Concrete Slab Calculator
Example: Slab length: 20 ft · Slab width: 12 ft · Slab thickness: 4 in · Waste allowance: 10 %
| Concrete to order | 3.26 |
| 80 lb bags | 147 |
| 60 lb bags | 196 |
Worked example
A 20 ft by 12 ft slab poured 4 inches thick is 20 x 12 x (4/12) = 80 cubic feet of concrete. Adding a 10% waste allowance brings it to 88 cubic feet, which is 88 / 27 = about 3.26 cubic yards to order from a ready-mix truck. If you mixed it by hand instead, an 80 lb bag yields roughly 0.60 cubic feet, so you would need about 147 bags, or about 196 of the 60 lb bags. That bag count is exactly why anything past a few cubic yards is worth a truck.
Frequently asked questions
How much waste should I add?
A 5 to 10% allowance is typical to cover uneven subgrade, spillage, and slightly over-dug forms. Bump it toward 10% for hand-dug footings or rough ground, and keep it lower for a formed, screeded slab on a flat base. The right number varies by how level and contained your pour is.
When should I order a truck instead of bags?
Mixing bags by hand is reasonable up to roughly half a cubic yard. Beyond about one cubic yard the bag count and mixing time become punishing, and most ready-mix suppliers have a minimum load of around one yard. This tool shows both so you can see where the crossover lands for your slab.
Does a 4-inch slab need to be exactly 4 inches?
Slab thickness drives volume directly, so measure your actual formed depth. A common residential slab is 4 inches, but driveways and load-bearing pads are often 5 to 6 inches. Enter the real thickness because even a half-inch across a large slab changes the yardage.
How many 80 lb bags are in a cubic yard?
Since an 80 lb bag of mix yields about 0.60 cubic feet and a cubic yard is 27 cubic feet, it takes roughly 45 of the 80 lb bags to make one cubic yard. That is why the calculator's bag counts climb quickly as the slab grows.