Soil Calculator

What is a soil calculator?

A soil calculator works out how much topsoil, compost, or garden-bed soil you need to fill a rectangular bed or planting area. You enter the bed’s length and width in feet and the depth of soil you want in inches, and the calculator returns the volume in both cubic feet and cubic yards.

Soil — like gravel, mulch, and concrete — is sold and delivered by the cubic yard, but garden beds are almost always measured on site by their footprint in feet and the desired depth in inches. This tool bridges those two units so you can order the right amount in one go.

How does it work?

A rectangular bed has three dimensions: length, width, and depth. Multiplying them gives the volume. The catch is that length and width are measured in feet while depth is entered in inches, so the depth has to be converted to feet first by dividing by 12.

Multiplying all three measurements in feet gives the volume in cubic feet:

$ft^3 = L \times W \times \frac{d}{12}$

Here $L$ and $W$ are the bed length and width in feet and $d$ is the depth in inches, so $d/12$ is the depth in feet. Because one cubic yard equals 27 cubic feet (a yard is 3 feet, and $3 \times 3 \times 3 = 27$), dividing by 27 converts the volume to cubic yards:

$yd^3 = \frac{L \times W \times (d/12)}{27}$

Worked examples

Example 1 — a 10 ft by 4 ft bed, 6 inches deep. The depth in feet is $6/12 = 0.5$ ft, so the volume in cubic feet is $10 \times 4 \times 0.5 = 20$ ft³. Dividing by 27 gives:

$yd^3 = \frac{20}{27} \approx 0.7407 \text{ yd}^3$

You would order roughly 0.74 cubic yards of soil, rounding up to allow for settling.

Example 2 — a 12 ft by 3 ft bed, 4 inches deep. The depth in feet is $4/12 \approx 0.3333$ ft, so the volume in cubic feet is $12 \times 3 \times (4/12) = 12$ ft³. Dividing by 27 gives:

$yd^3 = \frac{12}{27} \approx 0.4444 \text{ yd}^3$

About 0.44 cubic yards of soil.

Practical notes

• Fresh soil and compost settle once watered and worked, so order 5–10% extra to keep the bed topped up after the first season.
• For a deeper raised bed you only need to fill the part you actually plant into. Many gardeners fill the bottom of a tall bed with cheaper bulk material and reserve good topsoil for the top 8–12 inches.
• If your bed is an irregular shape, measure its footprint first with the square footage calculator, then use that area with a depth to find the volume.
• The same area-and-depth method covers other bulk materials. To estimate stone or aggregate instead, use the gravel calculator, and to convert any footprint and depth straight to cubic yards, see the square feet to cubic yards calculator.

FAQ

How many cubic yards of soil do I need?

Multiply the bed length and width in feet by the depth in feet (inches divided by 12), then divide by 27. For example, a 10 ft by 4 ft bed filled 6 inches deep needs $10 \times 4 \times 0.5 / 27 \approx 0.74$ cubic yards.

Why is the depth divided by 12?

The length and width are in feet, so every dimension in the volume formula must also be in feet. Since depth is entered in inches and there are 12 inches in a foot, dividing the inches by 12 converts the depth to feet before it is multiplied by the area.

How much does a cubic yard of topsoil cover?

A cubic yard is 27 cubic feet. Spread 1 inch deep it covers about 324 square feet; at 3 inches it covers about 108 square feet; and at 6 inches it covers about 54 square feet. Deeper layers cover proportionally less area.