How to calculate concrete
Work out the volume of each part in cubic feet, add them up, and divide by 27 for cubic yards. Thickness and depth are usually in inches, so divide them by 12 first.[4]
- Slab: length × width × thickness. 10 × 10 ft × (4 ÷ 12) = 33.3 cu ft = 1.23 yd³.
- Footing: length × width × depth. 56 ft of footing 12 in wide and 12 in deep = 56 cu ft = 2.07 yd³.
- Wall: length × height × thickness. 10 × 3 ft × (8 ÷ 12) = 20 cu ft.
- Round column or tube: 3.1416 × diameter² ÷ 4 × height. 12 in × 4 ft = 3.14 cu ft.
- Round slab: 3.1416 × diameter² ÷ 4 × thickness. A 10 ft circle 4 in thick = 26.2 cu ft.
- Post hole: the cylinder minus the post. A 10 in hole with 26 in of concrete around a 3½ in post is 1 cu ft.
- Steps: width × rise × run × n(n + 1) ÷ 2 for a solid flight from the ground. 3 steps 4 ft wide with 7 in risers and 11 in treads = 12.8 cu ft.
- Curb and gutter: length × (curb width × (curb height + gutter thickness) + gutter width × gutter thickness). 10 ft of a 6 in curb, 6 in high, with an 18 in gutter 6 in thick = 12.5 cu ft.
How many bags of concrete do I need?
Divide the cubic feet by the bag yield: about 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb and 0.30 for 40 lb.[1] The table rounds each count up.
| Slab | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 10 × 10 ft, 4 in | 1.23 | 56 | 75 | 112 |
| 10 × 12 ft, 4 in | 1.48 | 67 | 89 | 134 |
| 12 × 12 ft, 4 in | 1.78 | 80 | 107 | 160 |
| 12 × 24 ft, 5 in | 4.44 | 200 | 267 | 400 |
| 20 × 20 ft, 4 in | 4.94 | 223 | 297 | 445 |
| 24 × 24 ft, 4 in | 7.11 | 320 | 427 | 640 |
How many 80 lb bags of concrete in a yard?
45 bags of 80 lb make one cubic yard, because each yields about 0.60 cu ft and a yard is 27 cu ft.[1] The same yard takes 60 bags of 60 lb or 90 bags of 40 lb, about 3,600 lb of dry mix in every case.
| Bag | Yield | Per cubic yard | Per cubic meter |
|---|---|---|---|
| 80 lb (36.2 kg) | 0.6 cu ft (17 L) | 45 | 59 |
| 60 lb (27.2 kg) | 0.45 cu ft (12.7 L) | 60 | 79 |
| 50 lb (22.6 kg) | 0.375 cu ft (10.6 L) | 72 | 94 |
| 40 lb (18.1 kg) | 0.3 cu ft (8.5 L) | 90 | 118 |
Concrete footing calculator: footings and walls
A footing is a long, narrow slab: its length × width × depth. For a shed with a 12 × 16 ft slab and a 12 × 12 inch footing around the edge (56 ft), the slab is 64 cu ft and the footing 56 cu ft, 4.44 cubic yards together. Footing and wall sizes for buildings are set by your local code and design, so enter the dimensions from your plans; the calculator only adds up the volume.
Concrete post hole calculator
Each post hole needs the hole volume minus the post, above any gravel at the bottom. With a 10 inch hole, 4 inches of gravel under a 30 inch hole and a 3½ inch post, each post takes about 1 cu ft, so 1.7 bags of 80 lb. Choose Post holes and set how many; the fence calculator also counts the posts for a fence line.
Steps
A solid flight of steps holds width × rise × run × n(n + 1) ÷ 2, because the bottom step is one rise tall, the next two rises, and so on. Three steps 4 ft wide with 7 in risers and 11 in treads take 12.8 cu ft (0.48 yd³), or 22 bags of 80 lb.
Bags or ready-mix?
A cubic yard is 45 bags of 80 lb, about 3,600 lb of dry mix to carry and mix, so compare the bag count with a ready-mix quote before deciding.[1] Enter both prices in the calculator to see the two totals side by side; the concrete slab cost calculator explains the cost math in more detail.
Waste and marginBag yields are approximate,[1] and an uneven base or flexing forms add volume. Use the 5% or 10% option rather than rounding dimensions up.
How much does concrete weigh?
About 140 lb per cubic foot for a standard packaged mix, which is about 3,780 lb per cubic yard.[1] The American Concrete Institute describes lightweight concrete as usually 90–115 lb per cubic foot,[3] so ordinary concrete is much heavier. More detail is on how much a yard of concrete weighs.
Common concrete calculator mistakes
The most common mistake is leaving thickness in inches: 10 × 10 × 4 is 400, but the slab is 33.3 cubic feet.
- Treating steps as a box. A flight from the ground holds n(n + 1) ÷ 2 step blocks, not n.
- Not subtracting the post. A 3½ inch post takes up about 15% of a 10 inch hole.
- Pouring a house floor too thin. Residential floor slabs must be at least 3½ inches.[2]
- Using bagged mix too thin. Standard bagged mix is rated for pours 2 inches thick or more.[1]
Metric concrete
Cubic meters = length (m) × width (m) × thickness (cm ÷ 100). A 3 × 3 m slab 10 cm thick is 0.9 m³, which is 53 bags of 36.2 kg (17 L each).[1] Switch the calculator to meters for metric inputs and bag labels.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
At 4 inches thick: 56 bags of 80 lb, 75 of 60 lb or 112 of 40 lb.[1]
How many 80 lb bags of concrete make a yard?
45 bags.[1]
How much concrete do I need for a fence post?
For a 10 inch hole with 26 inches of concrete around a 3½ inch post, about 1 cu ft, or 1.7 bags of 80 lb.
How many yards of concrete for a 24 × 24 garage slab?
At 4 inches: 24 × 24 × 0.333 = 192 cu ft, which is 7.11 cubic yards before any waste.
How do I calculate concrete for steps?
Width × rise × run × n(n + 1) ÷ 2 for a solid flight of n steps. Choose Steps in the calculator to do it automatically.
Sources
- QUIKRETE Concrete Mix No. 1101 data sheet (yields, unit weight, strength)
- 2025 California Residential Code, Section R506 Concrete Floors (on Ground), based on the International Residential Code
- Aggregate classification by density (Technical FAQ)
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement