Concrete Slab Calculator
Size a floor or ground slab: volume, ready-mix load, weight and area, including an edge thickening if there is one.
Formula
The slab body and the perimeter thickening are calculated separately and added, then increased by the wastage allowance. The thickening is an inward ring, so its corners are counted once.
- Slab body (m³) = L × W × T
- Ring area (m²) = L × W − (L − 2w) × (W − 2w), where w is the thickening width
- Thickening (m³) = Ring area × Extra edge depth
- Net volume (m³) = Slab body + Thickening
- Ordered volume (m³) = Net volume × (1 + wastage % ÷ 100)
- Weight (kg) = Net volume × Density
Worked example
A 10 m × 6 m slab, 150 mm thick, with a 300 mm wide thickening running 200 mm deeper around the perimeter, at 7% wastage.
- Slab body = 10 × 6 × 0.15 = 9.00 m³
- Inner rectangle = (10 − 0.6) × (6 − 0.6) = 9.4 × 5.4 = 50.76 m²
- Ring area = 60.00 − 50.76 = 9.24 m²
- Thickening = 9.24 × 0.20 = 1.848 m³
- Net = 9.00 + 1.848 = 10.848 m³
- Ordered = 10.848 × 1.07 = 11.61 m³
- Slab weight = 10.848 × 2400 = 26,035 kg
Order 11.61 m³. The slab itself weighs about 26.0 tonnes — the wastage is not part of it.
Assumptions and limitations
- The thickening is treated as a rectangular downstand of constant section forming an inward ring around the slab. A tapered or stepped thickening will use less concrete.
- The four corners are counted once. Treating the thickening as a strip of length equal to the perimeter would count each corner twice — on a small slab with a wide thickening that error reaches 14%.
- Density is a user input, not a fixed constant. 2400 kg/m³ is a common planning value for normal-weight reinforced concrete, but the mix design governs.
- The weight shown is the weight of the finished slab, calculated from the net volume. The ordered volume includes wastage, which does not end up in the slab.
What this calculator does not do
- Does not size the slab, check bearing pressure, or verify any structural requirement.
- Does not handle stepped slabs, downstand beams, upstands or penetrations — calculate those separately.
- Does not account for falls or cambers; use the average thickness.
Practical tips
- Check the slab thickness against the subgrade level, not the design drawing. Sites routinely pour thicker than drawn because the subbase was trimmed low.
- If the slab has a fall for drainage, calculate at the average thickness rather than the thin edge.
- Concrete weight matters for temporary works. A 10 m × 6 m slab at 150 mm is around 22 tonnes sitting on the ground before you add anything to it.
Frequently asked questions
How thick should a concrete slab be?
That is a design decision, not a calculation this tool can make. It depends on the loads, the subgrade, the reinforcement and the local code. Typical domestic floor slabs run 100–150 mm; industrial floors are considerably thicker. Use the project specification.
How much does a concrete slab weigh?
Multiply the net volume by the density. At around 2400 kg/m³, every cubic metre weighs roughly 2.4 tonnes. Use the net volume, not the volume you ordered — the wastage allowance is spillage and over-break, and never becomes part of the slab.
Do I need to include the edge thickening?
Yes, if the slab has one. On a 10 m × 6 m slab a modest 300 × 200 mm thickening adds about 1.85 m³ — more than most people expect, and more than the wastage allowance would cover.
How many truck loads is my slab?
Divide the total volume by the truck capacity. Most ready-mix trucks carry 6–8 m³, but the usable capacity depends on the plant, the mix and local axle-load rules, so confirm with your supplier.
Related calculators
- Concrete Volume Calculator Work out the volume of concrete needed for a rectangular pour, with an allowance for wastage.
- Concrete Footing Calculator Calculate concrete for strip footings or a set of identical pad footings.
- Concrete Bags Calculator Convert a concrete volume into the number of pre-mixed bags needed, for small pours where ready-mix is not practical.
- Rebar Weight Calculator Calculate the weight of reinforcement bars from diameter, length and quantity.
- Excavation Calculator Calculate excavation volume in bank measure and the swelled volume that has to be hauled away.
Estimating tool, not a design. This calculator works out quantities from the dimensions you enter. It does not size any structural member, verify a design, or confirm compliance with any building code. Check every result against your project specification and applicable local standards, and involve a qualified engineer where the work requires one.