Monolithic Slab Concrete Calculator
Enter outside slab dimensions and the edge depth from your plans.
Concrete
Concrete Calculator for Slabs with Thickened Edges
Quick answer: total concrete = slab L × W × T + edge cross-section × edge centerline length + interior beams, ÷ 27 for yd³. A 24 × 24 ft garage slab 4 in thick with a 12 × 12 in turndown is 192 + 61.3 = 253.3 ft³ = 9.38 yd³, or about 10.3 yd³ with 10% waste.
Built for monolithic slabs where the footing and floor are poured together: the field slab, a thickened perimeter edge (square or with a sloped inner face), and optional interior thickened beams under bearing walls or posts, with bags, trucks and cost.
Thickened-edge slab section
How to calculate concrete for a thickened-edge slab
Treat the pour as three solids: a flat slab across the full outside footprint, a ring of extra concrete hanging below the slab around the perimeter, and any interior strips. Counting the edge only below the slab bottom avoids paying twice for the top 4 inches, and measuring it along its centerline keeps the four corners from being counted twice.
L·W·T + Aedge × [2(L + W) − 8x̄] + n·ℓ·w·h
Aedge = (bottom W + top W) ÷ 2 × (edge depth − T) · x̄ = edge centroid distance from the outside face · ÷ 27 for yd³
- Field slab. Outside length × outside width × slab thickness, all in feet.
- Edge cross-section. Extra depth = total edge depth − slab thickness. For a square turndown, area = width × extra depth. With a sloped inner face, use the trapezoid average of bottom and top widths.
- Edge run. Start with the outside perimeter, 2 × (L + W). For a square turndown subtract 4 × the edge width; for a sloped one the calculator subtracts 8 × the section's centroid distance, which is the exact correction for the corners.
- Interior beams. Count × length × width × depth below the slab.
- Add, divide by 27, add waste. Trenched turndowns are dug by hand or bucket and are rarely exact; 10% is a sensible minimum.
Worked example: 24 × 24 ft garage
A detached garage slab is 24 × 24 ft, 4 in thick, with a 12 in wide turndown 12 in deep from the top of the slab. There are no interior beams.
- Field slab: 24 × 24 × 0.333 = 192.0 ft³
- Edge: extra depth 12 − 4 = 8 in (0.667 ft); section 1 × 0.667 = 0.667 ft²
- Edge run: 96 − 4 × 1 = 92 ft → 92 × 0.667 = 61.3 ft³
- Net: 253.3 ft³ ÷ 27 = 9.38 yd³; with 10% waste = 10.32 yd³ → two trucks at 10 yd³, about $1,703 at $165/yd³
The turndown alone is 2.27 yd³ — almost a third more concrete than the 7.11 yd³ flat slab. If the same edge had a sloped inner face widening to 24 in at the slab, the section grows to 1.0 ft² and the edge to 89.8 ft³, adding about another 1.05 yd³.
Extra concrete in a thickened edge, by size
Concrete below a 4 in slab only, for a straight run (corners ignored), before waste. Add these to the flat-slab volume.
| Edge (bottom width × total depth) | Depth below slab | ft³ per 10 ft | yd³ per 10 ft | yd³ per 100 ft |
|---|---|---|---|---|
| 12 in × 12 in | 8 in | 6.67 | 0.25 | 2.47 |
| 16 in × 12 in | 8 in | 8.89 | 0.33 | 3.29 |
| 12 in × 16 in | 12 in | 10.00 | 0.37 | 3.70 |
| 12 in bottom, 24 in top (sloped) × 12 in | 8 in | 10.00 | 0.37 | 3.70 |
| 16 in × 18 in | 14 in | 15.56 | 0.58 | 5.76 |
When thickened edges are used, and what to watch
- Typical uses. Detached garages, sheds, workshops, carports and slab-on-grade houses where frost depth is shallow. The thickened edge acts as the footing under the walls, so the whole foundation is formed and poured in one day.
- Size comes from the plans. 12 in wide × 12 in deep turndowns are common for small outbuildings; houses and heavier walls may call for wider or deeper edges. Follow your drawings and local code rather than a rule of thumb.
- Frost still matters. In cold regions an uninsulated turndown may not meet frost requirements. Options include digging the edge to frost depth, a frost-protected shallow foundation with rigid insulation, or a separate footing and stem wall — ask the building department what they accept.
- Interior thickenings carry point and wall loads. Strips under a bearing wall, a post, or a heavy appliance are drawn as interior beams or pads. Enter strips as beams; for square post pads, enter each pad as a beam with length equal to its width.
- Measure the dug trench, not the drawing. Machine-dug turndowns often come out wider and deeper than planned, and sloped faces are frequently the result of soil sloughing. Recheck after excavation and update the edge numbers.
- Steel sits in the edge too. Plans commonly show continuous bars in the turndown in addition to slab mesh or rebar. Use a rebar calculator for that; it does not change concrete volume meaningfully.
Thickened-edge slab FAQ
What is a thickened edge slab?
A thickened edge slab, also called a monolithic or turned-down slab, is a floor slab and its perimeter footing poured in one piece. The slab is typically 4 inches thick and the edge drops to a deeper, wider section that carries the wall loads. It is common for garages, sheds and slab-on-grade houses in mild climates.
How deep should the thickened edge of a slab be?
A 12 in wide by 12 in deep turndown, measured from the top of the slab, is common for sheds and detached garages. Houses and colder climates often need deeper or wider edges, or a frost-protected design, to reach the required bearing and frost protection. Your plans and local code set the actual size.
How much concrete do I need for a 24x24 garage slab with thickened edges?
A 24 ft × 24 ft slab at 4 inches is 192 cubic feet. A 12 in × 12 in turndown adds 61.3 cubic feet (8 in below the slab along a 92 ft centerline), for 253.3 cubic feet or 9.38 cubic yards. With 10% waste, order about 10.3 cubic yards.
Does a sloped inner edge use more concrete than a square turndown?
Yes, if the bottom width stays the same. On a 24 × 24 ft slab, widening the top of a 12 in turndown to 24 in with a sloped inner face raises the edge volume from 61.3 to 89.8 cubic feet, about 1.05 cubic yards more before waste.
How many bags of concrete for a shed slab with thickened edges?
An 8 × 10 ft shed slab at 4 inches with a 12 in × 12 in turndown needs 48 cubic feet of concrete, or 52.8 cubic feet with 10% waste. At about 0.60 cubic feet per 80 lb bag that is roughly 88 bags, so many builders order a short load of ready-mix instead.