Concrete Slab Calculator
Concrete for a slab is length times width times thickness — the arithmetic is easy, and the ordering is where it goes wrong. This works the pour out in cubic yards, cubic metres and bags, with a waste allowance on top.
Concrete needed
1.36cubic yards
- Cubic yards
- 1.36
- Cubic metres
- 1.04
- Cubic feet
- 36.7
- Weight
- 2.75 short tons
- 40lb bags
- 123
- 50lb bags
- 98
- 60lb bags
- 82
- 80lb bags
- 62
- Includes a 10% waste allowance. The bare figure is 1.23 cubic yards.
- Past about half a cubic yard, ready-mix is usually cheaper and much less work than mixing bags by hand.
- Weight assumes concrete at 2,400 kg per cubic metre, the mid-range figure the trade quotes. Real mixes vary.
About the concrete slab calculator
A slab is the most common concrete pour there is: a patio, a shed base, a garage floor, a footpath. It is also the one people most often get wrong at the ordering stage, because thickness is measured in inches while length and width are in feet, and mixing the two overstates the pour twelve-fold.
Enter the three dimensions and you get the volume in every unit a supplier might quote in, the weight, and how many bags it would take if you were mixing by hand rather than ordering ready-mix. The waste allowance defaults to ten per cent, which covers the uneven subgrade and spillage that every real pour has.
It calculates the slab itself. Thickened edges, footings and post holes are separate shapes with their own volumes, so add those before you place an order.
What it works out
- Volume in cubic yards, cubic metres and cubic feet
- Weight in short tons, for delivery limits
- Bag counts for 40, 50, 60 and 80lb pre-mix
- A waste allowance you can set yourself
The formula
Volume = Length × Width × Thickness
Concrete is sold by volume, so a slab is the simplest calculation on this site: multiply the three dimensions together. The only thing that trips people up is that thickness is measured in inches while length and width are in feet, and the three have to be in the same unit before they are multiplied.
Four inches is a third of a foot, so a ten by ten slab at four inches is 10 × 10 × 0.333, not 10 × 10 × 4. Getting that wrong overstates the pour by a factor of twelve.
Ready-mix is ordered in cubic yards in the US and cubic metres almost everywhere else. There are 27 cubic feet in a cubic yard, which is where the awkward-looking division comes from.
- Length
- The longer side of the slab, in feet or metres.
- Width
- The shorter side, in the same unit as the length.
- Thickness
- The depth of the pour. Four inches is standard for a patio or shed base; six for a driveway that will carry a vehicle.
A worked example
A patio slab, ten feet square, poured four inches thick.
That works out to 1.36 cubic yards.
- Cubic yards
- 1.36
- Cubic metres
- 1.04
- Cubic feet
- 36.7
- Weight
- 2.75 short tons
- 40lb bags
- 123
- 50lb bags
- 98
- 60lb bags
- 82
- 80lb bags
- 62
Questions
How thick should a concrete slab be?
Four inches is the usual thickness for a patio, shed base or footpath. Six inches is normal where a vehicle will drive or park on it, and heavier loads go thicker still. Local building rules may set a minimum, so check before you order.
How much extra concrete should I order?
Between 5 and 10% over the calculated volume. Subgrade is never perfectly level, some is lost to spillage, and a slab that runs short partway through a pour is a far more expensive problem than a little left over.
How many bags of concrete are in a cubic yard?
About 45 80lb bags, 60 60lb bags, or 90 40lb bags. Bagged concrete is reasonable for footings and small pads, but past roughly half a cubic yard ready-mix is usually cheaper and far less work.
Should I use bagged concrete or ready-mix?
Bags make sense up to about half a cubic yard. Beyond that the mixing becomes the whole job, and most ready-mix suppliers have a minimum load of around one cubic yard anyway.
Does this include the footings?
No. This calculates the slab only. Thickened edges, footings and post holes are separate pours with their own shapes — add them to the slab volume before you order.
What is a cubic yard of concrete?
A cube one yard on each side, which is 27 cubic feet. It weighs roughly two short tons and covers about 81 square feet at four inches thick, or 65 at five.