How to Figure Yards of Concrete
Concrete is sold by the cubic yard, so before you order a truck or buy a stack of bags, you need to turn your slab into a yard number. The math is short. Here is the whole thing:
Length (ft) x Width (ft) x Depth (ft) ÷ 27 = cubic yards
The 27 is there because one cubic yard is 27 cubic feet. That single division is what trips people up, so walk through it once and it sticks.
The three steps
1. Measure in feet. Get the length and width of the area. Depth is usually given in inches, so convert it to feet by dividing by 12. A 4-inch slab is 4 ÷ 12 = 0.33 feet.
2. Multiply the three numbers. Length times width times depth gives you cubic feet.
3. Divide by 27. That converts cubic feet into cubic yards, which is what you order.
A worked example
Say you are pouring a patio that is 12 feet long, 10 feet wide, and 4 inches thick.
- Depth in feet: 4 ÷ 12 = 0.33
- Volume: 12 x 10 x 0.33 = 39.6 cubic feet
- Yards: 39.6 ÷ 27 = 1.47 cubic yards
So you need about 1.5 yards of concrete for that patio.
Add a little extra
Never order the exact number. Ground is uneven, forms bow slightly, and some concrete gets lost to spillage and the wheelbarrow. Add 5 to 10 percent and round up. For the patio above, 1.47 yards becomes a comfortable 1.6 yards, and you would ask the supplier for that.
Running short in the middle of a pour is the one mistake you really want to avoid. A cold joint from waiting on a second truck weakens the slab, and a small emergency delivery is expensive. A little waste is cheaper insurance than coming up short.
Common sizes worked out for you
If your project matches one of these, here is the answer at a 4-inch depth, before the waste cushion.
| Slab size | Square feet | Yards needed (4") |
|---|---|---|
| 8 x 8 ft | 64 | 0.8 |
| 10 x 10 ft | 100 | 1.24 |
| 12 x 12 ft | 144 | 1.78 |
| 12 x 24 ft | 288 | 3.56 |
| 20 x 20 ft | 400 | 4.94 |
| 24 x 24 ft | 576 | 7.11 |
For a thicker pour, the yard count goes up. A 6-inch slab needs half again as much concrete as a 4-inch slab of the same footprint.
Skip the math
If you would rather not run the numbers by hand, the concrete calculator does every step for you. Enter length, width, and thickness, and it returns the yards to order along with a bag count if you are going that route. It also handles odd shapes like footings, columns, and stairs, which are fiddly to work out on paper.
Frequently asked questions
How do I calculate yards of concrete?
Multiply length by width by depth, all in feet, then divide by 27. Convert inch depths to feet first by dividing by 12.
How many yards of concrete do I need for a 20x20 slab?
About 4.94 yards at 4 inches thick. Round up to roughly 5.5 yards once you add a waste factor.
Why divide by 27?
Because a cubic yard contains 27 cubic feet. Your length times width times depth gives cubic feet, and dividing by 27 converts that to the cubic yards concrete is sold in.
How much extra concrete should I order?
Add 5 to 10 percent to your calculated volume to cover spillage and uneven ground, then round up.