Pouring a slab is one of the most common concrete projects, and it is also one of the easiest to get wrong at the ordering stage. Order too little and you get a cold joint where the first pour sets before the second arrives. Order too much and you have paid for concrete you have to dispose of. This guide walks through the exact math, then shows the shortcuts the pros use.
The basic formula
Concrete is sold by volume, in cubic yards. A slab is just a rectangular box, so the volume is length times width times thickness. The only trick is keeping your units straight, because thickness is measured in inches while length and width are in feet.
The formula is:
Cubic yards = (length ft × width ft × thickness in ÷ 12) ÷ 27
You divide the thickness by 12 to convert inches to feet, then divide the whole thing by 27 because there are 27 cubic feet in a cubic yard.
A worked example
Say you are pouring a 10 by 12 foot patio at the standard 4 inch thickness.
- Volume in cubic feet: 10 × 12 × (4 ÷ 12) = 10 × 12 × 0.333 = 40 cubic feet
- Convert to yards: 40 ÷ 27 = 1.48 cubic yards
So the raw number is about 1.48 cubic yards. But you never order the raw number, which brings us to the most important part.
Always add a waste factor
Real pours never use the exact calculated volume. Some concrete sticks to the mixer and the wheelbarrow, the subgrade is never perfectly level, and forms flex outward under the weight of wet concrete. The industry standard is to add 10 percent for a normal slab.
For our patio: 1.48 × 1.10 = 1.63 cubic yards. Round up and order 1.75 cubic yards from a ready-mix supplier, or plan for the equivalent in bags.
If your subgrade is rough or you are pouring over gravel that will absorb some slurry, bump the waste factor to 15 percent. It is far cheaper to have a little extra than to stop a pour halfway.
How many bags is that?
For small slabs you may be mixing bags by hand instead of ordering ready-mix. Here is the coverage for the common bag sizes:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
For our 1.63 cubic yard patio (that is 44 cubic feet with waste included), you would need about 44 ÷ 0.60 = 74 bags of 80 lb concrete. That is a lot of mixing. As a rule of thumb, once you pass 30 or 40 bags, ready-mix delivery is cheaper and far less work.
Choosing the right thickness
Thickness drives both strength and cost, so match it to the load:
- 4 inches for patios, walkways, and shed floors
- 4 inches for driveways used by passenger cars
- 5 to 6 inches for driveways that see trucks, RVs, or trailers
- 6 inches or more for garage floors and anything holding heavy equipment
Going from 4 to 6 inches increases your concrete volume by 50 percent, so do not over-build. A 4 inch residential patio does not need 6 inches.
Do not forget the base
Concrete is only as good as what sits under it. Most slabs need 4 to 6 inches of compacted gravel underneath for drainage and to prevent settling. That gravel is a separate order, and you can size it with our gravel calculator. Skipping the base is the number one cause of slabs that crack within a few years.
Quick recap
- Multiply length by width by thickness in feet to get cubic feet.
- Divide by 27 to get cubic yards.
- Add 10 percent waste, 15 percent for rough conditions.
- Round up to the nearest quarter yard when ordering ready-mix.
If you would rather skip the arithmetic, our concrete calculator does all of this instantly, including bag counts for 80, 60, and 40 pound bags and a ready-mix versus bagged cost comparison. For patio and floor pours specifically, the concrete slab calculator is set up for exactly this job.