How to Calculate Cubic Yards of Concrete for Slabs and Round Columns

The quick answer

For a rectangular slab, multiply length by width by thickness after converting the dimensions into compatible units. For a round column, use the cylinder formula with radius and height. If the volume is in cubic feet, divide by 27 to convert it to cubic yards.

The current Mycelgrid Concrete Calculator then applies the waste percentage entered by the user, if any, to the base geometric volume.

Calculate concrete for a rectangular slab

In US units, slab length and width are entered in feet while thickness is entered in inches. Convert the thickness to feet before multiplying.

For a 20 ft x 12 ft slab that is 4 in thick:

• 4 in / 12 = 0.333333 ft.
• 20 x 12 x 0.333333 = 80 ft³.
• 80 / 27 = 2.962963 yd³, displayed as about 2.96 yd³.

That is the base geometric volume before any optional waste allowance.

Calculate concrete for a round column

A round column is a cylinder, so the diameter must first become a radius.

For a 12 in diameter column that is 8 ft high:

• 12 in diameter = 1 ft diameter.
• Radius = 0.5 ft.
• pi x 0.5² x 8 = about 6.283 ft³.
• 6.283 / 27 = about 0.233 yd³.

Convert between cubic feet, cubic yards, and cubic meters

The Tool reports equivalent volume units so a result can be cross-checked against plans or supplier units.

For example, a 6 m x 4 m slab that is 10 cm thick has a base volume of 2.4 m³: 6 x 4 x 0.10 = 2.4.

Apply a user-entered waste allowance

The calculator can increase the base geometric volume by the percentage entered by the user.

Using the 2.962963 yd³ slab above with a 10% entered allowance:

• 2.962963 x 1.10 = 3.259259 yd³.
• The Tool displays the estimated adjusted volume as about 3.26 yd³.

Base volume and adjusted volume are different

The base volume is the geometric amount calculated from the dimensions. The adjusted volume is the base volume after the optional percentage is applied. Keeping those values separate makes the estimate easier to review.

What this calculation does not include

The current calculator is a geometric volume estimator. It does not calculate or automatically account for:

• bag yield or bags required;
• reinforcement displacement;
• over-excavation, overbreak, spillage, or uneven subgrade;
• mix design or product-specific yield;
• supplier minimums, delivery rules, or site tolerances.

Verify field dimensions and local supplier or job-site requirements before ordering concrete. The Tool also rejects dimensions whose arithmetic exceeds the browser’s supported finite calculation range rather than presenting an overflowed result.