Concrete Slab Calculator
Enter the length, width and thickness of the slab. You get the cubic yards to order from a ready-mix plant and the number of bags of each size if you are mixing it yourself.
How much concrete for a 10x10 slab?
A 10 ft x 10 ft slab 4 in thick is 33.3 cu ft, or 1.23 cubic yards. With 10 % for spillage and an uneven subgrade it is 1.36 cubic yards, which is 62 bags of 80 lb concrete mix or 82 bags of 60 lb.
Concrete for your slab
10 x 10 ft at 4 in is 1.36 cu yd with waste, or 62 bags of 80 lb. Past a yard, price a ready-mix truck against the bags.
- Bags of 80 lb
- 620.6 cu ft per bag
- Bags of 60 lb
- 820.45 cu ft per bag
- Bags of 50 lb
- 980.375 cu ft per bag
- Bags of 40 lb
- 1230.3 cu ft per bag
- Ready-mix order
- 1.5 cu ydrounded up to the next quarter yard
The numbers above are a worked example for a 10 ft x 10 ft slab 4 in thick with a 10 % waste allowance. Allow this page to run scripts in your browser to use your own dimensions, or follow the formula further down the page and do it by hand.
How the concrete slab calculator works, step by step
A slab is a box: length times width times thickness. The only traps are the units, because the thickness is measured in inches and the order is placed in cubic yards, and the rounding, because bags come whole. For footings, round columns, post holes and steps, the general concrete calculator handles each shape.
yields: 80 lb = 0.6 cu ft, 60 lb = 0.45, 50 lb = 0.375, 40 lb = 0.3
Worked example: a 10 ft x 10 ft slab, 4 in thick
- Volume: 10 x 10 x 4 / 12 = 33.33 cu ft
- With 10 % waste: 33.33 x 1.1 = 36.67 cu ft
- Cubic yards: 36.67 / 27 = 1.36 cu yd
- 80 lb bags: 36.67 / 0.6 = 61.1, so 62 bags
- 60 lb bags: 36.67 / 0.45 = 81.5, so 82 bags
Concrete for common slab sizes
Every slab below is 4 in thick with a 10 % allowance. The ready-mix column is rounded up to the next quarter yard; ask your plant how it sells small loads, since many add a fee under a minimum order.
| Slab | Cubic yards, net | With waste | 80 lb bags | 60 lb bags | Ready-mix |
|---|---|---|---|---|---|
| 10 x 10 ft pad | 1.23 | 1.36 | 62 | 82 | 1.5 cu yd |
| 12 x 12 ft patio | 1.78 | 1.96 | 88 | 118 | 2 cu yd |
| 10 x 20 ft | 2.47 | 2.72 | 123 | 163 | 2.75 cu yd |
| 12 x 24 ft driveway | 3.56 | 3.91 | 176 | 235 | 4 cu yd |
| 20 x 20 ft | 4.94 | 5.43 | 245 | 326 | 5.5 cu yd |
| 24 x 24 ft garage | 7.11 | 7.82 | 352 | 470 | 8 cu yd |
Thickness changes the order in proportion. The same 10 x 10 ft slab takes 1.70 cu yd, or 77 bags of 80 lb, at 5 in, and 2.04 cu yd, or 92 bags, at 6 in. The full table of bags per cubic yard for each bag size is on concrete bags per yard.
How thick should a concrete slab be?
Four inches is the usual thickness for patios, walkways, shed pads and a driveway that only sees passenger cars. Five to six inches is common for driveways that carry pickups, RVs or delivery trucks, and for slabs under heavy equipment. Garage floors are usually 4 in, thickened at the edges. These are common practice, not a code: your local building department, or the plans for the structure, set the thickness where it is regulated.
Whatever the design thickness, the forms decide what you actually pour. A form set 1/2 in high over a 20 x 20 ft slab adds about 0.6 cu yd, which is one reason the waste allowance is there.
The base under the slab
Most slabs on grade sit on 4 in of compacted gravel or crushed stone. The base gives an even, draining surface to pour on, and it keeps the slab thickness uniform. Its volume is the slab area times the base depth, plus an allowance for compaction; the gravel calculator turns it into cubic yards and tons. For a gravel driveway with no concrete at all, the gravel driveway calculator works out all three layers.
Bags or ready-mix?
An 80 lb bag yields 0.6 cu ft, so a cubic yard is 45 bags and 1.8 tons of dry mix to lift, mix and place. Up to about a yard, bags and a rented mixer are manageable for two people. Past that, a ready-mix truck is usually faster and often cheaper, and a single truckload pours the slab in one go without cold joints. If the truck cannot reach the forms, add a concrete pump or a line of wheelbarrows to the plan.
Cases the simple formula does not cover
Thickened edges
A garage slab or a slab that carries a wall is often thickened along its edges, typically to 8 or 12 in over a width of 8 to 12 in. Work out the thickened strip as a footing in the concrete calculator, using only the extra depth below the slab, and add it to this result.
Odd shapes
Split an L-shaped or stepped slab into rectangles, run each one, and add the cubic yards. Round the bag count once, at the end, rather than per rectangle.
Slopes
A slab with a fall for drainage is thicker at one end. Average the thickness at the high and low ends and enter that figure.
Frequently asked questions
How many bags of concrete for a 10x10 slab?
At 4 in thick with 10 % waste, 62 bags of 80 lb or 82 bags of 60 lb. At 6 in thick, 92 bags of 80 lb. Without the waste allowance, the 4 in slab is 56 bags of 80 lb exactly, which leaves nothing for a low spot.
How many yards of concrete for a 24x24 garage slab?
At 4 in thick, 7.11 cubic yards, or 7.82 with 10 % waste, so an 8 cubic yard order. Thickened edges come on top of that and are worked out separately.
How thick should a concrete driveway be?
Four inches is common for passenger cars, and five to six inches for pickups, RVs and delivery trucks. A 12 x 24 ft driveway 4 in thick takes 3.91 cubic yards with 10 % waste, and about 4.9 at 5 in.
Should I use bags or order ready-mix for a slab?
Bags are practical up to about one cubic yard, which is 45 bags of 80 lb. Past that, a ready-mix truck is usually faster, avoids cold joints between batches and is often cheaper once you count the mixer rental and the labor.
Source
Bag yields per QUIKRETE, Concrete Mix No. 1101 data sheet (bag yields). The volume arithmetic is the definition of a cubic foot and a cubic yard. Other brands publish their own yields, usually within a few percent of these figures; the bag you buy wins.