How It Is Calculated
This calculator helps you determine the amount of grass seed needed for a rectangular lawn, the number of bags to buy, and the total material cost. The calculation proceeds in several steps.
Lawn Area
First, compute the area of your lawn in square feet by multiplying its length by its width:
Area = L × W
where L is the length in feet and W is the width in feet.
Seed Needed Before Waste
Next, use the seeding rate for your chosen grass type. The rate is given in pounds per 1000 square feet. For example, Kentucky Bluegrass uses 3 lbs/1000 sq ft, while Bermuda Grass uses 1.5 lbs/1000 sq ft. The seed needed before accounting for waste is:
Seed before waste = (Area / 1000) × R
where R is the seeding rate in lbs/1000 sq ft.
Apply Waste Factor
To account for uneven coverage, overlaps, and errors, add a waste percentage. The final seed needed is:
Seed needed = Seed before waste × (1 + W / 100)
where W is the waste factor in percent. The result is rounded to one decimal place.
Bags Needed
Divide the total seed needed by the bag size and round up to the nearest whole number, because you cannot buy a fraction of a bag:
Bags needed = ceil(Seed needed / B)
where B is the bag size in pounds.
Total Cost
Finally, multiply the number of bags by the price per bag:
Total cost = Bags needed × P
where P is the price per bag in dollars. The result is rounded to two decimal places.
Assumptions
- The lawn is rectangular or can be approximated by length and width.
- Seeding rates are for new lawn establishment. For overseeding, reduce rate by half.
- Waste factor accounts for uneven coverage, overlaps, and errors.
- Bag size and price are as entered by user.
- The seed types and rates are typical for the US.
Worked Example
Suppose you have a lawn that is 50 ft long and 30 ft wide. You choose Fine Fescue (seed rate 5 lbs/1000 sq ft), a waste factor of 10%, bag size of 20 lbs, and price per bag of $50.
- Lawn area = 50 ft × 30 ft = 1500 sq ft.
- Seed before waste = (1500 / 1000) × 5 = 7.5 lbs.
- Seed needed = 7.5 × (1 + 10/100) = 7.5 × 1.1 = 8.25 lbs, rounded to 8.3 lbs.
- Bags needed = ceil(8.3 / 20) = ceil(0.415) = 1 bag.
- Total cost = 1 × $50 = $50.00.
Thus, you need 8.3 lbs of seed, which means buying 1 bag at a cost of $50.00.
Practical Tips
- Measure your lawn carefully. For irregular shapes, break the area into rectangles and sum them.
- If you are overseeding an existing lawn, use half the recommended seeding rate.
- Always buy a little extra seed to account for spills or patchy areas. The waste factor helps with this.
- Store unused seed in a cool, dry place for future touch-ups.
- Consider using the Mulch calculator if you plan to mulch after seeding, or the Concrete slab calculator if you are also installing a patio.
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