Garden Bed Soil Math: Rectangles, L-Shapes, and Circles

The short version

Soil volume is length × width × depth. The only tricky part is keeping your units consistent. If you measure the bed in feet and the depth in inches, convert the depth to feet first by dividing by 12. Multiply the three numbers and you have cubic feet. Divide by 27 for cubic yards.

Step 1: Measure the bed

Use a tape measure and write down the length and width at the widest points. If the edges aren't straight, take a few measurements and use the average. For new beds, stake out the corners with string first so you're measuring the bed you plan to build, not a rough guess.

Step 2: Decide on depth

The depth you need depends on what you're growing and what's already there.

• Topsoil under new sod or seed: 4–6 inches
• Annual flowers and most vegetables: 6–12 inches
• Shrubs and perennials in poor native soil: 12 inches or more, mixed with existing soil
• Top-up on an established bed: 1–2 inches

Step 3: Do the math

Here's a 10 ft × 4 ft bed that you want to fill 8 inches deep: 8 ÷ 12 = 0.667 ft. Then 10 × 4 × 0.667 = 26.7 cubic feet. Divide by 27 and you get just under 1 cubic yard. In 1.5 cu ft bags, that's 26.7 ÷ 1.5 = 17.8, so 18 bags.

Odd shapes

Split L-shaped or T-shaped beds into rectangles, calculate each, and add them. For a round bed, measure across the middle (the diameter), multiply it by itself, and multiply by 0.785. That gives the area; multiply by the depth in feet for volume. A kidney-shaped bed can usually be approximated as a rectangle about 80% of its overall length and width.

Allow for settling

Loose soil compacts after watering. If you need the finished bed to sit at a specific height, order 10–20% more than your calculated volume. The CalcAndPlan Soil Calculator lets you add this allowance automatically and shows the number before and after so you can decide.

Bags or bulk?

Bags are convenient for small jobs and hard-to-reach backyards. Once you're past one or two cubic yards — roughly 18 to 36 bags of 1.5 cu ft — bulk delivery is often cheaper even after the delivery fee. Prices vary a lot by region, so compare the delivered total.