Concrete slab calculator
The figure above is your slab's three dimensions multiplied together. It does not decide how thick the slab ought to be or whether it will carry what you park on it — those belong to your drawings, your engineer or your building authority. Ring your ready-mix supplier to confirm the order before the truck rolls.
Enter the length, width and thickness of the pad and you get its volume in cubic yards and cubic metres, plus roughly how many bags it would take. Volume is simply length × width × thickness; the waste factor sits on top so an uneven base does not leave you short.
Worked example
A 12 ft × 12 ft shed floor, poured 4 inches thick: four inches is a third of a foot, so the volume is 12 × 12 × 0.333 = 48 cubic feet. Divide by 27 and that is 1.78 yd³. Add 10% for waste and you would order about 2 yd³. Bagging it instead, at 0.6 ft³ per 80 lb bag, is roughly ninety bags with waste — which is exactly when a truck earns its keep.
The formula
volume = length × width × thickness
Keep the units in step: drop thickness into feet by dividing inches by 12 before multiplying, then divide the cubic feet by 27 for cubic yards. In metric a depth in centimetres becomes metres over 100, and the answer is already cubic metres. Bags are the volume divided by what one bag yields once mixed.
What this tool assumes
Only that a cubic yard is 27 cubic feet, which is a definition rather than an assumption. Everything that actually varies from slab to slab — the thickness, the waste percentage, the yield per bag — is a field you can edit, so no hidden number is steering your order.
Questions
- How much concrete do I need for a slab?
- Multiply the slab length by its width by its thickness, all in the same units, then read it off in cubic yards or cubic metres. A 12 ft by 12 ft pad poured 4 inches thick is 12 × 12 × 0.333 = 48 cubic feet, which is 48 ÷ 27 ≈ 1.8 yd³ before you add any waste. Enter your own figures above and the number lands instantly.
- How thick should a concrete slab be?
- That is a design call this tool leaves to you — you pour the thickness your plans or your local authority specify, and you type it in. As a rough guide people pour a shed floor or a patio around 4 inches, and a slab meant to take vehicles thicker again, but anything carrying a real load should follow an engineered detail rather than a rule of thumb.
- How many bags does a slab take versus a truck?
- The bag count is shown alongside the volume, using an editable yield per bag. As a feel for it, a single cubic yard is roughly forty-five 80 lb bags — a lot of mixing. Once a slab climbs past about ten bags, ordering ready-mix by the yard is usually cheaper, and always faster, than bagging it by hand.
- Why does the tool add a waste factor?
- Because a slab is never poured onto a perfectly flat, perfectly sized base. Sub-grade dips, forms bow out under the weight, and screeding leaves a little proud. The waste field defaults to 10% and you can push it higher on a rough hand-dug base — running a slab short halfway through a pour is a far worse problem than a little left in the barrow.
Related tools
New tools, now and then
We add calculators most weeks. Drop your email and get a short note when a new one lands — nothing else, no selling your address, one-click out.