Why slab volume is L × W × t, and why a pier is not
Why we divide by 27, and why a cylinder uses π
A rectangular pour is . Thickness is in inches on most tickets, so you divide by 12 to get feet, then divide cubic feet by 27 to get cubic yards — the unit the ready-mix plant sells. That is the only reason 27 appears. It is not a strength number and it is not a waste factor.
A round pier is . Treating it as length × width over-orders by about 27 percent. An 80 lb bag is taken as 0.60 cubic feet, a 60 lb bag as 0.45 — typical premix yields, not a lab test. Truck loads assume a 10 cubic yard mixer. That is our plant assumption, not a code. Rural drums are often 8 or 9. A 1.2 yard patio is often cheaper in bags than as a short load.
Worked field example
24 × 12 foot driveway, 5 inches thick, plus two 12-inch-diameter piers 4 feet deep. Not the 20 × 10 × 4 slab on the form.
Slab net is 24 × 12 × 5 ÷ 12 = 120 cubic feet, or 4.44 cubic yards. Ten percent subgrade waste takes that to 4.89 yards. Two 12-inch piers are not 1 × 1 × 4. Each is cubic feet; two piers are 6.28 cubic feet (0.23 yard). Adding them as a 2 × 1 × 4 rectangle would have invented an extra 1.7 cubic feet that is not in the hole.
At 80 lb bags the slab alone is already hundreds of sacks. This pour is a truck. Confirm drum size before you treat “1 truck” as 10 yards.
What IRC R506 and ACI 302 actually require
R506 is a residential slab-on-ground chapter: base, vapor retarder, thickness where the table applies. ACI 302 is a placing-and-finishing guide. Neither one sizes your cubic yards. They tell you what has to be under and in the slab after the truck leaves.
Placement rules — volume is still L × W × t
- Provision
- IRC R506.1
- Limit
- Slab-on-ground min thickness where the IRC table applies (often 3.5 in)
- Failure mode
- Ordering 3 inches and calling it a driveway
- Provision
- IRC R506.2
- Limit
- Base and vapor retarder as adopted
- Failure mode
- Counting crushed stone as concrete volume
- Provision
- ACI 302.1R
- Limit
- Joint spacing, curing, no cold joints in a single placement
- Failure mode
- Stopping a truck mid-slab and coming back tomorrow
| Provision | Limit | Failure mode |
|---|---|---|
| IRC R506.1 | Slab-on-ground min thickness where the IRC table applies (often 3.5 in) | Ordering 3 inches and calling it a driveway |
| IRC R506.2 | Base and vapor retarder as adopted | Counting crushed stone as concrete volume |
| ACI 302.1R | Joint spacing, curing, no cold joints in a single placement | Stopping a truck mid-slab and coming back tomorrow |
Where the field usually goes wrong
Putting the gravel base into the concrete volume is the first miss. A 4-inch slab on 4 inches of stone is 4 inches of concrete, not 8. The stone is a different ticket. The second miss is a short load: a 2-yard order on a 10-yard truck often costs more per yard than 5 yards, and the drum may not mix a 1-yard load well.
Cold joints happen when the first truck sets before the second arrives. This calculator does not schedule trucks. A thickened edge is extra concrete at the perimeter, not a thicker slab over the whole driveway — split it into slab plus footing. Cylinders and square piers must not share the same L × W box.
Frequently asked questions
Why divide by 27?
27 cubic feet make a cubic yard. Ready-mix is sold by the yard. The 27 is unit conversion, not waste or strength.
How many 80 lb bags are in a yard?
About 45 at 0.60 cubic feet per sack. A 60 lb sack at 0.45 cubic feet is about 60 per yard. The bag label wins if it differs.
Is a 10-yard truck a code requirement?
No. It is this page’s plant assumption. Rural mixers are often 8 or 9 yards.
Do I include the gravel base in the volume?
No. Base is a separate ticket. R506 wants the base there; it does not add it to the yards.
How do I take off a round pier?
Cylinder mode: πr²h. A 12-inch hole is not a 12 × 12 square.
What is a cold joint and why does it matter?
A hardened edge when a placement stops. ACI 302 wants a continuous pour. Volume math will not warn you.
Does this check compressive strength or rebar?
No. Yards, bags, and trucks only. Bars are on the rebar calculator. psi is the mix you order.