How much concrete for a 20 × 40 ft slab, 4 inches thick?

A 20 × 40 ft slab covers 800 square feet. At 4 inches thick it holds 9.88 cubic yards (266.67 cubic feet) of concrete. With 10% extra for spills and uneven ground, order 10.86 cubic yards.

Concrete to order

10.86 yd³

9.88 yd³ for the shape, plus 10% extra.

What to buy
Cubic yards	10.86
Cubic feet	293.33
80 lb bags	489
60 lb bags	652
40 lb bags	978
Ready-mix truckloads	1 to 2

What this slab needs

This is a ready-mix job: 1 to 2 truckloads at 8 to 12 cubic yards each. The same pour from bags would mean 489 bags of 80 lb mix, about 19.6 tons.

At 6 inches instead of 4, the same slab would need 16.3 cubic yards with the extra, 50% more.

By bag size: 489 × 80 lb, 652 × 60 lb or 978 × 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.