Concrete Calculator
Enter your slab dimensions to get the concrete volume in cubic feet and cubic yards, plus how many 60 lb or 80 lb bags you need — including an allowance for waste.
How to calculate how much concrete you need
- Measure the length and width of the area in feet, and the thickness (depth) in inches. For a slab, multiply length × width × thickness to get the volume. Because thickness is measured in inches, divide it by 12 to convert it to feet before multiplying.
- The result is a volume in cubic feet. Concrete is usually ordered by the cubic yard, so divide cubic feet by 27 to convert. One cubic yard equals exactly 27 cubic feet.
- If you are using bagged concrete instead of a ready-mix truck, divide the total cubic feet by the yield of one bag: an 80 lb bag yields about 0.60 cubic feet and a 60 lb bag about 0.45 cubic feet. Always round up — you cannot buy half a bag.
Volume (cu yd) = Length (ft) × Width (ft) × Thickness (in) ÷
12 ÷ 27. Bags = Volume (cu ft) × (1 + waste %) ÷ bag yield,
rounded up.
Bagged concrete yield (approximate)
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb bag | 0.30 cu ft | 90 |
| 60 lb bag | 0.45 cu ft | 60 |
| 80 lb bag | 0.60 cu ft | 45 |
| 90 lb bag | 0.68 cu ft | 40 |
Yields are manufacturer approximations — check the bag label for exact values.
Frequently asked questions
A 10×10 ft slab at 4 inches thick is about 33.3 cubic feet (1.23 cubic yards). At 0.60 cu ft per 80 lb bag that is 56 bags — or about 61 bags with a 10% waste allowance.
12 × 12 × (4 ÷ 12) = 48 cubic feet, which is 48 ÷ 27 ≈ 1.78 cubic yards.
One cubic yard equals 27 cubic feet. Ready-mix concrete is priced per cubic yard, while bag yields are listed in cubic feet.
Common practice: 4 inches for patios, walkways and shed floors; 5–6 inches for driveways and garage floors that carry vehicles. Local codes and soil conditions can require more.
Standard concrete weighs about 150 lb per cubic foot — roughly 4,050 lb (just over 2 tons) per cubic yard.
Yes. Add 5–10% to allow for spillage, uneven subgrade and over-excavation. Running short mid-pour is far more expensive than a small surplus.
Only the yield: an 80 lb bag makes about 0.60 cu ft of concrete versus 0.45 cu ft for a 60 lb bag. Fewer, heavier bags are usually cheaper per cubic foot; lighter bags are easier to carry.
A common rule of thumb: once a project exceeds about 1–2 cubic yards (roughly 45–90 bags of 80 lb concrete), a ready-mix delivery is usually cheaper and gives better quality than hand-mixing bags.