Lawn rates are written as pounds of nitrogen, but you buy the whole bag by weight. The N-P-K analysis on the label gives the percentage by weight of nitrogen (N), phosphate (P) and potash (K). The calculator turns a nitrogen rate into pounds of the full bag. Phosphate, potash and filler add weight but no nitrogen, so the product you spread is always heavier than the nitrogen it delivers.
How to use this calculator
- Measure the length and width of the grass in feet. Leave out beds, paths, the driveway and the patio, since the rate applies to grass only. For an odd-shaped yard, split it into rectangles and add the results.
- Enter the first figure of the N-P-K analysis as the nitrogen percentage. If the label lists nitrogen in more than one form, enter the total.
- Leave the rate at 1 lb of nitrogen per 1,000 sq ft, the default, or enter the rate from your product label or local extension guidance.
- Buy enough bags to cover the pounds shown, rounding up to the next whole bag.
- Set the spreader to the label's setting and make overlapping passes, so no strip gets more or less than the rest.
- Sweep any granules off paving and back onto the lawn.
Worked examples
Small front yard
Lawn: 30 × 40 ft · Nitrogen on the bag: 16%
- Fertilizer product
- 7.5 lb
- Product rate
- 6.3 lb per 1,000 sq ft
- Actual nitrogen applied
- 1.20 lb N
- Lawn area
- 1,200 sq ft
Backyard lawn
Lawn: 60 × 90 ft · Nitrogen on the bag: 32%
- Fertilizer product
- 16.9 lb
- Product rate
- 3.1 lb per 1,000 sq ft
- Actual nitrogen applied
- 5.40 lb N
- Lawn area
- 5,400 sq ft
Light feeding on a large lawn
Lawn: 100 × 120 ft · Nitrogen on the bag: 20% · Nitrogen to apply: 0.5 lb N per 1,000 sq ft
- Fertilizer product
- 30.0 lb
- Product rate
- 2.5 lb per 1,000 sq ft
- Actual nitrogen applied
- 6.00 lb N
- Lawn area
- 12,000 sq ft
Long side yard
Lawn: 150 × 20 ft · Nitrogen on the bag: 24%
- Fertilizer product
- 12.5 lb
- Product rate
- 4.2 lb per 1,000 sq ft
- Actual nitrogen applied
- 3.00 lb N
- Lawn area
- 3,000 sq ft
Pounds of product for common lawn sizes at the default rate
| Lawn size | Nitrogen on the bag: 16% | Nitrogen on the bag: 20% | Nitrogen on the bag: 32% |
|---|---|---|---|
| 30 × 40 ft | 7.5 | 6.0 | 3.8 |
| 50 × 40 ft | 12.5 | 10.0 | 6.3 |
| 60 × 90 ft | 33.8 | 27.0 | 16.9 |
| 100 × 120 ft | 75.0 | 60.0 | 37.5 |
Each cell: Fertilizer product (lb). Inputs not in the table use the calculator's starting values.
How the calculation works
The calculator multiplies length by width to get the area in square feet. It converts that area into blocks of the size the rate is written for, multiplies by the nitrogen rate, and divides by the nitrogen share of the bag. Dividing is the step people reverse: a bag with a lower nitrogen percentage holds less nitrogen per pound, so you need to spread more of it.
The results also show the actual nitrogen applied, which is the product's weight times its nitrogen share. Check it against your rate by hand. Treat the result as an estimate, since lawn edges and spreader overlap change what you really use.
Reading the N-P-K on the label
- The first figure is the nitrogen percentage by weight. It is the only figure this calculator uses.
- The second and third figures are phosphate and potash. This calculator does not use them, and a soil test shows whether your lawn needs them.
- If the label splits nitrogen into forms, such as quick-release and slow-release, use the total. Using only one part overstates the product you need.
Rates, labels and local rules
The product label comes first. It gives the spreader setting and the rate for that exact bag, so follow it where it differs from the default. Local and state fertilizer rules may also limit where or when you can apply, so check them before you spread. When in doubt, ask your local extension service or get a soil test.
The default rate is 1 lb of nitrogen per 1,000 sq ft. If you are seeding or laying new turf, the grass seed and sod calculators cover those quantities, and this page covers only the fertilizer.
Mistakes to avoid
- Measuring the whole property. Beds, paths, driveways and the house take no product, so measure only the grass.
- Reading the phosphate or potash figure as nitrogen. Only the first figure goes in the nitrogen percentage box.
- Copying a rate from a different bag. A label's rate is written for that product's analysis, so do not apply it to another bag.
- Adding more because the grass looks thin. Shade, compacted soil and the wrong grass can thin a lawn, and extra nitrogen does not fix them.
- Leaving granules on paving. Sweep spills back onto the lawn so they do not wash into storm drains.
Common questions
What do the three numbers on fertilizer mean?
They are the percentages by weight of nitrogen (N), phosphate (P) and potash (K), in that order. Lawn rates are written for nitrogen, so the first figure is the only one this calculator needs.
How do I calculate fertilizer for a lawn by hand?
Multiply the length by the width, convert the area into blocks of the size the rate uses, and multiply by the nitrogen rate. Then divide by the nitrogen share written as a decimal to get the pounds of product. The calculator follows the same steps.
How much nitrogen does a lawn need per application?
For a single feeding, a university extension's normal range is 0.5 to 1 lb of nitrogen per 1,000 sq ft. The calculator starts at 1 lb of nitrogen per 1,000 sq ft, the top of that range, and the product label sets the final rate for your bag.
Can I use this calculator for liquid fertilizer?
The nitrogen math is the same, but liquid products are sold by volume and this calculator returns pounds. Use the rate and the volume on the liquid's label, and take the nitrogen percentage from that same label.
Can I over-fertilize my lawn?
Yes. Too much nitrogen at once can burn the grass. Stay within 0.5 to 1 lb of nitrogen per 1,000 sq ft per feeding unless your bag's label says otherwise.
Do organic and slow-release fertilizers need a different calculation?
No, the math is the same. Enter the bag's total nitrogen percentage. A bag with a lower nitrogen percentage holds less nitrogen per pound, so the calculator will show more pounds of product to spread.
How often should I fertilize my lawn?
A cool-season lawn typically gets 2 to 6 feedings a year, and a warm-season lawn 1 to 4 feedings a year. Keep each feeding within 0.5 to 1 lb of nitrogen per 1,000 sq ft. Bermudagrass can be fed every 4 to 6 weeks in summer, and your bag's label sets the spacing for that product.
When is the best time to fertilize a lawn?
For cool-season grasses, September is the most important feeding, and November (around your last mowing) is the next best. Warm-season grasses should wait until they have fully greened up in spring and nights are around 70 °F. Your local extension office can fine-tune the dates for your area.
How much fertilizer do I use when seeding a new lawn (starter fertilizer)?
Starter fertilizer should supply no more than 1 lb of nitrogen per 1,000 sq ft at planting, so enter that bag's nitrogen percentage and use a rate at or under that limit. About 4 to 8 weeks later, a follow-up feeding of 0.5 lb of nitrogen per 1,000 sq ft helps thicken the new turf.