Fertilizer Calculator
A fertilizer calculator divides lawn area by 200 sq ft per lb, the 5 lb per 1,000 sq ft rate: a 50 × 20 ft lawn needs 5 lbs of granular fertilizer.
How much fertilizer do I need?
This fertilizer calculator divides your lawn area by 200 square feet per pound, which is the common rate of 5 pounds of granular product per 1,000 square feet. The formula is pounds = (length × width) ÷ 200. A 50 × 20 ft lawn is 1,000 sq ft and needs 5.00 lbs, which costs $4.50 at $0.90 per pound.
Enter the lawn’s length and width in feet and the price per pound of your fertilizer. Divide the bag price by its weight to get the price per pound.
The formula in plain words
- Area = length × width, in square feet.
- Pounds = area ÷ 200. That is the same as 5 lb for every 1,000 sq ft.
- Cost = pounds × price per pound.
The 5 lb rate is a starting point, and the right rate depends on the nitrogen in your bag. Five pounds of a 20% nitrogen product puts down 5 × 0.20 = 1 lb of nitrogen per 1,000 sq ft. If your bag has a different nitrogen percent, scale the result as shown in the table below.
Worked examples
- Default: 50 ft × 20 ft at $0.90/lb**
- Area: 50 × 20 = 1,000 sq ft
- Fertilizer: 1,000 ÷ 200 = 5.00 lbs
- Cost: 5 × $0.90 = $4.50
- Front and back lawn: 80 ft × 60 ft at $0.90/lb**
- Area: 80 × 60 = 4,800 sq ft
- Fertilizer: 4,800 ÷ 200 = 24.00 lbs
- Cost: 24 × $0.90 = $21.60
If your bag is 10-10-10 instead of a 20% nitrogen blend, you need twice as much product for the same nitrogen: 48 lbs for example 2.
Pounds of product for 1 lb of nitrogen per 1,000 sq ft
| Nitrogen on the bag | Product per 1,000 sq ft | For a 5,000 sq ft lawn |
|---|---|---|
| 10% (for example 10-10-10) | 10.0 lb | 50.0 lb |
| 16% (16-4-8) | 6.25 lb | 31.3 lb |
| 20% (20-5-10) | 5.0 lb | 25.0 lb |
| 24% (24-0-6) | 4.17 lb | 20.8 lb |
| 29% (29-0-4) | 3.45 lb | 17.2 lb |
| 32% (32-0-10) | 3.13 lb | 15.6 lb |
Many lawn programs use 0.5 to 1 lb of nitrogen per 1,000 sq ft per feeding. Follow the bag’s own rate when it differs, since it accounts for the formula and release type.
How to measure your lawn
- Measure each section. Front yard, back yard, and side strips separately, then add them.
- Subtract what you will not feed. Leave out the house, driveway, patio, and planting beds.
- For odd shapes, split curves into rectangles. For a circle, area = 3.14 × radius × radius.
- Check the bag. Bags often state coverage, like “covers 5,000 sq ft.” Compare it with your area to see how many bags to buy.
Setting the spreader
| Step | Why |
|---|---|
| Use the setting on the bag for your spreader model | Each spreader meters differently |
| Make a header strip around the edges first | Stops throwing granules into beds and onto pavement |
| Overlap passes slightly, wheel to wheel | Avoids stripes |
| Try a half rate in two directions | Evens out coverage on uneven ground |
| Water in if the bag says so | Moves nutrients into the soil |
Common mistakes
- Using one rate for every product. A 10% nitrogen bag and a 32% bag need very different amounts.
- Fertilizing to fix pH. If soil is too acidic, grass cannot use the nutrients. Test soil, then add lime if needed with the lawn lime calculator.
- Feeding dormant grass. Grass that is not growing cannot use the nitrogen.
- Spreading phosphorus where it is restricted. Some areas limit phosphorus on established lawns. Check local rules.
Related calculators
Seeding at the same time? Use the grass seed calculator for new lawns or the overseeding calculator for thin ones. For a compost layer, the lawn topdressing calculator gives cubic yards.
Frequently asked questions
How do I read the N-P-K numbers on a fertilizer bag?
They are the percent by weight of nitrogen, phosphorus (as P2O5), and potassium (as K2O), in that order. A 50 lb bag of 20-5-10 holds 50 × 0.20 = 10 lb of nitrogen, 2.5 lb of phosphate, and 5 lb of potash. The rest is filler and carrier that helps spread the nutrients evenly.
How much fertilizer does an average lawn need?
This calculator uses 5 lb of product per 1,000 sq ft. With a 20% nitrogen fertilizer, that delivers 1 lb of nitrogen per 1,000 sq ft, a common per-feeding target in extension guidance. A 10% nitrogen product needs 10 lb to deliver the same 1 lb, so always check the bag's rate.
How do I work out the rate for my bag?
Divide the nitrogen you want per 1,000 sq ft by the nitrogen percent as a decimal. For 1 lb of nitrogen from a 29-0-4 product: 1 ÷ 0.29 = 3.45 lb of product per 1,000 sq ft. For 0.75 lb from 16-4-8: 0.75 ÷ 0.16 = 4.7 lb.
When is the best time of year to fertilize a lawn?
Cool-season grasses such as fescue, bluegrass, and ryegrass respond best to feeding in early fall, with a lighter feeding in late spring. Warm-season grasses such as Bermuda, zoysia, and St. Augustine are fed during their active growth in late spring and summer. Your local extension office can give dates for your area.
What happens if I apply too much fertilizer?
Too much nitrogen can burn grass, leaving yellow or brown stripes where spreader passes overlapped, and unabsorbed fertilizer can wash off into storm drains. Sweep granules off driveways and sidewalks back onto the lawn, and do not apply right before heavy rain.
Estimates only. Verify quantities with your supplier before purchasing.