How much concrete for a 20 × 20 slab?
A 20x20 slab is 400 sq ft. At the standard 4-inch depth it takes 5.43 cubic yards of concrete (245 80 lb bags) including the recommended 10% waste factor.
| Thickness | Cubic yards (incl. 10% waste) | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 in (patio, shed base) | 5.43 yd³ | 245 | 326 |
| 5 in (light vehicles) | 6.79 yd³ | 306 | 408 |
| 6 in (driveway, garage) | 8.15 yd³ | 367 | 489 |
Estimates are for planning. Confirm quantities against your measured site and product packaging before ordering, and follow local building codes.
Bags or ready-mix for a 20x20 slab?
At 5.43 cubic yards for the 4-inch pour, ready-mix delivery is the realistic choice: that volume means hand-mixing 245 bags, which is a full day of brutal work and risks cold joints between batches. Expect roughly $815 – $1195 for the concrete delivered at 2026 prices ($150 to $220 per cubic yard), and ask about the short-load fee, which can add $50 to $150 on small pours. The formula behind these numbers is on the concrete calculator: length × width × depth, converted to cubic yards, plus your chosen waste factor.
Before ordering, prepare a compacted gravel base of 4 inches (see the gravel calculator for quantities) and read what a finished slab really costs for forms, labor, and finishing on top of the material price.
Frequently asked questions
How many bags of concrete do I need for a 20x20 slab?
At 4 inches thick, a 20x20 slab needs about 245 80 lb bags or 326 60 lb bags, including 10% waste. At 400 sq ft, ready-mix delivery is usually more practical than mixing bags.
How much does concrete for a 20x20 slab cost?
The concrete alone for a 20x20 slab at 4 inches (5.43 cubic yards) runs roughly $815 – $1195 delivered at typical 2026 ready-mix prices of $150 to $220 per cubic yard, before any short-load fee. A finished slab with labor, forms, and base costs several times the material.
Should a 20x20 slab be 4 or 6 inches thick?
Four inches suits patios, walkways, and shed bases. Go to 5 or 6 inches if the slab will carry vehicles or heavy point loads. The jump from 4 to 6 inches adds about 50% more concrete.