How much concrete do I need?
Concrete volume is length times width times thickness. Ready-mixed concrete is ordered by the cubic yard, so enter your slab size above to see the result in cubic yards, plus a waste allowance. For small jobs, you may prefer to buy bags and mix it yourself.
Typical slab thickness
| Project | Typical thickness |
|---|---|
| Walkway or path | 3.5 to 4 inches |
| Patio | 4 inches |
| Shed base | 4 inches, sometimes more |
| Driveway | 4 to 6 inches |
| Footings and foundations | Set by the design and local building rules |
Footings, foundations and anything that carries a load should follow your local building code and a proper design. If in doubt, ask a qualified contractor or your local building department.
Quick formula: cubic yards = length (ft) × width (ft) × depth (in) ÷ 324. For example, a 10 ft × 10 ft area at 3 inches deep is 10 × 10 × 3 ÷ 324 = 0.93 cubic yards. The calculator above does this for you and adds the waste allowance.
Worked examples
Patio slab. A patio 10 ft by 10 ft and 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. With a 10% allowance that is about 1.36 cubic yards.
Shed base. A base 8 ft by 10 ft and 4 inches thick is 26.7 cubic feet, or 0.99 cubic yards.
Round footing. A circle 2 ft across and 3 ft deep holds about 9.4 cubic feet, or 0.35 cubic yards.
How many bags of concrete mix?
Bag yields vary by brand, so check the label. As a rough guide, an 80 lb bag makes about 0.6 cubic feet, a 60 lb bag about 0.45 cubic feet, and a 40 lb bag about 0.3 cubic feet.
| Volume needed | 80 lb bags (approx.) |
|---|---|
| 5 cubic feet | 9 |
| 10 cubic feet | 17 |
| 27 cubic feet (1 cubic yard) | 45 |
| 33 cubic feet (10 ft by 10 ft slab at 4 in) | 56 |
Once a job needs more than about a cubic yard, ordering ready-mix delivered is usually easier and gives a more consistent result than mixing dozens of bags by hand.
How far does a cubic yard of concrete go?
| Depth | Area covered by 1 cubic yard | Cubic yards for 100 sq ft |
|---|---|---|
| 1 inch | 324 sq ft | 0.31 |
| 2 inches | 162 sq ft | 0.62 |
| 3 inches | 108 sq ft | 0.93 |
| 4 inches | 81 sq ft | 1.23 |
| 6 inches | 54 sq ft | 1.85 |
| 12 inches | 27 sq ft | 3.70 |
Tips for ordering concrete
- Add 5 to 10 percent for uneven ground, spillage and slightly deeper edges.
- Ready-mix suppliers often have a minimum order and charge extra for small loads, so ask before you book.
- Have the forms, base and any reinforcement ready before the truck arrives. Concrete is time-sensitive.
- Concrete weighs roughly 2 tons per cubic yard, so check access and ground conditions for delivery.
Frequently asked questions
How many cubic yards of concrete for a 10 by 10 slab? About 1.23 cubic yards at 4 inches thick, or 1.36 with a 10% allowance.
How many bags of concrete make a cubic yard? About 45 bags of 80 lb mix, depending on the brand.
How much does a cubic yard of concrete weigh? About 4,000 lb, or roughly 2 tons.
Is the result safe to use for structural work? This calculator estimates volume only. For footings, foundations and structural slabs, follow your plans and local rules.