Published on 06/26/03

Rough calculations easy with garden-variety math

By Wayne McLaurin
University of Georgia

Most lawn fertilizer is sold in bags that cover 5,000 square feet, or a rectangle that's 50 feet by 100 feet. But if your yard isn't 50-by-100, how do you know how much to use?

You need a ballpark figure of how many square feet each area has. The best way to get that is with a map.

To map your yard, you'll need something to write on, something to write with and the longest tape measure you can find. A well-rewarded assistant would be extremely helpful. This may be a good time for the kids to see that math can be useful.

Make the map a rough sketch with measurements for each area. It's easiest to start at a fixed object: the house, a fence or a curb. Move from one area to the next, showing the measurements of the driveways, walkways, decks, buildings and fences as well as the lawn and garden.

Make a map

Transfer the measurements to scale onto graph paper. Separate the areas into approximate rectangles, triangles or circles. For example, divide an "L" shaped lawn area into two rectangles and add the calculated areas.

If your triangular area isn't a 90-degree triangle, divide it into two right triangles and add the calculated areas. For a semicircular planting bed, figure the area of the full circle, then divide by two.

The formula for the area of a rectangle is length times width. The area of a circle is twice the radius (a line from the edge to the center) times 3.14. The area of a right triangle is one-half the base times the height. And you thought high school geometry was useless.

Not rocket science

Break down an irregularly shaped area into a combination of geometric shapes. Figure the area of each shape and add them together. You may have to fudge a little. It isn't rocket science, but you do want a good estimate.

Take a copy of the map when you shop for lawn seed, fertilizer, weed barriers and other items sold in quantities based on area.

(Add compass directions and a brief description of soil conditions to your map and you'll have a useful tool for choosing plants, too, and designing your landscape.)

Many bright people have a hard time calculating volume, such as figuring out how much mulch it will take to topdress a bed.

First figure the area. Then divide the area by 12 (the number of inches in a foot) to find out how many cubic feet it will take for 1 inch of mulch. Multiply that by however many inches of mulch you want to add.

Use this formula to see how much topsoil you'll need to fill a raised bed, too.

Buying by the bag

Most gardeners buy soil amendments, compost and mulches by the bag. Usually, these are measured in cubic feet. Some, however, are listed in dry quarts, most commonly in a 20-quart bag, or roughly three-fourths of a cubic foot. (There are about 25.75 dry quarts in a cubic foot.)

Bags are the cleanest and most convenient way to transport this kind of product. But it's not the cheapest.

You can buy larger quantities loose by the cubic yard. There are 27 cubic feet to a cubic yard (length times width times depth, or 3 feet by 3 feet by 3 feet).

Bulk quantities

You can usually pick up a yard of compost, mulch or topsoil for less than half the price of bagged products. It's messy, and you need a truck with an open bed.

Bulk quantities can be delivered, but there's usually a 10-cubic-yard minimum. That's a fairly stupendous quantity, the equivalent of 270 cubic-foot bags.

Doing the math isn't hard and will help you to be not only a better gardener but a savvy shopper, too.

For more garden math and formulas, see publication, "Conversion Tables, Formulas and Suggested Guidelines for Horticultural Use" ( or

(Wayne McLaurin is a horticulturist with the University of Georgia College of Agricultural and Environmental Sciences.)

Wayne McLaurin is a professor emeritus of horticulture with the University of Georgia College of Agricultural and Environmental Sciences.