How Much Concrete for a 12x12 Slab?
Concrete needed
1.96 cubic yards
A 12 by 12 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.
- Length12 ft
- Width12 ft
- Thickness4 in
- Waste allowance10 %
Concrete needed1.96 cubic yards
| Volume | 48 cu ft |
|---|---|
| Cubic yards before waste | 1.78 |
| Cubic yards with 10% waste | 1.96order to this number |
| Order in quarter yards | 2 cu yd |
| 60 lb bags, if bagging it | 118 |
| 80 lb bags, if bagging it | 88 |
The material list this produces
| Ready mix concrete, cubic yards | 2 |
|---|---|
| Rebar or mesh, sq ft of slab | 144 |
| Form lumber, linear ft | 48 |
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 | 1.96 cubic yards |
| 5 in | 2.44 cubic yards |
| 6 in | 2.93 cubic yards |
| 8 in | 3.91 cubic yards |
What else matters here
A hundred and forty four square feet at four inches lands almost exactly on two cubic yards with waste, which makes this an easy one to order and an easy one to check. If a supplier quotes you very far off two yards for this slab, something in the conversation is wrong.
Twelve feet is also the width at which control joints stop being optional. The rule of thumb is that a joint should fall every 24 to 36 times the slab thickness, so a four inch slab wants joints every eight to twelve feet. On a 12 by 12 that means one cut across the middle in each direction, giving four six foot squares.
Cut them, tool them or set them, but do it within the first day. Concrete is going to crack as it shrinks; a joint is simply you deciding where. A slab left uncut for a week has already made that decision for you.
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 many yards of concrete for a 12x12 slab?
1.78 cubic yards of volume, 1.96 with a ten percent waste allowance, so you order two yards. At six inches it is just under three.
How many 80 lb bags for a 12x12 slab?
Around 88 at four inches thick with waste in. At that count, ready mix is cheaper and takes a fifth of the time, so bags only make sense if a truck cannot reach the site.
Where do the control joints go on a 12x12 slab?
One cut in each direction through the middle, six feet from each edge, on a four inch slab. Cut to a quarter of the slab depth, so one inch, within the first 24 hours.
The concrete calculator takes any dimensions you like and hands back the same list.