20x40 Concrete Slab: Bags, Yards & Cost
A 20 × 40 ft slab (800 sq ft) 4 in thick is 266.7 cu ft, or 9.88 cu yd: 445 bags of 80 lb Quikrete Concrete Mix (467 with a 5% margin), 593 bags of 60 lb, or 290 bags of 80 lb Sakrete Maximizer. The 80 lb bags weigh 35,600 lb in total.
Your cost and bags vs a truck
Set up for: a 20 × 40 ft slab · Different project?
20x40 slab at 4, 5 and 6 inches
| Thickness | Cu yd | With 5% | 80 lb Quikrete | 60 lb Quikrete | 80 lb Maximizer | 80 lb 5000 Plus |
|---|---|---|---|---|---|---|
| 4 in | 9.88 | 10.4 | 445 | 593 | 290 | 445 |
| 5 in | 12.3 | 13.0 | 556 | 741 | 363 | 556 |
| 6 in | 14.8 | 15.6 | 667 | 889 | 435 | 667 |
Other sizes: 10x10 · 12x12 · 20x20 · 8x8 · 10x12 · 30x30 · 8x10 · 12x24 · 12x36 · 20x30
FAQ
How many bags of concrete for a 20x40 slab?
At 4 in thick: 445 bags of 80 lb Quikrete Concrete Mix or 593 of 60 lb (whole bags, no margin). At 6 in: 667 of 80 lb.
How many yards of concrete for a 20x40 slab?
9.88 cu yd at 4 in, 12.3 at 5 in and 14.8 at 6 in, before the 4–10% NRMCA suggests adding when you order.
How much does a 20x40 concrete slab cost?
We don't publish prices because they change by store and region. Multiply 445 bags by your store's 80 lb bag price, or 10.4 cu yd (4 in, with 5%) by your plant's price per yard plus delivery and short-load fees. Type those into the calculator to compare.
Sources
- Quikrete: QUIKRETE Concrete Mix (No. 1101) data sheet · fetched and reviewed 2026-11-10
- Quikrete: QUIKRETE online project calculator (calculator.js) · fetched and reviewed 2026-11-10
- Sakrete: Sakrete Maximizer Concrete Mix technical data sheet · fetched and reviewed 2026-11-10
- Sakrete: Sakrete 5000 Plus Concrete Mix technical data sheet · fetched and reviewed 2026-11-10
- NRMCA: CIP 31: Ordering Ready Mixed Concrete · fetched and reviewed 2026-11-10