How to calculate concrete
Concrete is ordered by volume. In the U.S., ready-mix trucks sell by the cubic yard, and one cubic yard equals 27 cubic feet. Measure length and width in feet and thickness in inches, then convert everything to feet before multiplying.
Worked example: 10 × 10 ft patio, 4 inches thick
10 × 10 × (4 ÷ 12) = 33.3 cu ft. Divided by 27, that is 1.23 cubic yards. Adding 10% for waste and uneven subgrade brings it to 1.36 cubic yards, or about 62 bags of 80 lb mix.
Bag yields
| Bag size | Yield per bag | 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 |
Recommended thickness
- Sidewalks, patios and shed floors: 4 inches
- Residential driveways: 4–5 inches for cars, 6 inches for trucks or RVs
- Garage floors: 4–6 inches
- Fence and deck post holes: about 3 times the post width in diameter, 1/3 of the post length deep (check local frost depth)
Frequently asked questions
How many bags of concrete are in a yard?
One cubic yard takes about 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix.
How much does a cubic yard of concrete cover?
At 4 inches thick, one cubic yard covers 81 square feet. At 6 inches thick it covers 54 square feet.
Should I order extra concrete?
Yes. Add 5–10% for spillage, uneven ground and form flex. Running short mid-pour creates a cold joint, which is weaker and visible.
When is ready-mix cheaper than bags?
For most jobs above about 1 cubic yard, a ready-mix delivery costs less and saves hours of mixing. Many suppliers add a short-load fee below 3–4 yards.