Concrete Slab Calculator (Cubic Yards, m³ & Bags)
Estimate concrete volume in cubic metres, cubic yards, and cubic feet, plus the number of 40, 60, or 80 lb pre-mix bags for a slab, pad, driveway, or footing.
Net volume before waste: 2.40 m³ (3.14 yd³). Bag count assumes 0.0170 m³ finished concrete per 80 lb bag — round up and buy a spare bag or two.
How this works
Concrete volume is length × width × thickness, multiplied by the number of slabs, then increased by a waste allowance:
net m³ = L × W × T × quantity
ordered = net × (1 + waste% / 100)
yd³ = m³ × 1.308
bags = ceil(ordered m³ / bag yield)Bag yields are the finished-concrete volume per bag from manufacturer data: about 0.017 m³ (0.6 ft³) for an 80 lb bag, 0.0127 m³ (0.45 ft³) for a 60 lb bag, and 0.0085 m³ (0.3 ft³) for a 40 lb bag.
Worked example
A 6 m × 4 m patio slab at 100 mm thick, one slab, with a 10% waste allowance:
- net =
6 × 4 × 0.1= 2.4 m³ - ordered =
2.4 × 1.10≈ 2.64 m³ (≈ 3.45 yd³) - 80 lb bags =
ceil(2.64 / 0.017)≈ 156 bags — clearly a ready-mix job, not a bag job
Above roughly 0.5 m³, ready-mix delivery is cheaper and stronger than mixing bags by hand.
Sources
- Bag yields: Quikrete / Sakrete concrete mix product data (finished volume per 40, 60, 80 lb bag).
- Unit conversions: 1 m³ = 1.30795 yd³ = 35.3147 ft³.
FAQ
How do I calculate how much concrete I need for a slab?
Multiply length by width by thickness in consistent units to get the volume, then add a waste allowance for spillage, uneven subgrade, and over-excavation. For a slab 6 m long, 4 m wide, and 100 mm (0.1 m) thick, the net volume is 6 × 4 × 0.1 = 2.4 m³ (about 3.14 cubic yards). Ready-mix concrete is ordered in cubic metres or cubic yards, so most homeowners buy in those units rather than counting bags.
Should I use bagged concrete or order ready-mix?
Bagged (pre-mix) concrete is practical for small pours — post footings, small pads, repairs, and slabs under roughly 0.5 m³ (about 0.65 cubic yards). Above that, mixing dozens of bags by hand becomes slow and inconsistent, and a ready-mix truck delivery is usually cheaper per cubic metre and gives a stronger, more uniform result. The calculator shows both a bag count and the ready-mix volume so you can compare.
How many 80 lb bags of concrete make a cubic yard?
An 80 lb (36 kg) bag yields roughly 0.6 cubic feet of finished concrete, and a cubic yard is 27 cubic feet, so you need about 45 bags of 80 lb concrete per cubic yard. A 60 lb bag yields about 0.45 ft³ (60 bags per yard) and a 40 lb bag about 0.3 ft³ (90 bags per yard). Always round up and buy one or two spare bags.
How much extra concrete should I order for waste?
A waste allowance of 5 to 10 percent is typical for a flat, well-formed slab on a compacted, level base. Increase it toward 10 to 15 percent when the subgrade is uneven, the formwork is rough, or you are pouring footings into hand-dug trenches where the walls slump. Running short mid-pour forces a cold joint, so it is safer to over-order slightly than to stop.
What thickness should a concrete slab be?
Thickness depends on the load, not on the calculator. A typical residential floor slab or patio is 100 mm (4 in), a driveway for cars is 100 to 125 mm (4 to 5 in), and a slab for heavy vehicles or a garage may be 150 mm (6 in) or more, often with reinforcement. Structural slabs, raft foundations, and anything carrying a building must be sized by a qualified engineer — this tool only turns confirmed dimensions into a material quantity.
Does this include reinforcement, formwork, and base material?
No. The result is the concrete volume only. A real slab also needs formwork, a compacted sub-base of crushed stone or gravel, a damp-proof membrane, reinforcement mesh or bars where specified, chairs or bar supports, and curing water or compound. Estimate the sub-base separately with the gravel and aggregate calculator, and confirm reinforcement from the structural drawings.
How is cubic metres converted to cubic yards?
One cubic metre equals about 1.308 cubic yards, and one cubic yard equals about 0.765 cubic metres. The calculator shows both plus cubic feet, so you can order in whichever unit your local supplier uses. Metric markets such as Australia, New Zealand, the UK, and most of Asia quote cubic metres; the United States quotes cubic yards.