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Concrete Slab Calculator

Work out concrete volume, pre-mix bag count, and cost for a slab — metric or imperial.

  • concrete
  • slab
  • volume
  • bags
  • cubic metres
  • cubic yards
  • metric
  • canada

About Concrete Slab Calculator

Pouring a slab starts with one question: how much concrete do I actually need? Order too little and the pour stalls halfway with a cold joint; order too much and you're paying for a half-load you have to dispose of. This calculator turns the slab's length, width, and thickness into a volume — shown in both cubic metres and cubic yards — then tells you how many pre-mix bags that works out to and, optionally, what it will cost.

It's built for the way slabs are actually specified in Canada: metric by default, with a one-tap switch to feet and inches. Bag counts cover the common 20 kg and 30 kg metric bags and the 60 lb and 80 lb bags sold at North American hardware stores, and it flags a slab that's too thin to be structural before you commit to the pour.

How to use

Pick metric or imperial at the top, then enter the slab's length, width, and thickness. If you're pouring several identical slabs (footpath sections, pads), set the slab count and the totals scale up. The volume, bag count, and cost update as you type.

To estimate cost, enter the price your supplier quotes per cubic metre (or per cubic yard in imperial) and choose a currency — it defaults to Canadian dollars. The waste reminder suggests ordering 5–10% extra to cover spillage and an uneven sub-base, which is standard practice. If your thickness drops below 75 mm (3″) the tool warns you, since that's generally too thin for a load-bearing slab.

Frequently asked questions

  • How much concrete do I need for a slab?

    Multiply length × width × thickness to get the volume. For a 4 m × 3 m slab at 100 mm thick that's 4 × 3 × 0.1 = 1.2 m³ (about 1.57 cubic yards). The calculator does this for you in both metric and imperial, and adds a recommended 5–10% margin so a slightly over-dug or uneven base doesn't leave you short.

  • How many bags of concrete are in a cubic metre?

    It depends on bag size. A standard 20 kg pre-mix bag yields roughly 0.009 m³, so you need about 108 bags per cubic metre; a 30 kg bag covers more, at about 72 per cubic metre. In imperial terms an 80 lb bag yields about 0.6 ft³, so roughly 45 bags fill a cubic yard, and a 60 lb bag needs about 60 per yard. Always check the yield printed on the bag — it varies by product.

  • When should I order ready-mix instead of bags?

    Bags are practical for small jobs — footings, post pads, a small landing. Once you're past roughly 1 m³ (around 110 × 20 kg bags) the mixing labour and cost tip in favour of a ready-mix truck delivery, which also gives a consistent mix and a continuous pour with no cold joints. Most suppliers have a minimum load, so for very small pours bags still win.

  • How thick should a concrete slab be?

    For a non-structural surface like a shed base or path, 75–100 mm (3–4″) is typical. A residential driveway or garage floor is usually 100 mm (4″), and slabs carrying heavier loads go to 150 mm (6″) or more with reinforcement. The calculator warns below 75 mm because that's generally too thin to resist cracking under load. Always check your local building requirements for structural work.

  • What's the difference between a cubic metre and a cubic yard of concrete?

    They're just different units of volume. One cubic metre equals about 1.31 cubic yards, so a cubic yard is the smaller measure. Canadian suppliers quote either depending on the region, which is why this tool always shows both — enter your dimensions once and read off whichever unit your concrete supplier uses.

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