Square Foot Calculator with Depth: Volume in Cubic Feet

This calculator works out the volume of a rectangular space when you know its length, width, and depth. You enter the three dimensions in feet (or inches, which are converted automatically), and you get back the volume in cubic feet and cubic yards. It is the standard way to estimate how much soil, mulch, gravel, or concrete you need for a project.

Formula

The formula is straightforward: Volume (cubic feet) = Length (ft) × Width (ft) × Depth (ft). Since depth is usually given in inches, you first convert it to feet by dividing by 12. The equation in symbols is:

V = L × W × D

• V = volume in cubic feet (ft³)
• L = length in feet (ft)
• W = width in feet (ft)
• D = depth in feet (ft), after converting from inches (D_in ÷ 12)

To get cubic yards, divide the cubic feet by 27 (since 1 yard = 3 feet, and 3 × 3 × 3 = 27). This is the unit that most bulk materials are sold in.

Worked Example

Using the default values from the calculator: length = 10 ft, width = 5 ft, depth = 3 in.

1. Convert depth to feet: 3 in ÷ 12 = 0.25 ft
2. Calculate cubic feet: 10 × 5 × 0.25 = 12.5 ft³
3. Convert to cubic yards: 12.5 ÷ 27 = 0.46 yd³ (rounded to two decimals)

The calculator shows 12.50 cubic feet and 0.46 cubic yards, matching the manual calculation.

Why Depth Matters (and the Common Mistake)

Many people confuse square feet with cubic feet. Square feet is a two-dimensional area (length × width), while cubic feet adds depth to give a three-dimensional volume. The most frequent error is entering depth in inches without converting to feet, which makes the volume 12 times too large. Another common mistake is mixing units—for example, measuring length in feet but width in inches. Always use the same unit for all three dimensions before multiplying.

When to Use This Calculator

This tool is for any project that involves filling a rectangular space to a certain depth:

• Landscaping: topsoil, mulch, compost, or sand for a garden bed or lawn
• Construction: gravel or crushed stone for a driveway base, concrete for a slab
• Pools and ponds: estimating water volume or liner fill
• Moving and storage: checking if a truck or storage unit can hold your boxes (though irregular shapes need a different approach)

For irregular shapes, break the area into rectangles, calculate each separately, and add the volumes.

How to Read the Result and Order Materials

The result in cubic feet tells you the exact volume of space you need to fill. When buying bulk materials, suppliers usually sell by the cubic yard. For example, 12.5 cubic feet equals about 0.46 cubic yards, so you would order at least 0.5 cubic yards. For bagged products, check the bag label—common sizes are 0.5, 1, or 2 cubic feet. Divide your total cubic feet by the bag size to find the number of bags. Always add 5–10% extra to account for settling, compaction, and measurement errors, especially for soil and gravel.

Limitations and Accuracy Tips

This formula assumes a perfectly rectangular prism with uniform depth. Real-world surfaces are rarely perfectly flat, so your actual needs may vary. For best accuracy:

• Measure length and width at the widest points, in feet (or inches and convert).
• Measure depth at several spots and average them if the ground is uneven.
• For sloped areas, the depth should be the average depth, not the deepest point.
• Remember that materials like mulch and soil settle after being spread, so a slightly larger order is often wise.

If you are calculating concrete, note that concrete is usually ordered by the cubic yard, and the depth is the slab thickness. For a 4-inch slab (common for patios), divide 4 by 12 to get 0.333 feet.

Frequently Asked Questions

How do I convert square feet to cubic feet with depth?

Multiply the square footage (length × width) by the depth in feet. If your depth is in inches, divide it by 12 first. For example, a 100 sq ft area at 3 inches deep is 100 × (3/12) = 25 cubic feet.

How many cubic feet are in a cubic yard?

There are 27 cubic feet in one cubic yard (3 feet × 3 feet × 3 feet). To convert cubic feet to cubic yards, divide by 27. This is the standard unit for bulk landscape materials.

What if my area is not a perfect rectangle?

Divide the area into smaller rectangles, calculate the volume for each, and add them together. For circular areas, use the formula π × radius² × depth. For very irregular shapes, consider using a different method or consult a professional.

How much mulch do I need for a 10×10 garden bed?

Decide on the depth first. For 3 inches deep: 10 × 10 × (3/12) = 25 cubic feet, which is about 0.93 cubic yards. For 2 inches deep, it is 16.67 cubic feet (0.62 cubic yards).

Does the calculator work for concrete slabs?

Yes, the same formula applies. A 10×10 slab at 4 inches thick is 10 × 10 × (4/12) = 33.33 cubic feet, or about 1.23 cubic yards. Order a little extra to account for waste.

Why do I need to add extra material?

Soil, mulch, and gravel compact and settle after installation, and measurements are rarely exact. Adding 5–10% ensures you don’t run short mid-project. For concrete, extra is needed to account for uneven subgrades and spillage.

Related

• Land Cent to Square Feet Calculator
• Horizontal Cylindrical Tank Volume Calculator
• Horizontal Dish End Tank Volume Calculator

in length | Length | ft | 10 | 0
in width | Width | ft | 5 | 0
in depth | Depth | in | 3 | 0
out cubic_ft | Volume | cu ft | length * width * (depth / 12) | 2
out cubic_yd | Volume | cu yd | (length * width * (depth / 12)) / 27 | 2