Concrete Calculator
For slabs, pads, footings and post holes. Enter your dimensions and get the volume in cubic metres and cubic yards, plus the number of 30 kg pre-mix bags, with a waste allowance built in. Results update as you type.
Your concrete
Bag count assumes one 30 kg bag yields about 0.0135 cubic metres of placed concrete, the standard yield printed on Canadian pre-mix bags. Ordering ready-mix instead? Ask the plant for the volume with waste, in cubic metres.
Worked example
A 3 m by 4 m shed pad, 100 mm thick
Volume: 3 × 4 × 0.1 = 1.20 m³, which is 1.20 × 1.30795 = 1.57 yd³.
Bags with no waste: 1.20 ÷ 0.0135 = 88.9, so 89 bags.
With the recommended 10 percent waste: 1.20 × 1.10 = 1.32 m³ (1.73 yd³), and 1.32 ÷ 0.0135 = 97.8, so 98 bags. Those are the numbers in the results panel with the default settings above.
How this calculator works
Concrete is sold by volume, so the math starts with the volume of your form: volume = length × width × depth, with all three in metres. The waste allowance is added next, because real forms are never perfect and the ground under them is never perfectly level.
Cubic yards are the volume in cubic metres multiplied by 1.30795. Ready-mix plants in Canada quote in cubic metres, but a cubic yard figure helps if you are comparing with an older quote or a United States supplier.
The bag count divides the waste-adjusted volume by the yield of one bag. Canadian 30 kg pre-mix bags yield about 0.0135 cubic metres (roughly 0.48 cubic feet) of placed concrete. Check the yield printed on your bag if you buy a different brand or size, as 25 kg and 36 kg bags are also common.
Reinforcing the slab? Run the rebar calculator. Pouring a block wall or foundation instead? Count blocks and mortar with the concrete block calculator.
Common questions
How many bags do I need for 1 cubic metre of concrete?
About 74 bags of 30 kg pre-mix at 0.0135 cubic metres per bag, before waste. With 10 percent waste, plan on about 82 bags. At that point, ready-mix delivery is usually cheaper and far less work, so compare both before buying bags.
When should I order ready-mix instead of bags?
As a rule of thumb, once a job passes about 1 to 1.5 cubic metres (roughly 75 to 110 bags), ready-mix wins on cost, consistency and effort. Note that many plants charge a short-load fee under about 4 cubic metres, so ask for the delivered price, not just the per-metre price.
How thick should my slab be?
100 mm for patios, walkways and shed pads. 125 to 150 mm for driveways and garage floors, usually with reinforcement. Footings must reach below the frost line, which in most of Canada means 1.2 m or deeper; check your municipal building department for the local requirement.
Why add a waste allowance at all?
Ground settles unevenly, forms bulge, and a little concrete always stays in the wheelbarrow. Running short mid-pour leaves a weak cold joint. Ten percent is cheap insurance, and leftover dry bags can usually be returned if you keep the receipt and the bags dry.
Estimates only, verify before you buy materials. This calculator plans quantities; it does not design structures. Follow local building codes for footing depth, thickness and reinforcement.