How Much Concrete Do I Need? Calculating Cubic Yards and Bag Counts

Concrete is ordered and sold by the cubic yard. One cubic yard equals 27 cubic feet, a cube measuring 3 feet on each side. Ready-mix trucks deliver in cubic yards; bagged mix yields fractions of a cubic foot per bag. To order correctly, you need the project volume in cubic yards before anything else (to automate these calculations, try our concrete calculator).

The calculation is straightforward, but it has one consistent error point: thickness. Most homeowners measure thickness in inches but need to convert it to feet before multiplying. Mixing inches and feet produces a wrong number by a factor of 12.

The Core Concrete Formula

A cubic yard is simply a cube measuring 3 feet on all sides. That means it contains exactly 27 cubic feet.

To find your project's volume in yards, you multiply the length, width, and thickness in feet, then divide that total by 27.

Cubic Yards = [Length (feet) × Width (feet) × Thickness (feet)] ÷ 27

The trickiest step for most homeowners is converting the slab's thickness from inches to feet. If you multiply feet by inches, your math will be completely off. Always divide the target inches by 12 first:

• 4 inches of thickness = 4 ÷ 12 = 0.33 feet
• 6 inches of thickness = 6 ÷ 12 = 0.50 feet

Bagged Mix vs. Ready-Mix: When to Use Which

Bagged Mix (60 lb or 80 lb bags): Best for small projects under 2 cubic yards. Post holes, stepping stones, small repairs, and garden borders. An 80 lb bag yields 0.60 cubic feet; a 60 lb bag yields 0.45 cubic feet. You'll need 45 bags of 80 lb mix to equal one cubic yard.

Ready-Mix Delivery: Best for slabs, driveways (which should be sealed after curing; see our driveway sealing cost guide), and large pads over 2 cubic yards. A standard ready-mix truck carries 8-10 cubic yards. For smaller orders (1-5 yards), expect a "short-load" fee of $15-$30 per yard under the minimum.

Common Mistakes to Avoid

• Forgetting the waste buffer: Ground isn't perfectly flat. Forms bow outward under pressure. Always add 10% to your calculation.
• Mixing inches and feet: If your slab is 4 inches thick, don't multiply by 4. Multiply by 0.33 (4 ÷ 12).
• Ordering exact amounts: See above. Always round up. The $50 extra for half a yard is cheaper than a ruined project.
• Not checking access: Ready-mix trucks are 8 feet wide and 12 feet tall. They need a clear path to within 10-15 feet of your pour site. If the truck can't reach, you'll be wheelbarrowing concrete, and that's exhausting work.

Example Calculations

10×10 Patio at 4 inches thick:
(10 × 10 × 0.33) ÷ 27 = 1.22 cubic yards
With 10% buffer: 1.34 cubic yards (round up to 1.5 yards)
In bags: 1.34 × 27 ÷ 0.60 = 60 bags of 80 lb mix

24×24 Driveway at 6 inches thick:
(24 × 24 × 0.50) ÷ 27 = 10.67 cubic yards
With 10% buffer: 11.74 cubic yards (round up to 12 yards)
This requires a full ready-mix truck order.

Access is often the overlooked constraint for ready-mix orders. Standard ready-mix trucks are 8 feet wide and 12 feet tall, and they need a clear path to within 10–15 feet of the pour site. If the truck can't get within range, the concrete must be wheelbarrowed to the forms, which adds labor, time, and the risk of concrete beginning to set during transport. Confirm site access before scheduling delivery. If access is limited, a smaller volumetric truck (which mixes on-site) may be available in your area.

Use our Concrete Calculator to compute cubic yards, bag counts, and cost estimates for your specific dimensions.