How much concrete is a 20×20 slab?
A 20-by-20-foot slab covers 400 square feet. At the standard 4-inch thickness that's about 4.94 cubic yards of concrete before waste, or roughly 5.43 yards once you add a 10% allowance. At about 5.43 cubic yards a single ready-mix truck (roughly 10 cubic yards) delivers this in one load with room to spare. The calculator above is pre-filled to these dimensions; change the thickness or price to match your job.
Concrete for a 20×20 slab by thickness
| Thickness | Concrete to order | 80 lb bags | Material cost |
|---|---|---|---|
| 4″ (standard) | 5.43 yd³ | 245 | $869 |
| 5″ | 6.79 yd³ | 306 | $1,086 |
| 6″ | 8.15 yd³ | 367 | $1,304 |
Material cost at $160/yd³ ready-mix, before delivery fees. Cost data updated June 2026.
Common questions
How many yards of concrete for a 20x20 slab?
A 20x20 slab at 4 inches thick needs about 4.94 cubic yards before waste. Add 5-10% extra so you don't come up short. Change the thickness in the calculator above to see how it affects the total.
How many bags of concrete for a 20x20 slab?
At 4 inches thick that's roughly 245 eighty-pound bags with a 10% allowance. Past about 1 cubic yard (around 45 bags), ready-mix delivery is usually cheaper and far less work than mixing bags by hand.
What does a 20x20 slab cost?
Material alone is a small share of the total. Installed, slabs this size run a wide range depending on thickness, finish, site prep, and region. Use the calculator's installed-cost range as a ballpark, then compare local quotes for a real price.