How to use this calculator
- Measure your lawn. Add each area of lawn separately (front, back, side) or enter one total. Length × width works for rectangles; for odd shapes, break the lawn into rough rectangles.
- Enter the fertilizer analysis, the three numbers on the bag such as 24-0-6, plus the bag weight.
- Choose a nitrogen rate. 0.5 to 1 pound of nitrogen per 1,000 square feet per application is the usual range.
- Pick quick- or slow-release. The calculator warns you if a quick-release rate is higher than extension guides recommend.
- Choose your grass type to see roughly how many feedings a year it needs.
The formula
- Product per 1,000 sq ft = nitrogen rate ÷ (nitrogen % ÷ 100)
- Product for your lawn = product per 1,000 sq ft × lawn area ÷ 1,000
- Bags = product for your lawn ÷ bag weight, rounded up
Lawn fertilizer recommendations are given in pounds of nitrogen, not pounds of product, because nitrogen is the nutrient lawns use most. That is why the same lawn needs very different amounts of a 10-10-10 and a 32-0-4.
Worked example
You want to apply 1 pound of nitrogen per 1,000 square feet to a 6,000 square foot lawn using a 24-0-6 fertilizer sold in 25-pound bags.
- Product per 1,000 sq ft: 1 ÷ 0.24 = 4.17 pounds
- Product for the lawn: 4.17 × 6,000 ÷ 1,000 = 25 pounds, exactly 1 bag
- That also delivers 4.17 × 0.06 = 0.25 pound of potash per 1,000 square feet
Iowa State University Extension uses the same method: its example of 1 pound of nitrogen from a 32-0-4 fertilizer works out to 3.125 pounds of product per 1,000 square feet.
How much nitrogen a year?
| Grass | Typical yearly nitrogen (lb per 1,000 sq ft) | When to feed |
|---|---|---|
| Cool-season (fescue, bluegrass, ryegrass) | About 2 with clippings left on the lawn; about 3 if clippings are removed | Late spring and early fall |
| Bermuda, St. Augustine | About 2 to 3.5, depending on how much growth you want | Late spring through summer |
| Zoysia, centipede | About 1 to 2 | Late spring through summer |
These are general figures from university extension guides. A soil test and your own state extension service will give the most accurate advice for your region.
Applying fertilizer evenly
- Use a spreader and make two passes at half the rate, the second at right angles to the first. This avoids stripes.
- Sweep granules off hard surfaces such as driveways and sidewalks back onto the lawn, so they don’t wash into storm drains.
- Water lightly afterward if the label says to, and don’t fertilize right before heavy rain.
- Skip dormant lawns. Feeding grass that isn’t growing, such as warm-season grass in winter or any lawn in a drought, wastes fertilizer.
- Check local rules on phosphorus. Several states restrict phosphorus on established lawns unless a soil test shows a need.
Sources
- Iowa State University Extension: How to calculate fertilizer rates for the lawn
- UConn Soil Nutrient Analysis Laboratory: Lawns
- Virginia Cooperative Extension: Soil test note for lawns
Frequently asked questions
How do I calculate how much fertilizer to put on my lawn?
Divide the pounds of nitrogen you want per 1,000 square feet by the nitrogen percentage on the bag (written as a decimal). For example, 1 pound of nitrogen from a 32-0-4 fertilizer is 1 ÷ 0.32 = 3.1 pounds of product per 1,000 square feet. Multiply by your lawn area in thousands of square feet for the total.
What do the three numbers on a fertilizer bag mean?
They are the percentages by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). A 50-pound bag of 24-0-6 contains 12 pounds of nitrogen, no phosphate and 3 pounds of potash.
How much nitrogen should I apply at one time?
Extension guides commonly cap quick-release (water-soluble) nitrogen at about 1 pound per 1,000 square feet per application, and Virginia Tech recommends no more than 0.9 pound. Slow-release products can be applied at somewhat higher rates following the label.
How many times a year should I fertilize my lawn?
It depends on your grass. UConn Extension suggests about 2 pounds of nitrogen per 1,000 square feet a year for cool-season lawns when clippings are left on the lawn, often split between late spring and early fall. Warm-season grasses are fed while they are actively growing in late spring and summer; centipede and zoysia need less than Bermuda or St. Augustine.
When is the best time to fertilize a cool-season lawn?
Late spring and early fall are the main windows for fescue, bluegrass and ryegrass. Fall feeding is especially valuable because it helps the grass recover from summer and store energy for spring.