How much concrete do I need?
Slab volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12
Round column (cu ft) = π × (diameter (in) ÷ 24)² × height (ft)
Cubic yards = cubic feet ÷ 27 · Bags = cubic feet ÷ yield per bag
Concrete is ordered by volume: in the US by the cubic yard (27 cubic feet), elsewhere by the cubic metre. For small jobs, premixed bags are sold by weight, and the bag states its yield. The calculator uses the yields printed on common premixed concrete bags:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Worked example: a 10 × 10 ft patio, 4 in thick
- Volume: 10 × 10 × 4 ÷ 12 = 33.33 cu ft = 1.23 cu yd (0.94 m³).
- With 10% extra: 36.67 cu ft = 1.36 cu yd.
- In 80 lb bags: 36.67 ÷ 0.60 = 61.1, so 62 bags. In 60 lb bags: 36.67 ÷ 0.45 = 81.5, so 82 bags.
At that size, ordering ready-mixed concrete from a truck is usually easier than mixing 62 bags by hand. Ask the supplier about their minimum load and any short-load fee.
Worked example: four fence or deck post holes
Four 12 in diameter holes, 4 ft deep. One hole is π × 0.5² × 4 = 3.14 cu ft, so four are 12.57 cu ft. With 10% extra that is 13.82 cu ft, or 24 bags of 80 lb (13.82 ÷ 0.60 = 23.0, rounded up). The post itself takes up some of the hole; for a 4 × 4 post (3.5 × 3.5 in actual) that is 0.34 cu ft per 4 ft hole, so you may have a bag or two left over.
How thick should a slab be?
| Use | Typical thickness |
|---|---|
| Walkways, patios, shed bases | 4 in (10 cm) |
| Driveways for cars | 4–5 in; 6 in for heavier vehicles |
| Garage floors | 4–6 in |
| Footings | Set by local code and the load; often 8–12 in deep or more |
These are common practice figures. Footings, structural slabs and anything carrying a building must follow your local building code and, where needed, an engineer's design.
Why order extra?
Ground is never perfectly level, forms bow slightly outward, and some concrete is always left in the mixer or wheelbarrow. Industry guidance commonly suggests ordering 5–10% more than the calculated volume; use more for uneven ground. Running out mid-pour leaves a cold joint, which is weaker and visible.
Step by step: planning a small pour
- Measure the length and width inside the forms, and the depth at several points.
- Use the average depth, or the deepest point if the ground slopes.
- Choose bags or ready-mix: bags for small jobs, ready-mix for about a cubic yard or more.
- Add 5–10% extra for uneven ground and spillage.
- Arrange enough helpers, tools and water to finish the pour in one go.
Metric example
A shed base 3 × 2.4 m and 10 cm thick is 3 × 2.4 × 0.10 = 0.72 m³. With 10% extra that is 0.79 m³, about 1.04 cu yd, or 47 bags of 80 lb (each about 0.017 m³).
Common mistakes
- Using inches as feet. A 4 in slab is 0.333 ft thick, not 4 ft. The calculator handles the conversion.
- Using the bag weight instead of its yield. An 80 lb bag makes only 0.6 cu ft.
- Measuring the hole diameter at the top only. Dug holes are often wider at the bottom; bell-shaped footings take more concrete.
Frequently asked questions
How many bags of concrete do I need for a 10×10 slab?
At 4 in thick the slab is 33.33 cu ft (1.23 cu yd). With 10% extra that is 36.67 cu ft: 62 bags of 80 lb or 82 bags of 60 lb.
How many 80 lb bags of concrete make a yard?
45 bags. Each 80 lb bag yields about 0.60 cu ft, and a cubic yard is 27 cu ft. For 60 lb bags it is 60, and for 40 lb bags 90.
How do I calculate cubic yards of concrete?
Multiply length × width in feet by the thickness in inches, divide by 12 for cubic feet, then divide by 27. A 10 × 10 ft slab 4 in thick is 33.33 cu ft, or 1.23 cu yd.
How much concrete for a post hole?
A 12 in hole 4 ft deep holds 3.14 cu ft, about 5.2 bags of 80 lb before subtracting the post. A 10 in hole 3 ft deep holds 1.64 cu ft, about 3 bags.
How much concrete do I need in cubic metres?
Multiply length × width × thickness, all in metres. A 3 × 2.4 m slab 10 cm thick is 0.72 m³, or about 0.79 m³ with 10% extra.
Sources
- Premixed concrete bag yields as printed by major US manufacturers (e.g. Quikrete, Sakrete): 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft.
- American Concrete Institute (ACI) 332, Residential Code Requirements for Structural Concrete; Portland Cement Association guidance on residential slabs.
Last reviewed 20 October 2026. Results are estimates; see our disclaimer.