Enter the slab the way you'd read it off a tape measure — 12' 6 1/2" works. You'll get cubic yards, bag counts, and a ticket you can print and hand to the yard.
Volume is length × width × thickness, so the only trick is getting thickness into the same unit as the other two. A 4 inch slab is 0.333 ft thick, not 4.
Concrete is sold by the cubic yard, and there are 27 cubic feet in a yard. So a 20 × 12.5 ft slab at 4 inches is 83.3 ft³, or 3.09 yd³ before waste.
A slab is poured onto a subgrade that is never perfectly flat. Every low spot pulls extra concrete, and you lose some to the chute and the wheelbarrow. Coming up half a yard short means a cold joint in the middle of the pour, so the extra is cheap insurance.
Bags stop making sense somewhere around 1 yd³ — that's roughly 45 eighty-pound bags, and mixing that by hand is a long day. Above that, call a ready-mix supplier. Most have a short-load fee under 3–4 yd³, so factor that into the comparison.
| Assumption | Value used |
|---|---|
| Yield of an 80 lb bag of concrete mix | 0.60 ft³ |
| Yield of a 60 lb bag | 0.45 ft³ |
| Yield of a 40 lb bag | 0.30 ft³ |
| Yield of a 25 kg bag | 0.0118 m³ |
| Full ready-mix truck | 10 yd³ (7.6 m³) |
| Supplier order increment | 0.25 yd³ |
Bag yields come off the manufacturer's printed yield and vary slightly by product — check your bag if you're close to a round number. Confirm the truck size and short-load policy with your own supplier.
45. A cubic yard is 27 ft³ and each bag yields 0.60 ft³.
4 inches is standard for a patio, shed floor or walkway. Go to 5–6 inches where vehicles will sit on it, and check your local code — some areas set a minimum.
Yes. 12' 6 1/2", 12ft 6.5in, 150" and 12.54 all read the same way.