen per 1,000 sq ft, divide 100 by the first number on the bag; 16-4-8: 100 ÷ 16 = 6.25 lb per 1,000 sq ft), retrieved 2026-10-03. https://content.ces.ncsu.edu/extension-gardener-handbook/9-lawns
- Iowa State University Extension and Outreach, How much fertilizer should I apply to my lawn? (13 June 2025: divide the nitrogen rate per 1,000 sq ft by the nitrogen in the analysis; 0.75 ÷ 0.20 = 3.75 lb of 20-5-10), retrieved 2026-10-03. https://yardandgarden.extension.iastate.edu/faq/how-much-fertilizer-should-i-apply-my-lawn