How Much Concrete for a 20x20 Slab?
Concrete needed
5.43 cubic yards
A 20 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.
- Length20 ft
- Width20 ft
- Thickness4 in
- Waste allowance10 %
Concrete needed5.43 cubic yards
| Volume | 133.3 cu ft |
|---|---|
| Cubic yards before waste | 4.94 |
| Cubic yards with 10% waste | 5.43order to this number |
| Order in quarter yards | 5.5 cu yd |
| 60 lb bags, if bagging it | 326 |
| 80 lb bags, if bagging it | 245 |
The material list this produces
| Ready mix concrete, cubic yards | 5.5 |
|---|---|
| Rebar or mesh, sq ft of slab | 400 |
| Form lumber, linear ft | 80 |
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 | 5.43 cubic yards |
| 5 in | 6.79 cubic yards |
| 6 in | 8.15 cubic yards |
| 8 in | 10.86 cubic yards |
What else matters here
Four hundred square feet is the two car garage that everybody builds a little too small and the patio that everybody builds about right. At four inches it is five and a half yards, and at six inches, which is what most people should be using under two cars, it is over eight.
Twenty feet across is past the reach of a screed board, so the slab needs either wet screeds pulled through the middle or a laser screed you are not going to rent for one pour. Two intermediate wet screeds, set to grade and struck first, turn one impossible twenty foot pull into three easy ones.
Joints on a slab this size go in a grid: cut it into four ten foot squares at minimum, and into nine squares if the slab is six inches and carrying vehicles. Ten feet between joints on a four inch slab is already at the limit of the thirty times thickness rule.
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 cubic yards is a 20x20 slab?
4.94 cubic yards of volume at four inches, 5.43 with ten percent waste, ordered as 5.5. At six inches it is about 8.1.
How thick should a 20x20 garage slab be?
Six inches if vehicles park on it. Four is enough for a patio or a covered outdoor space, but a two car garage floor at four inches is thin and will show it at the door where the wheels land.
How long does a 20x20 slab take to pour?
The concrete is off the truck in half an hour. Screeding, floating, edging and jointing take another two to four hours depending on crew and weather, and the pour has to be finished in one go.
The concrete calculator takes any dimensions you like and hands back the same list.