A monolithic slab pours the floor and its footing together: a thin slab in the middle, a thickened edge beam around the perimeter. This calculator adds both parts into one order figure.
Monolithic Slab Calculator
Total pour (with waste)
The edge is where monolithic orders go wrong: priced as a plain 4 in slab, this same 20 × 20 pour would come up almost 2 yards short, mid-pour, with the truck leaving.
A monolithic slab calculator adds two volumes: the flat slab, plus the extra depth of the thickened edge footing that runs around its perimeter, below the slab. Enter slab length, width and thickness plus the edge width and total depth above to get one combined order figure in yards, bags and cost.
How to Calculate a Monolithic Slab
“Monolithic” means poured in one piece: the slab and the footing under its edges go in together, in a single truck visit, with no cold joint between them. It is the standard foundation for sheds, garages, workshops and many slab-on-grade homes in frost-mild regions. The volume has two parts, and both must be in the order.
The Monolithic Formula
Edge extra = Perimeter × Edge width × (Edge depth − Slab thickness)
Total = Slab volume + Edge extra · Perimeter = 2 × (L + W)
Note the subtraction: the top slice of the edge trench is already counted inside the slab volume. Only the part of the edge that hangs below the slab is extra, counting the full edge depth twice is the classic monolithic over-order.
Worked Example: 20 × 20 ft Slab, 4 in Thick, 12 × 12 in Edge
Slab = 20 × 20 × 0.333 = 133.3 ft³ (4.94 yd³). Perimeter = 80 ft. Edge extra = 80 × 1 ft × (12−4)÷12 ft = 53.3 ft³ (1.98 yd³). Total = 186.7 ft³ = 6.91 yd³ exact, 7.60 yd³ with 10% waste, about $1,141 at $150/yd³.
Typical Monolithic Dimensions
| Building | Slab thickness | Edge (width × total depth) |
|---|---|---|
| Shed / small outbuilding | 4 in | 12 in × 12 in |
| Garage / workshop | 4–5 in | 12 in × 12–18 in |
| Slab-on-grade house | 4 in | 12–16 in × 18–24 in (to frost depth) |
Edge depth must reach below your local frost line where codes require it, in cold climates that is exactly why some regions use separate frost walls instead of a monolithic edge.
How to Measure and Calculate: Step by Step
The edge is where monolithic measurements go wrong, so measure it as its own trench: its width, and its total depth from the top of the slab to the bottom of the trench, then let the calculator subtract the slab thickness it has already counted. Walk the perimeter with a tape rather than trusting the drawing’s rectangle; a building that is 20 ft 6 in on one side carries that extra half-foot all the way around the edge trench, where it is worth real concrete.
- Measure slab length and width to the outside of the edge.
- Probe the edge trench: record its width and total depth.
- Enter slab thickness, edge width and edge depth above.
- Check the slab/edge split in the results, the edge is usually 25–40% of the pour.
- Confirm edge depth reaches the frost line your code requires, or rethink the design.
Worked Example 2: Garage Slab, 24 × 24 ft, 4 in Slab, 12 × 18 in Edge
Slab = 24 × 24 × 0.333 = 192 ft³ (7.11 yd³). Perimeter = 96 ft. Edge extra = 96 × 1 ft × (18−4)÷12 ft = 112 ft³ (4.15 yd³). Total = 304 ft³ = 11.26 yd³ exact, 12.38 yd³ with 10% waste, two trucks, and the edge alone is 37% of the pour. Deepening the edge from 12 to 18 inches added more concrete than many whole patio slabs contain, which is why cold-climate frost depths push designs toward separate frost walls instead.
Common Quantities at a Glance: Cheat Sheet
| Job / quantity | Concrete needed |
|---|---|
| 20 × 20 ft, 4 in, 12 × 12 edge | 6.91 yd³ exact · edge adds 1.98 yd³ |
| 24 × 24 ft, 4 in, 12 × 12 edge | 9.48 yd³ exact |
| 30 × 40 ft, 4 in, 12 × 12 edge | 18.27 yd³ exact |
| Edge share, small slabs | up to ~40% of total volume |
| Edge share, large slabs | falls toward ~20% as area outgrows perimeter |
Practical Tips From the Pour
Form and trench the edge first and pour the whole slab in one continuous operation, the entire point of ‘monolithic’ is no cold joint between edge and slab, and a pour that stalls halfway around the perimeter creates exactly the joint the design exists to avoid. Order for the combined figure with the follow-up load pre-agreed with the plant. Insulate or protect the edge where your climate demands it: the thickened edge is a thermal bridge as well as a footing, and several cold-climate codes now ask for edge insulation on heated slab-on-grade buildings.
Common Mistakes to Avoid
- Counting the full edge depth without subtracting the slab thickness, a systematic over-order.
- Pricing the pour as a plain slab and discovering the edge mid-pour, 2 yards short.
- Setting edge depth by habit instead of the local frost line.
- Assuming the slab dimensions are the inside of the edge when drawings show the outside.
- Splitting the pour across two days and creating the cold joint the design avoids.
What It Costs and How to Order Smart
Monolithic construction saves a whole operation, no separate footing pour, no second delivery, no second mobilisation, and that saving is real money even though the concrete volume is identical to the two-pour route. On the 20 × 20 example, 7.60 yd³ with waste at a $150 placeholder rate is about $1,141 of material in a single delivery. Set against it: deeper trenching and more careful forming in one pass. The Cost Calculator can price the combined yardage from this page against two separate deliveries if you want the comparison in numbers.
Who Uses This Calculator?
Builders in slab-on-grade country use this calculator as the foundation take-off: slab and edge in one figure, one delivery, one pour. Contractors use the slab/edge split to explain to clients why a ‘simple slab’ carries a footing’s worth of extra concrete around its edge. Engineers reviewing a monolithic design check the ordered volume against the edge depth the frost line demands. Architects coordinating slab-on-grade details can see how a deeper edge moves the total before the sections are drawn. Homeowners planning a garage or workshop get the complete number, including the part of the pour hidden underground that plain slab calculators miss.
Monolithic Slab FAQs
Monolithic slab vs separate footing and slab?
One pour instead of two: faster, no cold joint, and one delivery. The trade-off is that the edge depth is limited by how deep you can trench and form in a single pour, which is why cold-climate foundations often stay two-part.
How much extra does the edge add?
On the 20 × 20 example, the edge adds 1.98 yd³ on top of a 4.94 yd³ slab, about 40% more concrete. On smaller slabs the edge share is even bigger, because the perimeter shrinks slower than the area.
Does the edge run inside the slab dimensions?
This calculator treats the edge as sitting under the perimeter of the stated slab size, which matches how these slabs are normally formed and measured. If your drawings size the slab to the outside of the edge, you are already measuring the same way.
How much extra concrete does the thickened edge add?
On the 20 × 20 ft example, 4-inch slab, 12 × 12 inch edge, the edge adds 1.98 cubic yards on top of the 4.94 yd³ slab, about 40% more concrete. The share shrinks as slabs grow, because edge volume follows the perimeter while slab volume follows the area: on a 30 × 40 ft slab the same edge is about 26% of the total. Small buildings are therefore hurt most by forgetting the edge.
What is the difference between a monolithic slab and a regular slab?
A regular slab is a uniform-thickness plate, often poured after separate footings and a stem wall are already in place. A monolithic slab is poured in a single operation with its perimeter dug deeper to form an integral edge footing, so slab and footing are literally one piece of concrete, ‘monolithic’ means ‘one stone’. The volume calculation reflects that: slab volume plus only the edge depth that hangs below the slab.
How deep does a monolithic edge need to be?
Deep enough for its base to sit below your local frost line where the code requires frost protection, and deep enough to reach firm bearing soil everywhere. In mild climates a 12-inch total edge depth is common; the dimension table on this page shows 18–24 inches for house slabs in colder areas. Where frost depth runs to 3–4 feet, a monolithic edge becomes impractical and a separate frost wall is the standard solution.
Why do you subtract the slab thickness from the edge depth?
Because the top slice of the edge trench is already counted inside the slab’s own volume: the slab calculation covers the full footprint at slab thickness, including the strip above the edge. Only the part of the edge below that slice is extra concrete. Skipping the subtraction double-counts a perimeter strip, on the 20 × 20 example that error is 80 ft × 1 ft × 0.333 ft = about 0.99 yd³, a full phantom yard on the order.
Can a monolithic slab be used for a heated building?
Yes, slab-on-grade with a thickened edge is a standard heated-building foundation in suitable climates, often with rigid insulation under the slab and around the edge to cut the thermal bridge. Insulation does not change the concrete volume this calculator returns, but it does change trench preparation: the edge may be formed wider to fit insulation boards, so measure the trench you actually dig and enter its real width.
Is a monolithic slab cheaper than separate footing and slab?
Usually, on suitable sites: one excavation pattern, one forming setup, one delivery and one pour day instead of two, with no cold joint to prepare between footing and slab. The concrete itself costs the same either way, the volumes are identical. Where it loses is deep-frost country and difficult soils, where reaching frost depth in a single pour is impractical or where an engineer wants the footing and wall as separate, inspectable elements.
Related Calculators
Sources used for standards and conversions: National Ready Mixed Concrete Association (NRMCA) guidance on ordering and yield; American Concrete Institute (ACI) practice for mix and placement; Portland Cement Association (PCA) design and control guidance; US bagged-concrete manufacturers’ printed yields (80 lb bag = 0.60 ft³). Frost depths, footing sizes and step dimensions come from your local building code. If the code says something different from the general figures here, go with the code.
Disclaimer: Results are estimates for planning and ordering. Check the quantities with your ready-mix supplier before you pour. Footings, walls, columns and any other load-bearing work need a licensed engineer to look them over.