Estimate concrete volume for a rectangular slab, add a waste allowance, and convert the result to premix bag quantities.
Concrete volume, slabs, footings, blocks, bricks, pavers and related material quantities.
Enter the slab length and width in feet and thickness in inches. Add a waste allowance for spillage, uneven subgrade and ordering margin. The calculator reports cubic yards and cubic feet, then estimates the number of 40 lb, 60 lb or 80 lb premix bags when you select a bag size.
| Slab | Thickness | Volume before waste |
|---|---|---|
| 10 × 10 ft | 4 in | 1.23 yd³ |
| 10 × 20 ft | 4 in | 2.47 yd³ |
| 12 × 12 ft | 4 in | 1.78 yd³ |
| 20 × 20 ft | 4 in | 4.94 yd³ |
The bag count is a planning estimate based on approximate mixed yields of 0.30 ft³ for a 40 lb bag, 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag. Product yields vary, so use the coverage printed by the manufacturer when purchasing material.
For larger pours, cubic yards are usually more useful than bag counts. Confirm dimensions in the field and consider the shape of footings, thickened edges, grade changes and other features that a simple rectangular-slab calculation does not capture.
A slab 20 feet long, 10 feet wide, and 4 inches thick has volume 20 × 10 × (4 ÷ 12) = 66.67 cubic feet. Divide by 27 to obtain approximately 2.47 cubic yards. Adding a 10% waste allowance gives approximately 2.72 cubic yards to plan for.
Before ordering: field thickness and excavation depth can vary, and concrete suppliers may have minimum loads. A bag mix calculator uses estimated bag yield; verify the manufacturer's stated yield and project specifications. Reinforcement, foundation engineering, and curing requirements are separate from volume.
About 2.47 cubic yards before waste. With 10% extra, the estimate is about 2.72 cubic yards.
Using an approximate yield of 0.60 cubic feet per bag, one cubic yard is about 45 bags. Check the specific product label before buying.
A waste allowance can help account for uneven excavation and handling loss, but the appropriate amount depends on the project.
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