How Much Concrete for a 9x9 Slab?
Concrete needed
1.1 cubic yards
A 9 by 9 foot slab at four inches, which is exactly one cubic yard before the waste allowance.
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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.
- Length9 ft
- Width9 ft
- Thickness4 in
- Waste allowance10 %
Concrete needed1.1 cubic yards
| Volume | 27 cu ft |
|---|---|
| Cubic yards before waste | 1 |
| Cubic yards with 10% waste | 1.1order to this number |
| Order in quarter yards | 1.25 cu yd |
| 60 lb bags, if bagging it | 66 |
| 80 lb bags, if bagging it | 50 |
The material list this produces
| Ready mix concrete, cubic yards | 1.25 |
|---|---|
| Rebar or mesh, sq ft of slab | 81 |
| Form lumber, linear ft | 36 |
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 | 1.1 cubic yards |
| 5 in | 1.38 cubic yards |
| 6 in | 1.65 cubic yards |
| 8 in | 2.2 cubic yards |
What else matters here
Nine by nine at four inches is exactly twenty seven cubic feet, which is exactly one cubic yard. It is the cleanest example of the conversion this calculator exists to do, and it is worth remembering as a sanity check: one yard of concrete covers 81 square feet at four inches.
That figure is the one to carry around. Eighty one square feet per yard at four inches, fifty four at six inches, forty at eight. Any slab estimate can be checked against it in a few seconds, and an estimate that is out by a factor of three usually turns out to have treated four inches as four feet.
Order one and a quarter yards rather than one. The exact yard has no allowance in it at all, and a slab poured to the exact calculated volume will come up short somewhere, because the sub base is never perfectly flat and the forms are never perfectly straight.
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 9x9 slab?
Exactly one cubic yard at 4 inches before waste, and about 1.1 yards with a ten percent allowance.
How many square feet does a yard of concrete cover?
81 square feet at 4 inches, 54 at 6 inches and about 40 at 8 inches. It is the quickest way to sanity check any slab estimate.
Should I order extra concrete?
Yes, around ten percent. The sub base is never perfectly flat and the forms are never perfectly straight, so the exact calculated volume always runs short somewhere.
The concrete calculator takes any dimensions you like and hands back the same list.