To calculate how much concrete you need, measure the finished length and width, choose the specified thickness and convert the result into cubic yards. The dimensions should describe the concrete itself, not the full excavation or formwork area.
Concrete volume formula
For a 20-by-12-foot slab that is 4 inches thick, the theoretical volume is 80 cubic feet, or about 2.96 cubic yards. A 10% ordering allowance brings the planning quantity to about 3.26 cubic yards.
Calculate separate sections separately
A project may include a main slab, thickened edge, steps or isolated footing sections. Calculate each rectangular section on its own and add the volumes. This is easier to check than forcing different thicknesses into one average.
| Input | Use this measurement | Common mistake |
|---|---|---|
| Length and width | Finished concrete dimensions | Using excavation size without checking forms |
| Thickness | Specified concrete thickness | Choosing thickness from a generic online example |
| Allowance | Site-appropriate extra quantity | Ordering the exact geometric volume |
| Bag yield | Yield printed on the selected bag | Assuming every bag size yields the same volume |
Ready-mix or bags?
Bags can be practical for repairs and small pads. As volume grows, the labor and consistency of hand mixing become important. Compare the calculated bag count with ready-mix delivery minimums, short-load charges, access and the time available to place the concrete.
Thickness is a design decision
The calculator does not choose slab thickness, reinforcement, base preparation, joints or concrete mix. Those details depend on loads, soil, climate and local requirements. Confirm them before converting dimensions into an order.
Before ordering
- Recheck the inside dimensions of the forms.
- Confirm the subgrade and base are at the planned elevation.
- Ask the supplier about minimum order, delivery and rounding increments.
- Plan labor, access and finishing tools before the truck or mixed bags are ready.
After calculating volume, use the concrete cost per yard guide to compare complete project quotes.
Calculate concrete yards and bags →
Verify the project specification
Local conditions, product requirements and project rules can change the final quantity or installation method. For additional background, consult Concrete Masonry & Hardscapes Association resources, along with the selected manufacturer and local authority.
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