Monolithic Concrete Slab Calculator
Use this monolithic concrete slab calculator to estimate the concrete volume needed for a slab that includes a thickened perimeter edge or integral footing. This is useful for slab foundations, shed bases, garages, and similar pours where the slab body and edge thickening are cast together. The calculator helps with quantity planning by estimating the total concrete needed for the combined shape.
How this calculator works
A monolithic slab combines a main slab area with a deeper thickened edge around the perimeter. This calculator works out the volume of the main slab section and adds the extra concrete in the thickened edge section. The total result is shown in cubic metres (metric) or cubic yards (imperial), with litres or cubic feet as a secondary unit. A waste allowance is then applied to give the final total.
Formulas used
base volume = length × width × slab thickness
extra depth = max(edge total depth − slab thickness, 0)
perimeter = (2 × length) + (2 × width)
edge volume = perimeter × edge width × extra depth
total = base volume + edge volume
final total = total × (1 + waste % / 100)
The edge extra depth is the portion of the edge that goes below the main slab thickness. Only that extra portion is included in the edge volume calculation — the concrete at slab thickness depth is already counted in the base slab volume. When custom sides are selected, only those sides contribute to the perimeter.
Worked example
Take a slab measuring 4 m by 3 m with a slab thickness of 100 mm. The base slab volume is 4 × 3 × 0.1 = 1.2 m³. If the thickened edge has an edge width of 300 mm and an edge total depth of 300 mm, the extra depth below the slab is 300 − 100 = 200 mm (0.2 m). The full perimeter is (2 × 4) + (2 × 3) = 14 m. The edge volume is then 14 × 0.3 × 0.2 = 0.84 m³. Total before waste is 1.2 + 0.84 = 2.04 m³. With a 10% waste allowance, the final total is 2.04 × 1.10 = 2.244 m³.
What a monolithic slab is
A monolithic slab is poured in one operation so the slab surface and the thickened edge or footing form a single continuous concrete element. This can simplify some foundation pours and makes accurate quantity planning more important, because the total volume is not just a flat slab. This calculator helps estimate that combined concrete volume for planning purposes only.
Frequently asked questions
Related tools
- Concrete Slab Bag Calculator — how many bags of concrete for a slab
- Gravel Under Concrete Slab Calculator — sub-base volume, tonnes and bulk bags
- Weight of Concrete Slab Calculator — estimate slab weight in kg, tonnes and lb
- Dry Pour Concrete Calculator — volume, bags and cost for dry-pour slabs
Guides worth reading
- How to Pour a Monolithic Slab: Shed & Garden Building Bases (UK) — when to use one, depths, mix and a worked example
- How Much Sub-Base Under a Concrete Slab? UK Guide — the layer that stops it cracking
Disclaimer: This calculator is for planning and estimating only. Actual concrete quantities vary with dimensions, edge geometry, waste, and site conditions. Use it as a guide only and check project requirements before ordering.