How much concrete for a 18 × 18 ft slab, 3.5 inches thick?
A 18 × 18 ft slab covers 324 square feet. At 3.5 inches thick it holds 3.5 cubic yards (94.5 cubic feet) of concrete. With 10% extra for spills and uneven ground, order 3.85 cubic yards.
Concrete to order
3.85 yd³
3.5 yd³ for the shape, plus 10% extra.
Concrete to order. 3.85 yd³. 3.5 yd³ for the shape, plus 10% extra.
| Cubic yards | 3.85 |
|---|---|
| Cubic feet | 103.95 |
| 80 lb bags | 174 |
| 60 lb bags | 231 |
| 40 lb bags | 347 |
| Ready-mix truckloads | 1 |
What this slab needs
Mixing it from bags would take 174 bags of 80 lb mix, about 7 tons to lift and mix by hand. One ready-mix truck, which carries 8 to 12 cubic yards, can bring all 3.85 cubic yards in a single delivery.
At 4 inches instead of 3.5, the same slab would need 4.4 cubic yards with the extra, 14% more.
By bag size: 174 × 80 lb, 231 × 60 lb or 347 × 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.