How Much Concrete for a Thickened Edge Slab?
How much concrete for a thickened edge slab is the flat slab volume plus the extra concrete in the deeper perimeter. For example, a 24x24 garage slab poured 4 inches thick takes about 7.1 cubic yards. If the outer edge is thickened to 12 inches deep and 12 inches wide, that adds about 2.3 cubic yards, for a total near 9.4 yards before waste. Edges with sloped sides, wider bottoms, or deeper frost-protected designs add more.
Concrete calculator →How to calculate it
- Flat slab: length × width × slab thickness (ft) ÷ 27.
- Extra edge depth: edge depth minus slab thickness.
- Edge volume: perimeter (minus corner overlap) × edge width (ft) × extra depth (ft) ÷ 27.
- Add them and allow 5 to 10 percent.
For a 24x24 slab: perimeter is 96 feet; subtract 4 feet so corners are not counted twice, leaving 92 feet. Extra depth is 12 − 4 = 8 inches (0.667 ft). Edge volume is 92 × 1 × 0.667 = 61.3 cubic feet, or 2.27 yards.
Examples
| Slab | Flat slab (4 in) | Edge 12 x 12 in | Total |
|---|---|---|---|
| 12 x 16 shed | 2.37 yd | 1.10 yd | 3.47 yd |
| 16 x 20 shop | 3.95 yd | 1.41 yd | 5.36 yd |
| 24 x 24 garage | 7.11 yd | 2.27 yd | 9.38 yd |
| 24 x 30 garage | 8.89 yd | 2.57 yd | 11.46 yd |
Edges with a 45-degree sloped inner face hold more concrete than a square edge. A rough adjustment is to add half the edge width again to the bottom width when estimating.
Why builders thicken the edge
A thickened edge, also called a turndown or monolithic slab, acts like a built-in footing under the walls. It stiffens the slab edge where loads concentrate, resists frost-related cracking at the perimeter, and saves forming a separate footing. Sheds, garages, and pole barns often use it. In cold climates, codes may require deeper footings or frost-protected shallow foundation details instead, so check with your building department.
Digging the edge trench
Machine-dug trenches run wider than planned, which adds concrete fast. A trench that ends up 16 inches wide instead of 12 adds a third more edge volume. Measure the trench after digging, not the drawing, and recalculate before ordering.
Reinforcement
Thickened edges usually get two continuous rebar bars near the bottom, tied at corners, plus mesh or rebar in the flat slab. Keep steel off the dirt with chairs or bricks so it ends up covered by concrete.
Base and vapor barrier
The flat part of the slab sits on compacted gravel, typically 4 inches. The trench bottom should be firm, undisturbed soil or compacted stone. A vapor barrier can run under the slab and down into the edge trench. See gravel base under a concrete slab.
Bottom line
To estimate a thickened edge slab, calculate the flat slab, then add the edge volume: perimeter times edge width times extra depth. A 12 by 12 inch edge adds roughly 1 to 2.5 yards to common shed and garage slabs. Measure the real trench width before ordering, since a wider trench is the most common reason for running short.
Questions people ask
How much extra concrete does a thickened edge add?
For a 12 by 12 inch edge on a 4-inch slab, about 0.67 cubic feet per foot of perimeter, or roughly 2.3 yards on a 24x24 garage.
How deep should a thickened slab edge be?
Often 12 to 18 inches, but local code and frost depth decide. Check with your building department.
Is a thickened edge slab the same as a monolithic slab?
Yes, the terms are often used for a slab poured in one piece with its perimeter footing.
Do thickened edges need rebar?
Typically, yes. Two continuous bars near the bottom of the edge is a common detail.
Why did my thickened edge slab need more concrete than planned?
The trench was likely wider or deeper than the plan. Measure after digging and order from those numbers.
Related guides
- How Much Concrete for a 24x24 Garage Slab?
- Gravel Base Under a Concrete Slab: Depth and Tons
- Garage Slab Thickness: 4 Inches, 5, or 6 for Vehicles
- Rebar vs Wire Mesh in a Slab: Which One to Use
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