Monolithic Slab Calculator: Slab Plus Thickened Edge
A 24×20 ft monolithic slab, 4 in thick with a 12 in wide edge 8 in deeper, needs 8.80 cubic yards of concrete with 10% extra.
Advanced options: waste, prices
NRMCA advises ordering 4 to 10 percent over the estimate.
Concrete needed (with 10% extra)
8.80 yd³
6.73 m³ · 216 ft³ before extra
Monolithic slab: 24 ft × 20 ft × 4 in × 12 in × 8 in. Over about 1 yd³: order ready-mix.
Premix bags
- 80 lb396
- 60 lb528
- 50 lb634
- 40 lb792
- Ready-mix cost
- $1,408
- Weight
- 32,400 lb
Full breakdown
| Volume before extra | 8 yd³ · 6.12 m³ |
|---|---|
| With 10% extra | 8.80 yd³ |
| Weight (150 lb/ft³) | 32,400 lb · 16.20 tons |
| Pallets of 40 lb (80 bags each) | 10 |
| Pallets of 60 lb (56 bags each) | 10 |
| Pallets of 80 lb (42 bags each) | 10 |
| Truckloads at 12 yd³ | 1 |
| Truckloads at 8 yd³ | 2 |
Bag counts round up, as Quikrete's own calculator does. Cost uses your prices; delivery and short-load fees are extra.
Key takeaways
- Monolithic = slab and thickened-edge footing poured in one piece.
- Edge at least 12 in wide × 6 in deep (IRC footing minimum).
- Slab at least 3½ in (IRC R506.1).
- A 24×20 ft slab with a 12×8 in edge = 7.18 yd³ with extra.
How Much Concrete for a Monolithic Slab?
You need the slab volume plus the volume of the thickened edge around it. A 24×20 ft slab at 4 in holds 160 ft³; a 12 in wide edge 8 in below the slab adds 56 ft³, for 8 yd³ before extra and 8.80 yd³ with 10%.
What Is a Monolithic Slab?
A monolithic slab is a concrete floor and its perimeter footing poured together in one piece, so the edge of the slab is thickened to act as the footing. It is common for garages, sheds and slab-on-grade houses in mild climates.
Monolithic Slab Formula
ft³ = L × W × T + (2 × (L + W) − 4 × edge width) × edge width × edge depth below slabThe corner term stops the four corners from being counted twice.
Monolithic Slab Examples: Garage and Shed
| Slab (4 in, 12×8 in edge) | ft³ | yd³ with 10% | Trucks at 12 yd³ |
|---|---|---|---|
| 24×24 ft | 253.33 | 10.32 | 1 |
| 24×20 ft | 216 | 8.80 | 1 |
| 12×16 ft | 98.67 | 4.02 | 1 |
| 10×12 ft | 66.67 | 2.72 | 1 |
How Deep and Wide Is the Thickened Edge?
The edge acts as a footing, so the 2021 IRC footing minimum of 12 in (305 mm) wide and 6 in (152 mm) deep applies (R403.1.1), and the floor slab itself must be at least 3½ in thick (R506.1). Frost depth and soil set deeper edges locally.
Monolithic Slab vs Slab and Stem Wall
A monolithic slab pours the footing and floor at once; a stem-wall foundation pours a footing, builds a short foundation wall, then pours the slab inside it. Stem walls suit deeper frost lines and sloped lots; the choice follows local code and the site.
Gravel and Vapor Barrier Under a Monolithic Slab
A monolithic house slab follows the same base rules as any slab: a 4 in base course below grade and a 10-mil vapor retarder (2021 IRC R506.2). See base gravel calculator.
How to Pour a Monolithic Slab
- Excavate the perimeter trench for the thickened edge below frost depth.
- Prepare and compact the base inside the slab area (IRC R506.2).
- Set forms and any reinforcement the plans call for.
- Order the full volume and pour the edge and slab together.
Monolithic Slab Cost
Concrete for a 24×24 ft monolithic garage slab (4 in, 12×8 in edge) costs about $1,651–$2,013 at $160–195 per yard; excavation, forms, base and finishing add labor.
Monolithic Slab vs Floating Slab
A floating slab rests on the ground without an integral footing, used for sheds and patios; a monolithic slab adds the thickened edge so it can carry walls.
Monolithic Slab for a Shed or Pole Barn
A 12×16 ft shed slab with a 12×8 in edge needs 4.02 yd³ with extra; a 30×40 ft pole barn slab with the same edge needs about 19.99 yd³. Enter your own edge size in the calculator.
Worked Examples: Two Monolithic Slabs
24×24 ft garage, 4 in slab, 12×8 in edge. Slab 192 ft³ + edge (96 − 4) × 1 × 0.667 = 61.3 ft³ = 253.3 ft³ = 9.38 yd³; with 10% extra, about 10.32 yd³.
10×12 ft shed, 4 in slab, 12×8 in edge. Slab 40 ft³ + edge (44 − 4) × 0.667 = 26.7 ft³ = 66.7 ft³ = 2.47 yd³; with 10% extra, about 2.72 yd³.
Monolithic Slab Calculator FAQ
What is a monolithic slab?
A monolithic slab is a concrete floor and its perimeter footing poured as one piece.
Is a monolithic slab better?
It is faster to build in mild climates; stem-wall foundations suit deep frost lines and sloped lots.
How much gravel under a monolithic slab?
A 4 in base course under the slab area, per IRC R506.2.2 for slabs below grade; see gravel under a concrete slab.
Next step
Check the base on gravel under a concrete slab and price the pour on the concrete price calculator.
Official sources
Last reviewed 2026-11-05. Numbers come from our tested engine; see how we calculate and our sources.