How to calculate concrete
Concrete is ordered by volume. Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. A 10 × 10 ft patio, 4 inches (0.333 ft) thick, needs 10 × 10 × 0.333 = 33.3 cu ft = 1.23 cubic yards. For round columns and post holes, the volume is π × radius² × height.
How many bags of concrete do I need?
| 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 |
Bags make sense for small jobs like post holes, steps and pads under about one cubic yard. For larger slabs, ready-mix delivered by truck is usually cheaper and much less work. Many suppliers have a minimum order and charge a “short load” fee under a certain amount.
Typical slab thickness
| Project | Thickness |
|---|---|
| Sidewalks, patios, shed floors | 4 in (10 cm) |
| Driveways for cars | 4–5 in (10–13 cm) |
| Driveways for trucks, RVs | 6 in (15 cm) |
| Garage floors | 4–6 in |
| House footings | Follow local building code — often 8–12 in deep |
Why add extra?
Ground is never perfectly level, forms bow and some concrete is always lost. Order 5–10% extra — running short in the middle of a pour leaves a weak cold joint. Find the area of irregular shapes first with the square footage calculator, then enter it as length × 1.
Frequently asked questions
How many cubic yards is a 10×10 slab?
At 4 inches thick, about 1.23 cubic yards (33.3 cubic feet).
How many 80 lb bags of concrete make a yard?
About 45 bags, since each 80 lb bag yields roughly 0.6 cubic feet.
How thick should a concrete patio be?
Usually 4 inches. Use 5–6 inches for driveways that carry heavy vehicles.
How much extra concrete should I order?
Add 5–10% for uneven ground, spillage and form movement.