How to Calculate Concrete for a Slab (With Worked Examples)

A step-by-step way to work out the cubic yards and bags of concrete for a patio, shed base or garage slab, with two worked examples and common mistakes.

Whether you are pouring a patio, a shed base or a garage floor, the amount of concrete you need comes down to one volume calculation. This guide walks through it, shows two worked examples, and explains how to turn the answer into cubic yards or bags.

Step 1: Measure the slab

Measure the length and width in feet. For an L-shaped or irregular slab, split it into rectangles, work out each one, and add the results. Then decide the thickness in inches. Four inches is common for patios and walkways, while driveways and garage floors are often thicker. Local building rules may set a minimum for your project.

Step 2: Convert to cubic feet

Volume in cubic feet = length × width × thickness in inches ÷ 12. The division by 12 turns inches into feet so all three measurements use the same unit.

Step 3: Convert to cubic yards

Ready-mix trucks sell by the cubic yard. One cubic yard is 27 cubic feet, so divide your cubic feet by 27.

Worked example 1: a 10 × 12 foot patio, 4 inches thick

• Volume: 10 × 12 × 4 ÷ 12 = 40 cubic feet
• Cubic yards: 40 ÷ 27 = 1.48
• Add 10% for waste: 1.48 × 1.1 = 1.63 cubic yards

Worked example 2: a 20 × 20 foot pad, 6 inches thick

• Volume: 20 × 20 × 6 ÷ 12 = 200 cubic feet
• Cubic yards: 200 ÷ 27 = 7.41
• Add 10% for waste: 7.41 × 1.1 = 8.15 cubic yards

How many bags is that?

Bag yields differ by brand, so read the bag. A typical 80-lb bag yields about 0.6 cubic feet and a 60-lb bag about 0.45 cubic feet. For the 10 × 12 patio: 40 cubic feet plus 10% is 44 cubic feet, and 44 ÷ 0.6 is about 74 bags of 80 lb. At that quantity most people compare the price of a ready-mix delivery.

Why add waste?

The ground under a slab is never perfectly level and forms can bow slightly, so the pour usually takes more than the geometry says. An allowance of 5 to 10 percent is common. Running short in the middle of a pour is far more expensive than ordering a little extra.

Common mistakes

• Measuring thickness from the dirt instead of from the top of the compacted base.
• Mixing units, such as using inches for thickness but feet for length and width.
• Forgetting the gravel base, reinforcement, forms and finishing tools, which are separate costs.
• Ordering exactly the calculated amount with no allowance.

Before you dig or pour near utilities, call 811 (in the United States) so underground lines can be marked.