How much concrete for a 20 x 20 slab?
Short answer
- A 20 x 20 ft slab is 400 square feet. At 4 inches thick it needs 4.94 cubic yards (133.3 cubic feet) of concrete.
- With 10% extra that's 5.43 cubic yards, so you'd order 5.5 cubic yards of ready-mix (plants sell in quarter-yard steps). By the bag it's 245 bags of 80 lb.
- It's typically a two-car parking pad.
Concrete for a 20 x 20 slab at 4, 5 and 6 inches
Bag counts include 10% extra for uneven ground and spillage. Cubic yards are shown without extra so you can compare quotes.
| Thickness | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags | Weight with extra |
|---|---|---|---|---|---|
| 4 in | 4.94 | 245 | 326 | 489 | 10.3 to 11 tons |
| 5 in | 6.17 | 306 | 408 | 612 | 12.8 to 13.8 tons |
| 6 in | 7.41 | 367 | 489 | 734 | 15.4 to 16.5 tons |
Open this slab in the concrete calculator to change the size, thickness, waste or bag size.
How the math works
20 ft × 20 ft × 4 in ÷ 12 = 133.3 cubic feet. Divide by 27 to get 4.94 cubic yards. Add 10% and you have 146.7 cubic feet. An 80 lb bag of premixed concrete yields about 0.60 cubic feet, so 146.7 ÷ 0.60 rounds up to 245 bags.
Bags or ready-mix?
At 4.94 cubic yards, a 4 inch 20 x 20 slab means mixing 245 bags by hand. Price a ready-mix load too, and ask about the minimum order and short-load fees before you decide.
Forms and gravel base
The slab's edge is 80 feet, so that's how much form board you need around it, plus stakes. Under it, Quikrete's patio steps call for 5 inches of loose gravel tamped down to 4. For a 20 x 20 slab that's 166.7 cubic feet of loose gravel, about 6.17 cubic yards or 8.8 tons at 105 lb per cubic foot. The slab planner adds rebar or mesh and totals the cost with your local prices.