20×20 Concrete Slab Calculator
Start with a typical 4-inch slab below, compare common thicknesses, then customize the measurements in the full visual calculator.
20×20 slab at 4 inches thick
A 20×20 slab at 4 inches thick contains about 4.94 cubic yards before extra material. With a 10% allowance, plan for about 5.43 yd³; rounded to a quarter-yard ordering increment, that is 5.50 yd³.
How thickness changes the order
| Thickness | Concrete + 10% | Ready-mix order | 80-lb bags |
|---|---|---|---|
| 4 in | 5.43 yd³ | 5.50 yd³ | 245 |
| 5 in | 6.79 yd³ | 7.00 yd³ | 306 |
| 6 in | 8.15 yd³ | 8.25 yd³ | 367 |
Thickness should be selected for the actual project and loading requirements; this comparison is for quantity planning, not structural design.
Use the quantity to plan how you will buy concrete
At 4 inches, this project is approximately 245 80-lb bags, 326 60-lb bags, or 489 40-lb bags. Large bag counts mean a substantial amount of mixing and handling, so compare bagged material with ready-mix delivery before purchasing.
The full BuildGauge Concrete Calculator also shows 40, 50, 60 and 80-lb bags, project weight, waste allowance and a visual slab diagram.