How Much Concrete for a 12x20 Slab?
Concrete needed
3.26 cubic yards
A 12 by 20 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.
- Length12 ft
- Width20 ft
- Thickness4 in
- Waste allowance10 %
Concrete needed3.26 cubic yards
| Volume | 80 cu ft |
|---|---|
| Cubic yards before waste | 2.96 |
| Cubic yards with 10% waste | 3.26order to this number |
| Order in quarter yards | 3.5 cu yd |
| 60 lb bags, if bagging it | 196 |
| 80 lb bags, if bagging it | 147 |
The material list this produces
| Ready mix concrete, cubic yards | 3.5 |
|---|---|
| Rebar or mesh, sq ft of slab | 240 |
| Form lumber, linear ft | 64 |
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 | 3.26 cubic yards |
| 5 in | 4.07 cubic yards |
| 6 in | 4.89 cubic yards |
| 8 in | 6.52 cubic yards |
What else matters here
Two hundred and forty square feet and about three and a half yards ordered. This is a two car parking pad, a large patio or a single bay garage floor, and it is the size where the crew matters as much as the concrete.
Three and a half yards arrives in one load and has to be placed, screeded, bull floated and edged before it sets. Two people can do a slab this size in good weather. In July with a hot mix and a breeze, two people will be behind from the moment the chute opens, and being behind on a slab is not recoverable.
Thickness depends entirely on what parks on it. Four inches for cars, six for a truck, an RV or anything with a jack stand under it, and six throughout rather than a thickened patch, because the transition between two thicknesses is exactly where it will crack.
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 a 12x20 slab?
About 3.26 cubic yards at 4 inches with waste, so order 3.5 yards.
How thick should a driveway or parking slab be?
Four inches for cars, six for trucks, RVs or anything heavy. If part of it needs six, pour the whole slab at six rather than thickening one area.
How many people does a 12x20 pour need?
Two in good weather, three in hot weather. Three and a half yards comes in one load and all of it has to be placed and floated before it sets.
The concrete calculator takes any dimensions you like and hands back the same list.