Concrete Slab Chart: Yards and Bags by Slab Size
Seventeen slab sizes from a 4 x 4 pad to a 30 x 40 shop floor, at three thicknesses, with the bag counts next to the yardage.
Concrete is ordered by the cubic yard and there are 27 cubic feet in one, so a slab is length times width times thickness in feet, divided by 27. The arithmetic is easy and the mistake is always the same one: thickness gets left in inches.
Every number below includes a 10 percent waste allowance, because a slab poured on grade over a base that is not perfectly flat always takes more than the geometry says.
Cubic yards and bags by slab size and thickness
| Slab | Area | 4 in | 5 in | 6 in | 60 lb bags at 4 in | 80 lb bags at 4 in |
|---|---|---|---|---|---|---|
| 4 x 4 ft | 16 sq ft | 0.22 | 0.27 | 0.33 | 14 | 10 |
| 4 x 8 ft | 32 sq ft | 0.43 | 0.54 | 0.65 | 27 | 20 |
| 6 x 6 ft | 36 sq ft | 0.49 | 0.61 | 0.73 | 30 | 22 |
| 8 x 8 ft | 64 sq ft | 0.87 | 1.09 | 1.3 | 53 | 40 |
| 8 x 10 ft | 80 sq ft | 1.09 | 1.36 | 1.63 | 66 | 49 |
| 10 x 10 ft | 100 sq ft | 1.36 | 1.7 | 2.04 | 82 | 62 |
| 10 x 12 ft | 120 sq ft | 1.63 | 2.04 | 2.44 | 98 | 74 |
| 12 x 12 ft | 144 sq ft | 1.96 | 2.44 | 2.93 | 118 | 88 |
| 12 x 16 ft | 192 sq ft | 2.61 | 3.26 | 3.91 | 157 | 118 |
| 12 x 20 ft | 240 sq ft | 3.26 | 4.07 | 4.89 | 196 | 147 |
| 16 x 16 ft | 256 sq ft | 3.48 | 4.35 | 5.21 | 209 | 157 |
| 20 x 20 ft | 400 sq ft | 5.43 | 6.79 | 8.15 | 326 | 245 |
| 20 x 24 ft | 480 sq ft | 6.52 | 8.15 | 9.78 | 392 | 294 |
| 24 x 24 ft | 576 sq ft | 7.82 | 9.78 | 11.73 | 470 | 352 |
| 20 x 30 ft | 600 sq ft | 8.15 | 10.19 | 12.22 | 489 | 367 |
| 24 x 40 ft | 960 sq ft | 13.04 | 16.3 | 19.56 | 783 | 587 |
| 30 x 40 ft | 1,200 sq ft | 16.3 | 20.37 | 24.44 | 978 | 734 |
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The concrete calculator takes your own measurements and returns the number for your job, plus the material list as a printable document. Every cell in the table above came out of that same calculator, run at build time.
Where the line is between bags and a truck
Around one cubic yard. Below it, bagging is cheaper and you are not paying a short load fee. Above it, bagging is a bad afternoon: one cubic yard is 60 sixty pound bags or 45 eighty pound bags, which is 3,600 pounds of material to carry, open and mix by hand before any of it starts setting.
Most ready mix suppliers have a minimum load and charge a short load fee under it, usually somewhere between one and three yards. Ask what theirs is before deciding, because it often makes two yards cost nearly the same as three.
Thickness is a code question, not a preference
Four inches is the usual residential slab on grade: patios, walks, a garage floor for passenger vehicles. Five and six inches are for driveways that take a loaded truck, shop floors, and anything a dumpster or an RV will sit on.
Slab thickness, reinforcement, base preparation and control joints are all governed by local code and by what the soil under them is. This table sizes an order. It does not design a slab.
Order to the quarter yard
Ready mix is batched and billed in quarter yard increments, so a calculated 3.26 yards is a 3.5 yard order. Rounding down to save a quarter yard is how a pour ends three feet short of the far form with a truck already leaving.
The concrete calculator also gives the rebar and the form lumber for the same slab, which is the part of the list that gets forgotten until the morning of the pour.
Questions people ask
How many bags of concrete for a 10 x 10 slab?
At 4 inches thick, about 82 sixty pound bags or 62 eighty pound bags with waste included, which is 1.36 cubic yards. That is well past the point where a ready mix truck is easier and usually cheaper.
How many cubic feet are in a cubic yard of concrete?
Twenty seven. A cubic yard is three feet by three feet by three feet, which is also why a 4 inch slab covers about 81 square feet per yard.
How much does a 60 lb bag of concrete cover?
About 0.45 cubic feet, so it takes 60 of them to make a cubic yard. An 80 lb bag yields about 0.60 cubic feet and it takes 45 to make a yard.
Should I add waste to a concrete order?
Yes. Ten percent is normal on a slab on grade, because the base is never perfectly level and the forms are never perfectly straight. On a footing trench dug by machine, allow more.