Volume first, then convert to cubic yards
Concrete is ordered by the cubic yard, but a slab is measured in feet and inches. Get the volume in cubic feet, then divide by 27 (a cubic yard is 3 × 3 × 3 ft). Forgetting that divide-by-27 step is the most common ordering mistake.
Thickness goes in as feet: a 4-inch slab is 4 ÷ 12 = 0.333 ft, a 6-inch slab is 0.5 ft. So 20 ft × 10 ft × 0.333 ft = 66.7 ft³, and 66.7 ÷ 27 = 2.47 yd³.
Slab size, volume and 80-lb bags
| Slab (4″ thick) | Cubic yards | 80-lb bags |
|---|---|---|
| 8 × 8 ft | 0.79 yd³ | 36 |
| 10 × 10 ft | 1.23 yd³ | 56 |
| 12 × 12 ft | 1.78 yd³ | 80 |
| 20 × 10 ft | 2.47 yd³ | 112 |
| 20 × 20 ft | 4.94 yd³ | 223 |
| 24 × 24 ft | 7.11 yd³ | 320 |
Bags use 45 per cubic yard for the 80-lb size, before any waste allowance. A 6-inch slab needs 50% more of everything than the 4-inch figures above.
Getting the order right
- Add a waste factor. 10% is normal for a first pour, 5% for experienced crews. It covers spillage, an uneven base, and settling.
- Bags vs ready-mix. A 2.5 yd³ slab is over 110 80-lb bags to mix by hand — a job for ready-mix delivery. Bags suit small pads and repairs.
- Base and joints matter more than a bit of extra concrete. Pour over 3 to 4 inches of compacted gravel, and cut control joints every 4 to 6 ft so shrinkage cracks land where you want them.
Common questions
How do I work out cubic yards of concrete?
Multiply length by width by thickness, all in feet, to get cubic feet, then divide by 27 to get cubic yards. A 20 by 10 ft slab at 4 inches thick is 20 x 10 x 0.333 = 66.7 cubic feet, which is 2.47 cubic yards.
How many 80-lb bags of concrete are in a cubic yard?
About 45 bags of 80-lb pre-mix, or 60 bags of the 60-lb size. Bags only make sense for small jobs; past roughly one cubic yard, ready-mix delivery is cheaper and easier.
Should I order a bit extra?
Yes. Add 5 to 10% for spillage, uneven subgrade, and settling. Running short mid-pour leaves a cold joint, so it is safer to over-order slightly.
Do I use 4-inch or 6-inch thickness?
Four inches suits patios, walkways, and car driveways. Six inches is for driveways that carry trucks, and for heavier loads. Going to 6 inches raises the concrete volume by half.
