20 × 20 Slab: How Much Concrete?
A 20 × 20 ft slab at the standard 4 inch thickness needs 4.94 cubic yards of concrete — 133.33 ft³, or 245 bags of 80 lb premix with 10% waste included.
Bag counts use the waste-adjusted volume and always round up — you can’t buy part of a bag. Ready-mix is usually ordered to the nearest quarter yard; your plant may add short-load or delivery fees.
20 × 20 slab at 4, 5 and 6 inches
| Thickness | Cubic feet | Cubic yards | Yards +10% waste | 80 lb bags* | 60 lb bags* | 50 lb bags* | 40 lb bags* |
|---|---|---|---|---|---|---|---|
| 4 in | 133.33 | 4.94 | 5.43 | 245 | 326 | 392 | 489 |
| 5 in | 166.67 | 6.17 | 6.79 | 306 | 408 | 489 | 612 |
| 6 in | 200.00 | 7.41 | 8.15 | 367 | 489 | 587 | 734 |
*Bag counts include the 10% waste allowance and round up. Exact counts with no waste at 4 inches: 223 × 80 lb · 297 × 60 lb · 356 × 50 lb · 445 × 40 lb. Which thickness do you need? Patios and walkways are typically 4 in, driveways 5–6 in — see the slab thickness guide and always check local code.
The math for a 20 × 20 slab
Volume = length × width × thickness, with thickness in feet. At 4 inches: 20 × 20 × (4 ÷ 12) = 133.33 ft³. Divide by 27 for cubic yards: 4.94 yd³. Bags come from the yield printed on the bag — 0.60 ft³ per 80 lb bag — so 133.33 ÷ 0.60 = 222.2 bags, rounding up to 223 before waste and 245 with 10% added. Change any dimension in the calculator above and every figure updates.
Other slab sizes
Frequently asked questions
How many bags for a 20 × 20 slab at 4 inches?
223 bags of 80 lb premix exactly, or 245 bags with the standard 10% waste allowance. In 60 lb bags: 297 exact, 326 with waste.
Should a 20 × 20 slab be 4 or 6 inches thick?
It depends on the load, not the footprint. Foot traffic and patio furniture: 4 inches is the common standard. Cars and trucks: 5–6 inches over a compacted gravel base is typical. Your local building code and soil conditions have the final say.
Bags or ready-mix for this size?
At 4 inches this slab is 4.94 yd³ — past the one-yard mark, so a ready-mix truck is usually the practical choice unless access is impossible. Compare the two on the cost page.