How this calculator works
Worked example: a slab 20 × 10 ft and 4 in thick is 200 sq ft × 0.3333 ft = 66.7 cubic feet, and with 10% extra 73.3 cubic feet, which is 2.72 cubic yards. In 80 lb bags (0.6 ft³) that is 123 bags. At $6 a bag the bags cost $738.00, and ready-mix at $160 a yard plus $100 of fees costs $534.57. With 60 lb bags (0.45 ft³) the count is 163. A 6 in slab needs 4.07 cubic yards, and a 30 × 30 ft slab 12.22 cubic yards.
Concrete for common slabs, 4 in thick, 10% extra
Cubic yards and bags for a slab, by size.
| Slab | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 × 10 ft | 1.36 | 62 | 82 |
| 12 × 12 ft | 1.96 | 88 | 118 |
| 20 × 10 ft | 2.72 | 123 | 163 |
| 20 × 20 ft | 5.43 | 245 | 326 |
| 24 × 24 ft | 7.82 | 352 | 470 |
| 30 × 30 ft | 12.22 | 550 | 734 |
Tips before you buy
- Get a written quote from a ready-mix plant, with the delivery charge and the fee for a small load, before you compare.
- Over about two thirds of a cubic yard, mixing bags by hand is hard work: the break-even is often between 1 and 2 yards.
- Order the extra: running short in the middle of a pour is the most expensive mistake.
- Add the price of the base, the forms, the reinforcing and the tool hire to the total, since those are not in this number.
- Ask the plant about the mix, the slump and a trial of the access for the truck.
Tools & materials for this project
Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How much does a concrete slab cost?
It is the volume times the price, plus the base and the labor. For a slab of 200 sq ft at 4 in the volume is 2.72 cubic yards, and the costs come from your own prices.
Are bags or ready-mix cheaper?
Bags are cheaper for a small job, and ready-mix usually wins as the volume rises and the delivery fee is spread over more yards. Enter your prices to see where the break-even is for your slab.
How many bags of concrete make a cubic yard?
About 45 bags of 80 lb, or 60 bags of 60 lb, since a bag yields 0.6 and 0.45 cubic feet and a yard is 27 cubic feet.
How thick should a slab be?
Four inches is usual for a patio, walkway or shed, and more for a driveway or a garage floor. Check the code and the soil.
Does this include the labor and the base?
No. It is the concrete only. Add the gravel base, the forms, the reinforcing, the tools and any labor.
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.