How Much Concrete for a 20x30 Slab?
Concrete needed
8.15 cubic yards
A 20 by 30 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
- Width30 ft
- Thickness4 in
- Waste allowance10 %
Concrete needed8.15 cubic yards
| Volume | 200 cu ft |
|---|---|
| Cubic yards before waste | 7.41 |
| Cubic yards with 10% waste | 8.15order to this number |
| Order in quarter yards | 8.25 cu yd |
| 60 lb bags, if bagging it | 489 |
| 80 lb bags, if bagging it | 367 |
The material list this produces
| Ready mix concrete, cubic yards | 8.25 |
|---|---|
| Rebar or mesh, sq ft of slab | 600 |
| Form lumber, linear ft | 100 |
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 | 8.15 cubic yards |
| 5 in | 10.19 cubic yards |
| 6 in | 12.22 cubic yards |
| 8 in | 16.3 cubic yards |
What else matters here
Eight and a quarter yards, six hundred square feet, and past the point where this is a do it yourself pour. A slab this size needs a crew who have done it before, because everything happens at once and none of it waits.
The joint layout is now a plan rather than a decision. Six hundred square feet at ten foot spacing means panels of roughly 10 by 10, so two cuts one way and one the other, giving six panels. Mark them before the pour rather than working them out while the saw is running.
Concrete at this volume is also where the mix design stops being a detail. Ask for the strength, the slump and the air content rather than accepting whatever is on the truck: 4,000 psi, a slump that suits placing by chute rather than by pump, and air entrainment if it will ever see a freeze. All three are ordered, not assumed.
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 20x30 slab?
About 8.15 cubic yards at 4 inches with waste, so order 8.25 or 8.5 yards.
How many control joints does a 20x30 slab need?
Panels of around 10 by 10, so two cuts across the thirty foot direction and one across the twenty, giving six panels. Lay it out before the pour.
What concrete mix should I order for a slab?
Specify it rather than accepting the default: commonly 4,000 psi, a slump suited to how it is being placed, and air entrainment anywhere that freezes.
The concrete calculator takes any dimensions you like and hands back the same list.