Concrete Slab Calculator
Enter the dimensions of a slab, path or footing — or the diameter of a round column or post hole — to get concrete volume in cubic yards or cubic metres, the number of pre-mixed bags needed, and the total weight, with a waste allowance already applied.
The calculation behind the numbers
Concrete is sold by volume, so every estimate reduces to length × width × thickness. The only trap is unit mixing: slab faces are measured in feet or metres while thickness is quoted in inches or centimetres.
- Imperial: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet. Divide by 27 for cubic yards.
- Metric: length (m) × width (m) × thickness (cm) ÷ 100 = cubic metres.
Bag yields are fixed by the manufacturer: an 80 lb bag makes 0.60 cubic feet, a 60 lb bag 0.45, a 40 lb bag 0.30. A 25 kg bag yields about 0.0125 cubic metres. Cured concrete weighs roughly 150 lb per cubic foot, or 2,400 kg per cubic metre — worth checking before you load a trailer.
Common slab sizes
| Slab | Thickness | Cubic yards | 80 lb bags |
|---|---|---|---|
| 8 × 8 ft | 4 in | 0.79 | 36 |
| 10 × 10 ft | 4 in | 1.23 | 56 |
| 12 × 12 ft | 4 in | 1.78 | 80 |
| 20 × 20 ft | 5 in | 6.17 | 278 |
| 24 × 24 ft | 6 in | 10.67 | 480 |
Volumes are exact; bag counts include no waste allowance. Add 10% for a real pour.
Frequently asked questions
How much concrete do I need for a 10x10 slab?
A 10 by 10 foot slab poured 4 inches thick contains 33.3 cubic feet, which is 1.23 cubic yards. With the standard 10% waste allowance that becomes 1.36, so order 1.5 — suppliers sell in quarter yards. That same slab needs 56 eighty-pound bags at the exact volume if you mix on site, which is why anything above about half a cubic yard is normally ready-mixed.
Why add a waste allowance?
Sub-grade is never perfectly level, forms bow outward under the weight of wet concrete, and some material is always lost in the chute and wheelbarrow. Ten percent is standard for a formed slab on a prepared base. Use 15% for uneven ground or footings dug straight into soil, where over-excavation is unavoidable.
How thick should a concrete slab be?
Four inches (100 mm) is standard for patios, walkways and shed bases. Go to five or six inches (125–150 mm) for a driveway carrying passenger vehicles, and six inches with reinforcement for anything taking a truck. Thickening the edge of a slab by two inches adds far more strength than adding the same volume across the middle.
Is it cheaper to use bags or ready-mix?
Bagged concrete costs roughly three to four times more per cubic yard than ready-mix, but ready-mix suppliers charge a short-load fee below about one cubic yard and expect the pour to be finished within the truck's unloading window. The crossover point is usually around half a cubic yard — below it use bags, above it order a truck.
Does this include reinforcement or base material?
No. This calculates concrete volume only. A slab also needs a compacted sub-base of typically four inches of crushed stone, plus rebar or welded wire mesh, and a vapour barrier under anything that will be enclosed. Those are separate material takeoffs.
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