🧱 Concrete Calculator
Enter your slab, footing or post-hole dimensions to find the concrete volume and how many bags of pre-mixed concrete you need.
How the volume is worked out
Length and width are in feet, depth in inches, because that is how slabs are actually specified. The depth is converted to feet, the three are multiplied, and the result is divided by 27 to give cubic yards, which is the unit concrete is ordered in.
cubic feet = length(ft) x width(ft) x (depth(in) / 12)
cubic yards = cubic feet / 27A 10 x 10 ft slab at 4 inches deep is 33.3 cubic feet, or 1.23 cubic yards.
Bags, and when to stop using them
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Bags make sense for post holes, small pads and repairs. Past roughly one cubic yard the arithmetic turns against you: that is 45 eighty-pound bags, about 1.8 tonnes to carry, and every one has to be mixed and placed before the first ones start setting.
Order more than the calculation says
The figure here is the exact geometric volume. Real pours consume more, for reasons that are normal rather than avoidable:
- Subgrade is never flat. A dip of half an inch across a 10 x 10 slab is another 4 cubic feet.
- Forms bow outwards under the weight of wet concrete.
- Spillage and what stays in the mixer is real volume.
Add 5 to 10% for a slab on prepared ground, and closer to 15% for footings or anything poured against bare earth. Running out mid-pour is far more expensive than a little waste.
Depths worth knowing
- 4 inches — patios, walkways, shed bases, foot traffic.
- 5 to 6 inches — driveways for ordinary cars.
- 6 inches or more, with reinforcement — anything taking heavy vehicles.
Local building rules and frost depth override all of these. Check before you order, particularly for footings.
Frequently asked questions
How many 80 lb bags are in a cubic yard?
45, since each yields about 0.6 cubic feet and a cubic yard is 27 cubic feet. A 60 lb bag yields 0.45 cu ft, so it takes 60 of those.
How much concrete do I need for a 10 x 10 slab?
About 1.23 cubic yards at 4 inches deep, or 1.85 at 6 inches. Add 5 to 10% on top for the subgrade and spillage.
At what size should I order ready-mix instead of bags?
Around one cubic yard. Below that, bags are convenient. Above it you are mixing 45 or more bags against a clock, and the risk of a cold joint starts to outweigh the convenience.
Why is my slab using more concrete than calculated?
Almost always an uneven subgrade. A shallow dip adds volume across the whole area, and forms that bow outwards add more. Both are why the 5 to 10% allowance exists.
Does this include reinforcement?
No. Rebar and mesh displace so little volume that they are ignored in the estimate. They matter a great deal structurally, but not to the amount of concrete you order.