Some basic math calculations can help you know how much soil, compost and/or fertilizer is needed to maintain one’s garden. Photo by Sandy Cortez

Some basic math calculations can help you know how much soil, compost and/or fertilizer is needed to maintain one’s garden. Photo by Sandy Cortez

Get It Growing: Ready, set, measure!

It is that time of year again – time to get back out to the garden and figure out how much new soil, compost or fertilizer is needed.

The following basics will help make those calculations easier:


You need to know the area of a garden or lawn to apply appropriate amounts of fertilizer or other supplements, or to calculate the volume of a raised bed. For rectangular or square gardens, multiply the length times the width to get the area. For example, 10-feet-wide by 10-feet-long = 100 square feet.

If your garden is triangular, multiply the base width by the height at highest point; then divide by 2. For a circular garden, measure from the center of the circle to the edge (this is the radius); multiply the radius times itself, and then multiply the resulting number by 3.14 (pi). For example: with a radius of 10 feet, (10 feet by 10 feet) x 3.14 = 314 square feet of a circular garden.

For an irregular garden shape, divide this irregular shape into rectangles, triangles and circles, then calculate the area of each and add them together.


You need to know volume to purchase sufficient amounts of compost to cover a garden bed or soil for a raised bed. To calculate volume, multiply the area of the bed times the depth of compost desired.

For example, to determine how much compost is needed to add 3 inches of compost over a 100-square-foot garden, you start by converting the 3 inches to feet: 3 inches divided by 12 inches is 0.25 feet.

Then, multiply the area (100 square feet) times the desired compost depth (0.25 feet). The amount of compost needed is 25 cubic feet.

More about volume: Compost, soil, and bark are typically sold by the cubic yard. One cubic yard is 3 feet wide by 3 feet long by 3 feet high, or 27 cubic feet. To spread 1 foot of compost over a 100 square foot garden, nearly 4 cubic yards is needed (100 square feet times 1 foot high divided by 27 cubic feet per cubic yard = 3.7 cubic yards) or 100 cubic feet.

Think of the compost being in blocks that are 12 inches x 12 inches x 12 inches (or 1 cubic foot). One hundred of these blocks are needed to cover the 100-square-foot garden. To spread 3 inches of compost instead of 12 inches would be 25 percent of 3.7 cubic yards or 0.93 cubic yards (or 25 cubic feet) because 3 inches is 25 percent of 12 inches (see sidebar).

Amount of fertilizer

Fertilizer application recommendations are usually given in pounds of a particular nutrient needed per 100, or 1,000 square feet of lawn or garden. Most commercial fertilizers contain a combination of nitrogen, phosphorus and potassium, with the percentage of each by weight identified on the bag. For example, if a bag is labeled 5-0-0, the bag contains 5 percent nitrogen (N), 0 percent phosphorus (P), and 0 percent potassium (K) by weight.

If the soil test indicates 4 pounds of nitrogen are needed per 1,000 square feet of garden, divide the amount of nitrogen recommended (4 pounds) by the percentage of nitrogen in the fertilizer (5 percent) to determine the total amount of fertilizer needed. In this example, you will need 20 pounds of the 5-0-0 fertilizer per 1,000 square feet.

If the fertilizer contained 100 percent nitrogen rather than 5 percent, you would need 4 pounds.

For more

Those who read this column know that Master Gardeners always suggest soil testing to understand your soil requirements; for information, call Clallam Conservation District 360-452-1912. The report will give information about the type, and the amount, of amendments needed.

Our hope is that the math in this article will make application easy. It’s wise to add only the amendments needed. Your garden will be healthier and you will save money and time.

This is a good “clip and save” article to have on hand next week when this column will talk about soil.

Judy English is a Washington State University-Certified Clallam County Master Gardener.

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