How much concrete for a 4 × 6 ft slab, 3.5 inches thick?
A 4 × 6 ft slab covers 24 square feet. At 3.5 inches thick it holds 0.26 cubic yards (7 cubic feet) of concrete. With 10% extra for spills and uneven ground, order 0.29 cubic yards.
Concrete to order
0.29 yd³
0.26 yd³ for the shape, plus 10% extra.
Concrete to order. 0.29 yd³. 0.26 yd³ for the shape, plus 10% extra.
| Cubic yards | 0.29 |
|---|---|
| Cubic feet | 7.7 |
| 80 lb bags | 13 |
| 60 lb bags | 18 |
| 40 lb bags | 26 |
| Ready-mix truckloads | 1 |
What this slab needs
From bags, that is 13 bags of 80 lb mix (1,040 lb in all) or 18 bags of 60 lb. At this size bagged mix is practical: a ready-mix truck carries 8 to 12 cubic yards, many times what this slab needs.
At 4 inches instead of 3.5, the same slab would need 0.33 cubic yards with the extra, 14% more.
By bag size: 13 × 80 lb, 18 × 60 lb or 26 × 40 lb. An 80 lb bag of concrete mix makes about 0.60 cubic feet.
How it's calculated
A slab or footing is a box: length × width × thickness. A round column is π × radius² × height. Solid steps on the ground are stacked blocks: the bottom step is one block tall, the next two, and so on, so n steps hold n × (n + 1) ÷ 2 blocks of width × rise × run.
Volumes are worked out in cubic feet and converted exactly: 27 cubic feet make a cubic yard, and a foot is 0.3048 m.
Bags are rounded up to whole bags of the order volume. An 80 lb bag of concrete mix makes about 0.60 cubic feet, a 60 lb bag 0.45 and a 40 lb bag 0.30.
Ready-mix trucks carry 8 to 12 cubic yards, so the truckload count is a range. Most suppliers also have a minimum order, so check before ordering a small amount.
Sources
- QUIKRETE Concrete Mix No. 1101 product data sheet, Quikrete International (2022-10)
- Sakrete High-Strength Concrete Mix technical data sheet, Sakrete (Oldcastle APG) (2023)
- CIP 31: Ordering Ready Mixed Concrete, National Ready Mixed Concrete Association (2023)
- PCC Truck Mixer (citing AASHTO), Pavement Interactive (2026)
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement, National Institute of Standards and Technology (2026)
Last reviewed October 2026.