When you need this calculation
Ordering bulk mulch by the cubic yard, working out how many bags to buy at a garden center, or refreshing beds in spring. Bulk suppliers sell by the cubic yard in the US and by the cubic meter in the UK, Europe and Australia, while stores sell bags in cubic feet or liters, so the same bed has to be measured three ways.
How the volume is worked out
Multiply the bed’s length by its width to get the area, then by the depth. In US units the depth is usually in inches, so divide it by 12 to get feet first. The result is in cubic feet; divide by 27 for cubic yards, because a cubic yard is 3 × 3 × 3 feet.
For beds that aren’t rectangles, split them into rectangles and add the results, or measure the area another way and use a length of that area and a width of 1. For a round bed, the area is 3.1416 × radius².
How deep to mulch
University extension services generally recommend 2 to 4 inches of organic mulch around trees and shrubs and 1 to 2 inches around annuals, perennials and vegetables. Deeper isn’t better: thick layers can mat down and keep water and air from reaching the roots. Keep mulch pulled back a few inches from trunks and stems.
When topping up an existing bed, measure the depth already there and calculate only the difference.
Common mistakes
Forgetting to convert inches to feet. 20 × 10 × 3 is 600, but that’s cubic feet only if all three numbers are in feet. With a 3 inch depth, the bed needs 50 cubic feet, not 600.
Ordering exactly the calculated amount. Mulch settles and beds are rarely perfect rectangles. Rounding up to the next half yard, or one more bag, avoids a second trip.
For soil, use the topsoil calculator; for stone, the gravel calculator.