Concrete Calculator
Calculate how much concrete you need for a slab, footing or column — enter your dimensions to get the volume in cubic yards, the number of 40, 60 or 80 lb bags, and an estimated cost.
How to calculate concrete
Concrete is ordered by volume — cubic yards for ready-mix, or bags for smaller jobs. Find the volume by multiplying length × width × thickness with every dimension in feet, then divide by 27 to convert cubic feet to cubic yards. The calculator above handles three common shapes — flat slabs, wall footings and round columns or post holes — and adds a waste allowance.
cubic feet = length (ft) × width (ft) × thickness (ft) · cubic yards = cubic feet ÷ 27
How many bags of concrete in a yard?
One cubic yard is 27 cubic feet, so the number of bags depends on each bag's yield. Use this to cross-check the calculator:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | ≈ 0.60 cu ft | 45 bags |
| 60 lb | ≈ 0.45 cu ft | 60 bags |
| 40 lb | ≈ 0.30 cu ft | 90 bags |
Concrete for a slab
| Slab size | 4 in thick | 6 in thick |
|---|---|---|
| 10 × 10 ft | 1.23 yd³ | 1.85 yd³ |
| 12 × 12 ft | 1.78 yd³ | 2.67 yd³ |
| 20 × 20 ft | 4.94 yd³ | 7.41 yd³ |
| 24 × 24 ft | 7.11 yd³ | 10.67 yd³ |
Volumes before waste — add 5–10% for spillage and uneven subgrade.
Pricing the pour? Use the concrete cost calculator, or the concrete bags calculator if you're bagging it yourself.
Frequently asked questions
How much concrete do I need?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. For a slab, convert the thickness from inches to feet first (4 inches = 0.333 ft). The calculator above does this for slabs, footings and round columns, then converts the volume into bags and ready-mix yards with a waste allowance.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. Since an 80 lb bag yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.30 cu ft, one cubic yard takes roughly 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags. The calculator shows the bag count for each size.
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab is 100 sq ft. At 4 inches thick that is 100 × 0.333 = 33.3 cubic feet, or about 1.23 cubic yards — roughly 56 × 80 lb bags. At 6 inches it is about 1.85 cubic yards. Enter your exact thickness above for the precise figure.
Should I use bags or order ready-mix?
Bagged concrete is usually the cheaper, simpler choice for small jobs up to about half a cubic yard — fence posts, small pads, repairs. For anything from roughly one cubic yard up, ready-mix delivered by truck is normally cheaper per yard and far less work than mixing dozens of bags. The calculator shows both so you can compare.
How much extra concrete should I add for waste?
Add about 5–10% extra to cover spillage, uneven subgrade and over-excavation. The calculator defaults to a 10% allowance, which you can adjust. Running a little short mid-pour means a cold joint, so ordering slightly over is the safer choice.