# How to Calculate Volume for Topsoil Gardening

Calculating how much topsoil you need to fill your raised bed involves a simple mathematical formula. Topsoil is usually sold by the cubic foot or the cubic yard, so it's best to have both measurements for your space when you arrive at the garden supply store. Instead of just measuring the length and width, you also need to measure the depth so you can calculate how much topsoil it would take not just to cover the bottom of the garden area but also to fill the raised bed to the desired depth.

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Measure the length and width of your flower or vegetable garden bed in inches. If it's a raised bed, measure from the inside edges of the sides. Measure the depth as well. This isn't usually the same as the depth of your raised bed sides. If your sides are 10 inches deep, for example, the soil should be about 8 inches deep to allow the sides to hold the soil in place.

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Multiply the three measurements together. If your bed is 48 inches by 48 inches by 8 inches deep, for example, you have 18,432 cubic inches.

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Convert your inch measurement into cubit feet, which is how bags of topsoil are often sold. Divide your cubic inches by 1,728, which is the number of inches in a cubic foot. (Multiply 12 by 12 by 12 to get 1,728.) If you have 18,432 cubic inches, you have 10.67 cubic feet.

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Find the cubic yard measurement as well in case your garden supply store sells the topsoil by the cubic yard instead of the cubic foot. Divide the number of cubic feet by 27, which is how many cubic feet are in a cubic yard. If you have 10.67 cubic feet, you have 0.4 cubic yards, which means you need less than half a cubic yard for your garden bed.

References

Tips

- Garden bed measurements rarely fall exactly in line with a topsoil quantity. For example, if you need 10.67 cubic feet of topsoil but the bags cover 3 cubic feet each, you must choose between buying three bags and covering 9 cubic feet or buying four bags and covering 12 cubic feet. It's always best to buy more than you think you'll need. You can hold the extra topsoil in reserve to replenish the bed for fall plantings or use it to increase the size of other existing flower beds.

Writer Bio

Based outside Atlanta, Ga., Shala Munroe has been writing and copy editing since 1995. Beginning her career at newspapers such as the "Marietta Daily Journal" and the "Atlanta Business Chronicle," she most recently worked in communications and management for several nonprofit organizations before purchasing a flower shop in 2006. She earned a BA in communications from Jacksonville State University.