Concrete Slab Calculator — Volume, Bags & Thickness
Calculate the concrete volume for a slab in cubic metres or cubic yards, plus the bag count if you're mixing yourself. Enter slab dimensions and thickness to get the amount to order, with waste factor and a ready-mix vs bags comparison.
Free to try — no sign-up
Plan the whole job — not just this calculation
Save results to a project: budget and shopping list build themselves, and the whole plan exports as a PDF. Draw your floor plan and room sizes pre-fill the calculators.
How the slab volume is calculated
- 1Volume = length × width × thickness
- 2Thickness by use: patios and paths 100 mm (4 in), driveways 100–125 mm (4–5 in), garage floors 100–150 mm
- 3Add 5–10% for waste, uneven sub-base and spillage
- 4Bags: volume ÷ yield per bag (0.012 m³ for 25 kg; 0.017 m³ for 80 lb)
- 5Ready-mix: order in m³ or yd³ — usually more economical above 1 m³
- 6Reinforcement (mesh or rebar) is design-dependent — specify it separately
Worked example
Single-car garage slab: 6 × 3 m (≈ 20 × 10 ft) at 100 mm (4 in) thick = 1.8 m³. With 8% waste: 1.95 m³.
Order 2.0 m³ of ready-mix (C25/30) — mixing this by hand would take about 162 bags of 25 kg premix, which is rarely practical above 1 m³.
Frequently asked questions
How thick should a concrete slab be?
Pedestrian patios: 75–100 mm. Driveways: 100–125 mm. Garage floors: 100 mm. Structural floor slabs: 150–200 mm with reinforcement. Always check local building codes for vehicle loads.
How many yards of concrete for a 24×24 garage?
A 24×24 ft slab at 4 in thick is 192 ft³ = 7.1 yd³ (5.4 m³). With 8% waste, order about 7.5–8 yd³ (≈ 6 m³) of ready-mix.
Do I need rebar or mesh in a slab?
Light-duty patios on well-compacted base often use unreinforced concrete or crack-control mesh. Driveways and garage floors normally get welded mesh or rebar. Reinforcement is a structural design decision — the calculator estimates volume only.
How long before I can drive on a new slab?
Foot traffic after 24–48 hours; vehicles after 7 days at the earliest. Concrete reaches about 70% of design strength at 7 days and full design strength at 28 days.
Do I need a vapor barrier under a slab?
Under any interior or enclosed slab (garage, basement, extension floor): yes — a polyethylene membrane prevents ground moisture migrating through. Exterior patios generally don't need one.