About this calculator
Work out how much concrete a slab, patio, shed base or driveway needs, then see it as a ready-mix order in cubic yards, a bag count, or a site-mix recipe. Add the result to a job and it carries straight into your quote.
How it’s calculated
Volume = length × width × thickness. A circular slab uses π × radius² × thickness. The waste allowance is then added on top.
Cubic yards = cubic feet ÷ 27. Bag yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft.
Worked example
With these inputs:
- Slab shape
- Rectangle / square
- Length
- 20 ft
- Width
- 12 ft
- Thickness
- 4 in
- Number of slabs
- 1
- Supply as
- Pre-mixed bags
- Bag size
- 80 lb
- Waste allowance
- 10%
the result is:
- Concrete needed
- 3.26 cu yd
- Slab area
- 240 sq ft
- Net volume (no waste)
- 2.96 cu yd
- Ready-mix order (to ¼ yd)
- 3.5 cu yd
- 80 lb bag
- 147 bags
Tips from the trade
- Measure thickness at several points on the sub-base. A slab that is ½" thicker than planned across the whole area adds up fast.
- 5–10% waste is normal for a formed slab on a level base. Use 10–15% if the ground is uneven or the edges are hand-dug.
- Most suppliers deliver in ¼ cu yd steps and charge a short-load fee under about 3–4 yards.
- Past about 1 cubic yard (around 45 bags of 80 lb), ready-mix or a mix-on-site lorry is usually cheaper and much faster than bags.
Common questions
How thick should a concrete slab be?
Walks and patios are typically 4", driveways 4–6", and garage floors 4–6" with reinforcement. Local code and soil conditions take priority.
How many bags of concrete do I need for a slab?
Divide the volume by your bag yield. The calculator shows every common bag size at once so you can compare prices.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and local building codes before ordering.