About this calculator
Work out how much concrete a slab, patio, shed base or driveway needs, then see it as a ready-mix order in cubic metres (with cubic yards alongside), a bag count, or a site-mix recipe. Add the result to a job and it carries straight into your quote.
How it’s calculated
Volume = length × width × thickness. A circular slab uses π × radius² × thickness. The waste allowance is then added on top.
Ready-mix is sold by the cubic metre in Canada: 1 m³ = 1.31 cu yd = 35.3 cu ft. Bag yield used: a 30 kg bag of concrete mix makes roughly 0.014 m³ (0.5 cu ft), so a cubic metre takes about 71 bags. Check the yield on your brand’s bag.
Worked example
With these inputs:
- Slab shape
- Rectangle / square
- Length
- 20 ft
- Width
- 12 ft
- Thickness
- 4 in
- Number of slabs
- 1
- Supply as
- Pre-mixed bags
- Bag size
- 30 kg
- Waste allowance
- 10%
the result is:
- Concrete needed
- 2.49 m³
- Slab area
- 240 sq ft
- Net volume (no waste)
- 2.27 m³
- Ready-mix order (to 0.25 m³)
- 2.5 m³ (3.27 cu yd)
- 30 kg bag
- 178 bags
Tips from the trade
- Measure thickness at several points on the base. A slab that is ½" thicker than planned across the whole area adds up fast.
- 5–10% waste is normal for a formed slab on a level base. Use 10–15% if the ground is uneven or the edges are hand-dug.
- Ready-mix plants sell by the cubic metre, usually in 0.25 or 0.5 m³ steps, and charge a short-load fee below their minimum load. Ask your plant for its minimum.
- Past about 1 m³ (around 71 bags of 30 kg), ready-mix or a mix-on-site (volumetric) truck is usually cheaper and much faster than bags.
Common questions
How thick should a concrete slab be?
Walks and patios are typically 4" (100 mm) and driveways 4–6" (100–150 mm), on a compacted granular base. Anything outdoors should be air-entrained 32 MPa concrete, because freeze-thaw cycles and de-icing salt break up ordinary concrete. Your provincial building code and soil conditions take priority.
How many bags of concrete do I need for a slab?
Divide the volume by your bag yield. The calculator shows every common bag size at once so you can compare prices.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the building code adopted in your province before ordering.