Units:
Rectangular Slab
Length × Width × Thickness
ft
ft
in
Total volume needed
—
cubic yards
Cubic Yards
—
Cubic Feet
—
Cubic Meters
—
Bags of pre-mixed concrete needed
40 lb bag
—
bags
0.30 cu ft / bag
60 lb bag
—
bags
0.45 cu ft / bag
80 lb bag
—
bags
0.60 cu ft / bag
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Coverage Reference
| Thickness | Coverage per yard | Coverage per 80lb bag | 80lb bags per yd³ |
|---|---|---|---|
| 3 inches | 108 sq ft | 5.4 sq ft | 20 bags |
| 4 inches | 81 sq ft | 4.1 sq ft | 45 bags |
| 5 inches | 65 sq ft | 3.3 sq ft | 55 bags |
| 6 inches | 54 sq ft | 2.7 sq ft | 65 bags |
| 8 inches | 40 sq ft | 2.0 sq ft | 90 bags |
| 10 inches | 32 sq ft | 1.6 sq ft | 112 bags |
| 12 inches | 27 sq ft | 1.4 sq ft | 135 bags |
For large pours above 1 cubic yard, consider ordering ready-mix concrete by the yard from a local supplier rather than mixing bags — it's more economical and far less labour-intensive. For small repairs and footings under 1 yard, pre-mixed bags are perfectly practical.
Always add 5–10% extra to your calculated volume. Running short mid-pour is a serious problem — you can't just stop and continue later, as the joint will be visible and structurally weak. The waste toggle above handles this automatically.
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