How Much Concrete for an 8x12 Slab?
Concrete needed
1.3 cubic yards
An 8 by 12 foot slab at four inches, with ten percent on top for waste.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Length8 ft
- Width12 ft
- Thickness4 in
- Waste allowance10 %
Concrete needed1.3 cubic yards
| Volume | 32 cu ft |
|---|---|
| Cubic yards before waste | 1.19 |
| Cubic yards with 10% waste | 1.3order to this number |
| Order in quarter yards | 1.5 cu yd |
| 60 lb bags, if bagging it | 79 |
| 80 lb bags, if bagging it | 59 |
The material list this produces
| Ready mix concrete, cubic yards | 1.5 |
|---|---|
| Rebar or mesh, sq ft of slab | 96 |
| Form lumber, linear ft | 40 |
Thickness compared
Thickness is the one number on this page worth arguing about, so here is the same slab at each of the usual four.
| Thickness | Concrete needed |
|---|---|
| 4 in | 1.3 cubic yards |
| 5 in | 1.63 cubic yards |
| 6 in | 1.96 cubic yards |
| 8 in | 2.61 cubic yards |
What else matters here
One and a third yards, comfortably a truck pour, and long enough that the joint layout needs a moment's thought. Twelve feet in one direction is past the eight to ten foot rule, so this slab wants a joint across the short way, dividing it into two panels of roughly 8 by 6.
Keep the panels close to square. A long thin panel cracks across its middle regardless of the joints around it, so the goal is panels with a length to width ratio under about one and a half, not simply joints at regular spacing.
Cut the joints early. Saw cut joints go in within six to eighteen hours depending on the weather, and a slab that is left until the second day has usually cracked where it wanted to rather than where the saw was going to go.
What this calculator assumes
- One cubic yard is 27 cubic feet. A 4 inch slab is one third of a foot thick, not 4.
- A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.60. Bags are for small pours only; past about one cubic yard, ready mix is cheaper and far less work.
- Ready mix is usually sold in quarter yard steps with a short load fee below one yard.
Questions people ask
How much concrete for an 8x12 slab?
About 1.3 cubic yards at 4 inches thick with a ten percent waste allowance.
Where do the control joints go on an 8x12 slab?
One across the twelve foot direction, splitting it into two panels around 8 by 6. Panels should stay close to square rather than long and thin.
When should control joints be cut?
Within six to eighteen hours of the pour, sooner in hot weather. Left until the second day the slab has usually already cracked where it wanted to.
The concrete calculator takes any dimensions you like and hands back the same list.