Fertilizer Calculator
Fertilizer Calculator
Work out how much fertilizer to spread from the numbers on the bag: the pounds of product per 1,000 sq ft that deliver the nitrogen, phosphate or potash you want, the total for your lawn or garden, the number of bags, and what else each application adds.
What each application delivers
Product needed by fertilizer
The three numbers on a fertilizer bag are the percent by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). Pounds of product = the nutrient rate divided by its percent, so 1 lb of nitrogen from a 32 percent product takes 1 / 0.32 = 3.13 lb. Elemental phosphorus is 0.436 of the phosphate figure and elemental potassium 0.830 of the potash figure. Yearly nitrogen by grass type is from NC State Extension lawn guidance; many labs and extension services recommend other amounts for other regions. A soil test is the best guide to phosphate and potash, and some states restrict phosphorus on lawns. The CSV, image and printed report contain the values you entered, nothing preloaded.
How to use this tool
Enter the area and pick the unit, then type the three numbers from the bag. Choose which nutrient the rate is for, usually nitrogen, and the rate you want; 1 lb of nitrogen per 1,000 sq ft is a typical single feeding. Add the bag size to see how many bags to buy and how much is left over. The amount per 1,000 sq ft and the total appear at the top.
Pick your grass to see how many feedings a year the rate works out to. The first chart shows everything the application puts down, and the second shows how much of other common fertilizers would deliver the same feeding.
How the amount is worked out
The numbers on a fertilizer bag are percents by weight: nitrogen, then phosphate (P2O5), then potash (K2O). A bag of 32-0-4 is 32 percent nitrogen, so every pound of it holds 0.32 lb of nitrogen. To put down 1 lb of nitrogen you need 1 divided by 0.32, which is 3.13 lb of product.
That is the rule for any product and any nutrient: the pounds of nutrient wanted divided by its percent as a decimal. The total is that rate times the area, and the other two numbers on the bag tell you what else comes along with each feeding.
Pounds of product per 1,000 sq ft
| Fertilizer | 0.5 lb N | 0.75 lb N | 1 lb N | Per 50 lb bag, sq ft at 1 lb N |
|---|---|---|---|---|
| 46-0-0, urea | 1.09 lb | 1.63 lb | 2.17 lb | 23,000 |
| 34-0-0, ammonium nitrate | 1.47 lb | 2.21 lb | 2.94 lb | 17,000 |
| 32-0-4, lawn food | 1.56 lb | 2.34 lb | 3.13 lb | 16,000 |
| 29-0-4, lawn food | 1.72 lb | 2.59 lb | 3.45 lb | 14,500 |
| 21-0-0, ammonium sulfate | 2.38 lb | 3.57 lb | 4.76 lb | 10,500 |
| 16-4-8, lawn and garden | 3.13 lb | 4.69 lb | 6.25 lb | 8,000 |
| 13-13-13, balanced | 3.85 lb | 5.77 lb | 7.69 lb | 6,500 |
| 10-10-10, balanced | 5.00 lb | 7.50 lb | 10.00 lb | 5,000 |
| 18-24-12, starter | 2.78 lb | 4.17 lb | 5.56 lb | 9,000 |
| 5-3-4, organic | 10.00 lb | 15.00 lb | 20.00 lb | 2,500 |
Pounds of product per 1,000 sq ft for each nitrogen rate, and how far a 50 lb bag goes at 1 lb of nitrogen per 1,000 sq ft.
Nitrogen a year by grass type
| Grass | Nitrogen a year, lb per 1,000 sq ft | Feedings at 1 lb |
|---|---|---|
| Tall fescue | 2 | 2 |
| Kentucky bluegrass | 2 | 2 |
| Fine fescue | 2 | 2 |
| Bermudagrass | 3 | 3 |
| Zoysiagrass | 2 | 2 |
| St. Augustinegrass | 2 | 2 |
| Centipedegrass | 1 | 1 |
Yearly amounts from NC State Extension lawn guidance. Lawns on sandy soil, irrigated lawns and other regions may use more or less; a local extension office or soil test gives the best figure.
Cool season grasses, the fescues and bluegrass, are fed mostly in fall with a lighter feeding in spring. Warm season grasses, bermuda, zoysia, St. Augustine and centipede, are fed once they green up in late spring and through summer, and not in fall.
A worked example you can check on paper
A 5,000 sq ft tall fescue lawn gets 1 lb of nitrogen per 1,000 sq ft from a 32-0-4. The product rate is 1 divided by 0.32, 3.125 lb per 1,000 sq ft, and over 5,000 sq ft that is 15.6 lb. A 15 lb bag is not quite enough, so buy two and keep 14.4 lb for the next feeding.
Each 1,000 sq ft also gets 3.125 times 0.04, 0.125 lb of potash, and no phosphate. Tall fescue needs about 2 lb of nitrogen a year, so this is one of two feedings.
Mistakes that catch people out
Reading the numbers as pounds. They are percents; a 10-10-10 is not 10 lb of anything per bag unless the bag weighs 100 lb.
Measuring the area loosely. Pace it or use a tape; take off the house, drive and beds. A 20 percent error in area is a 20 percent error in fertilizer.
Putting it all down at once. Quick release nitrogen above about 1 lb per 1,000 sq ft can burn grass and wash away. Split bigger yearly amounts into several feedings.
Skipping the soil test. Phosphate and potash are only worth adding when the soil is short of them, and pH may matter more than either.
Popular calculations
Each opens the calculator with its own address, and every value can be changed once it is open.
Lawns with 32-0-4 at 1 lb N
5,000 sq ft with other fertilizers
Lighter feedings
Phosphate and potash
Acres
Metric
How this was checked
The engine was tested in Node against hand calculations and the Rutgers Cooperative Extension example: a 25 percent nitrogen product at 2 lb of nitrogen per 1,000 sq ft needs 8 lb per 1,000 sq ft. Acre, square meter and hectare rates were checked by converting to pounds per 1,000 sq ft by hand, and 50 lb of nitrogen an acre from urea gives the expected 108.7 lb of product.
Every address in the lists above was then opened and checked for a distinct title, the right answer, charts that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.
Common questions
How much fertilizer do I need for 1,000 sq ft?
Divide the pounds of nitrogen you want by the nitrogen percent on the bag. For a typical feeding of 1 lb of nitrogen per 1,000 sq ft, that is 3.13 lb of a 32-0-4, 4.76 lb of a 21-0-0, 10 lb of a 10-10-10 or 2.17 lb of urea, 46-0-0.
What do the three numbers on a fertilizer bag mean?
They are the percent by weight of nitrogen (N), phosphate (P2O5) and potash (K2O), always in that order. A 50 lb bag of 10-10-10 holds 5 lb of each; the rest is carrier and filler.
How much nitrogen does a lawn need a year?
NC State Extension suggests about 2 lb per 1,000 sq ft a year for tall fescue, Kentucky bluegrass, zoysiagrass and St. Augustinegrass, 3 lb for bermudagrass and 1 lb for centipedegrass, split into feedings of about 1 lb or less.
How much of a 50 lb bag of 10-10-10 per 1,000 sq ft?
At 1 lb of nitrogen per 1,000 sq ft, 10 lb, so a 50 lb bag covers 5,000 sq ft. At half a pound of nitrogen, it covers 10,000 sq ft.
Can I use a lb per acre rate?
Yes. Choose acres as the unit and enter the rate in pounds per acre; 1 lb per 1,000 sq ft is about 44 lb per acre. For metric, 1 lb per 1,000 sq ft is about 4.9 g per m² or 49 kg per hectare.
Do I need phosphorus?
Many established lawns have enough, which is why lawn fertilizers often have a middle number of 0, and several states restrict phosphorus on lawns except for new seeding or when a soil test shows a need. A soil test tells you for sure.
Related Calculators and Reference
- Grass Seed Calculator: seed for a new lawn or overseeding.
- Sod Calculator: pallets of sod for an instant lawn.
- Mulch and Soil Calculator: cubic yards and bags for beds.
- Square Footage Calculator: the area of the lawn from its shape.
- Rutgers NJAES: How to Calculate the Amount of Fertilizer Needed for Your Lawn: the extension fact sheet on the method.