Fertilizer Calculator
How much fertiliser to spread for a nitrogen, phosphorus or potassium rate, from the N-P-K on the bag, with all three nutrients it delivers and the bags to buy.
Result
Buy 50 lbEstimate
Rates come from your label or soil test; none is stored.
- Nitrogen delivered
- 5 lb (2.268 kg)Estimate
- Phosphate delivered
- 5 lb (2.268 kg)Estimate
- Potash delivered
- 5 lb (2.268 kg)Estimate
- Note
- More than the label rate does not help the plants, and the excess can wash into streams and lakes (US EPA).
https://calcgallery.com/landscaping/fertilizer-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Fertilizer Calculator
- Enter the area and the three N-P-K numbers from the bag.
- Choose which nutrient to size by, usually nitrogen, and the target rate from the label or a soil test.
- Read how much to spread, all three nutrients it delivers, and, if you enter your bag's weight, how many bags to buy.
The arithmetic
nutrient needed = area × target ratefertilizer = nutrient needed ÷ that nutrient’s percentagedelivered = fertilizer × each percentagebags = fertilizer ÷ bag weight, rounded upBags round up, since you cannot buy part of one; the page shows what is left in the last bag. Opened fertilizer can take up moisture and cake, so store the remainder sealed and dry.
Rates by area
Rates are given per area: in the US usually pounds of nutrient per 1,000 square feet, elsewhere grams per square metre. One pound per 1,000 square feet is about 4.9 grams per square metre.
With your numbers
- Fertilizer = area × target rate ÷ that nutrient’s percentage
Quick reference
Rates come from your label or soil test; none is stored.
| Job | Fertilizer to spread |
|---|---|
| 5,000 ft², 10-10-10, 1 lb N | 50 lb |
| 5,000 ft², 20-0-10, 1 lb N | 25 lb |
| 5,000 ft², 10-10-10, 0.5 lb N | 25 lb |
| 10,000 ft², 16-4-8, 1 lb N | 62.5 lb |
| 500 m², 10-10-10, 5 g N | 25 kg |
| By potash, 0-0-60 | 8.33 lb |
Common questions about the Fertilizer Calculator
- How many bags of fertilizer do I need?
- 2 bags of 40 lb for 5,000 square feet of 10-10-10 at 1 lb of nitrogen per 1,000 square feet. That rate needs 5 lb of nitrogen, or 5 ÷ 0.10 = 50 lb of product, and 50 ÷ 40 = 1.25 bags, rounded up to 2 with 30 lb left over. A different rate, product or bag weight changes the count, so enter the ones on your label.
- What do the three numbers on a fertilizer bag mean?
- The percentage by weight of nitrogen (N), phosphate (P) and potash (K), always in that order. A 40 lb bag of 10-10-10 holds 4 lb of each. A 20-0-10 bag holds twice the nitrogen, no phosphate and the same potash per pound.
- How much fertilizer do I need?
- Divide the nutrient you need by its percentage. To put down 1 lb of nitrogen per 1,000 square feet on 5,000 square feet you need 5 lb of nitrogen; from 10-10-10 that is 5 ÷ 0.10 = 50 lb of product, and from 20-0-10 only 25 lb.
- Why does the page show all three nutrients?
- Because sizing a product by one nutrient delivers the others too. The 50 lb of 10-10-10 above also puts down 5 lb each of phosphate and potash, which may be more than the soil needs. A soil test tells you which nutrients are actually short, and a product matched to it avoids spreading what is not needed.
- What rate should I use?
- The one on the product label, or from a soil test. It depends on the plants, the soil and the season, so this page supplies none. More than the label rate does not help and the excess can run off into waterways.