Monolithic Slab Calculator
Enter the slab here. Then choose Add another area, pick Footing, and enter the thickened edge: its length is the slab perimeter and its depth is the extra depth below the slab.
Last updated November 13, 2026
How to calculate a monolithic slab
A monolithic slab is a flat slab with a thicker edge, poured in one go. The total is the slab plus the extra concrete in the edge.
- Slab: length × width × thickness.
- Thickened edge: perimeter × edge width × extra depth. The extra depth is the edge depth minus the slab thickness.
Example: a 20 ft × 20 ft slab, 4 inches thick, with an edge 12 inches wide and 12 inches deep overall. Slab: 20 × 20 × 0.333 = 133.3 cubic feet. Edge: the perimeter is 80 ft and the extra depth is 8 inches, so 80 × 1 × 0.667 = 53.3 cubic feet. Total 186.7 cubic feet, or 6.91 cubic yards.
How much the edge adds
Figures are for a 4 inch slab with an edge 12 inches wide and 12 inches deep overall, no waste.
| Slab size (ft) | Slab only | Edge | Total |
|---|---|---|---|
| 10 × 12 | 1.48 yd³ | 1.09 yd³ | 2.57 yd³ |
| 12 × 16 | 2.37 yd³ | 1.38 yd³ | 3.75 yd³ |
| 20 × 20 | 4.94 yd³ | 1.98 yd³ | 6.91 yd³ |
| 24 × 24 | 7.11 yd³ | 2.37 yd³ | 9.48 yd³ |
| 30 × 40 | 14.81 yd³ | 3.46 yd³ | 18.27 yd³ |
On small slabs the edge can add half as much concrete again. This estimate treats the edge as a square trench. A sloped inner face uses a little less, and corners overlap slightly, so the figure runs a touch high.
Interior footings
If the plan has thickened strips under interior walls or posts, add each one as another Footing area in the same way.
Frequently asked questions
What is a monolithic slab?
A slab and its footing poured at the same time as one piece. The edges of the slab are thickened to act as the footing.
How do I calculate concrete for a monolithic slab?
Work out the flat slab, then add the extra concrete in the thickened edge: perimeter × edge width × the extra depth below the slab.
How deep should the thickened edge be?
That is set by your local code and depends on the building, the soil and the frost depth. Use the dimensions on your plan.