How to calculate concrete
Concrete is ordered by the cubic yard. Multiply the length, width and depth of the pour in feet to get cubic feet, then divide by 27.
Cubic yards = length × width × depth (all in feet) ÷ 27
Convert the depth from inches to feet by dividing by 12. Four inches is 0.333 feet.
Worked example
A 10 by 10 foot slab that is 4 inches thick: 10 × 10 × 0.333 = 33.3 cubic feet. 33.3 ÷ 27 = 1.23 cubic yards. With 10% extra, order about 1.4 cubic yards.
How many bags of concrete
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
The 10 by 10 slab above takes about 56 bags of 80-pound mix (62 with 10% extra). For more than about one cubic yard, ready-mix delivery is usually easier than bags.
Typical slab thickness
- Walkways and patios: 4 inches.
- Driveways for cars: 4 to 5 inches.
- Driveways for heavy vehicles: 6 inches.
How to use this calculator
Enter the length, width and thickness of the slab, footing or patio, plus an extra allowance for spillage. The concrete calculator returns the volume in cubic yards and cubic feet and the number of 80, 60 or 40 pound bags that volume takes. Add a price per cubic yard if you want to work with a cost. For a pour with several parts, such as a slab with a separate footing, run each part and add the results.
More examples
Driveway
A driveway 12 feet wide and 20 feet long at 5 inches thick: 12 × 20 × 5 ÷ 12 = 100 cubic feet. 100 ÷ 27 = 3.70 cubic yards. With 10% extra, order about 4.1 cubic yards.
Footing
A footing 40 feet long, 1.5 feet wide and 1 foot deep: 40 × 1.5 × 1 = 60 cubic feet, or 2.22 cubic yards.
Small pad with bags
A 4 by 4 foot pad that is 4 inches thick is 5.33 cubic feet. Divide by the yield of the bag and round up: 9 bags of 80-pound mix, 12 bags of 60-pound mix, or 18 bags of 40-pound mix.
Round post hole
A hole 12 inches across has a radius of 0.5 feet. At 2 feet deep, 3.1416 × 0.5 × 0.5 × 2 = 1.57 cubic feet. That is 3 bags of 80-pound mix per hole, before allowing for the space the post takes up.
Square slabs at two thicknesses
| Slab size | 4 inches thick | 6 inches thick |
|---|---|---|
| 10 × 10 ft | 1.23 cu yd | 1.85 cu yd |
| 12 × 12 ft | 1.78 cu yd | 2.67 cu yd |
| 20 × 20 ft | 4.94 cu yd | 7.41 cu yd |
| 24 × 24 ft | 7.11 cu yd | 10.67 cu yd |
These volumes do not include extra for uneven ground or spillage.
What changes the result
- Thickness. Going from 4 inches to 6 inches adds 50% to the volume.
- Area. Doubling both the length and the width makes four times the concrete.
- The base. If the ground under the forms is uneven, the real depth is greater than the planned depth in the low spots.
- Bag size. Smaller bags are easier to lift, but you need more of them for the same volume.
Common mistakes
- Entering the depth in inches as if it were feet. A 4 inch slab is 0.333 feet deep, not 4.
- Ordering the bare volume with nothing extra.
- Rounding the number of bags down.
- Leaving out thickened edges, steps or footings that are poured with the slab.
- Using the yield of one bag size to count bags of another.
Related calculators
If you already know the area, square feet to cubic yards does the same conversion from square feet and a depth. The square footage calculator helps with slabs that are not simple rectangles, and the fence calculator counts the posts you will be setting in concrete. Results are estimates, so confirm the quantity with your supplier before you order ready-mix.
Frequently asked questions
How much does a cubic yard of concrete cover?
At 4 inches thick, one cubic yard covers about 81 square feet.
How much extra concrete should I order?
Add 5% to 10% for uneven ground and spillage.
How do I calculate concrete for a round hole or post?
Volume = 3.1416 × radius × radius × depth, all in feet, then divide by 27.
How many cubic feet are in a cubic yard of concrete?
27. A cubic yard is 3 feet by 3 feet by 3 feet.
How much does a cubic yard cover at 6 inches thick?
About 54 square feet.
How many bags of concrete do I need for a 4 by 4 slab?
At 4 inches thick, a 4 by 4 foot slab takes 9 bags of 80-pound mix or 12 bags of 60-pound mix.
How do I calculate concrete for steps or an odd shape?
Break the shape into rectangular blocks, calculate the cubic feet of each, add them together and divide by 27.