How to Calculate Concrete Volume: Slabs, Footings & Bag Counts

For any flat concrete pour (patios, shed pads, driveways, sidewalks), calculate cubic feet and convert to cubic yards:

Step 1: Measure Length (ft) and Width (ft) inside the wooden forms.

Step 2: Measure thickness in inches and divide by 12 (e.g. 4 inches = 4 ÷ 12 = 0.333 ft).

Step 3: Multiply Length × Width × Thickness (ft) = Total Cubic Feet.

Step 4: Divide Cubic Feet by 27 to find Cubic Yards.

Deck piers, post footings, and pole barn foundations frequently use round cardboard Sonotubes. Use the cylinder volume formula: Volume = π × Radius² × Depth.

For a 12-inch diameter sonotube (radius = 0.5 ft) that is 4 feet deep: Volume = 3.1416 × (0.5)² × 4 = 3.14 cubic feet per pier.

Bagged concrete mixes (such as Quikrete or Sakrete) have fixed yields when water is properly added:

• 80-lb Bag: Yields 0.60 cu ft (Takes 45 bags to equal 1 cubic yard).

• 60-lb Bag: Yields 0.45 cu ft (Takes 60 bags to equal 1 cubic yard).

• 50-lb Bag: Yields 0.375 cu ft (Takes 72 bags to equal 1 cubic yard).

Excavated ground is never perfectly laser-flat. Wooden 2×4 form boards flex slightly outward under hydraulic concrete weight, and sub-base gravel absorbs small amounts of slurry.

Always order 10% extra concrete. Having a small wheelbarrow load leftover is a minor inconvenience; running out 10 square feet short leaves an ugly, weak cold joint.