Short answer: a 20×20 slab at the standard 4-inch thickness needs 4.94 cubic yards of concrete. With the usual 10% safety margin, order 5.5 yards of ready-mix. At roughly $150 per yard, budget about $815 for the concrete alone.
The numbers by thickness
| Thickness | Concrete needed (before safety margin) |
|---|---|
| 4 inches (standard) | 4.94 cubic yards — 223 × 80 lb bags (297 × 60 lb) |
| 6 inches (driveways, garages) | 7.41 cubic yards — 334 × 80 lb bags (445 × 60 lb) |
Add 10% to either figure for real-world ground — that means ordering 5.5 yards at 4 inches, or about 8.25 yards at 6 inches. With the margin, that is 245 × 80 lb bags (326 × 60 lb) at 4 inches, or 367 × 80 lb bags (489 × 60 lb) at 6 inches.
Why bags stop making sense here
Those bag counts are not a typo. A 20×20 slab in bags means mixing and moving roughly 10 tons of concrete — 223 heavy bags at 4 inches — before it starts to set. Beyond about 1 cubic yard, a ready-mix truck is cheaper in money and massively cheaper in labor, and the concrete arrives at a consistent mix.
Do not forget what sits under the slab
Budget separately for 4 inches of compacted gravel base, rebar or mesh reinforcement for vehicle loads, and form boards. The concrete is often only half the real cost of a finished slab.
Pouring a different size or thickness? Get exact yards, bags and cost in our concrete calculator.
Related guides
Frequently asked questions
How long until I can use the slab?
Walking on it after 24–48 hours is normal; wait about 7 days before driving on it, and concrete reaches full strength after roughly 28 days.
Can I pour it in sections?
Better in one pour; if sections are unavoidable, plan control joints in advance rather than leaving random cold joints, which weaken the slab.