How the math works
The volume of a rectangular slab is length × width × thickness. Divide by 27 to convert cubic feet to cubic yards — the unit ready-mix producers price and deliver by. Contractors quote slab thickness in inches (a “4-inch slab” is universal), so we convert thickness from inches to feet before multiplying.
On top of the exact volume, we add a waste factor (default 10%) to cover over-dig, spillage, and pump-line prime. Industry convention is 5–10%; we default to the conservative end because under-ordering ready-mix means paying a short-load fee or waiting for a second truck.
When to order ready-mix vs. bags
For most residential slabs — driveways, garage floors, patios larger than about 100 square feet — delivered ready-mix is cheaper, faster, and better quality. Mixing several dozen bags by hand is slow, back-breaking work, and the pour is only as consistent as the last-batched wheelbarrow.
Bags win for small, isolated pours: setting a post, patching a section, or fixing a step. The calculator applies a short-load fee when your yardage is below the local producer’s minimum load (typically 2–4 yards) — that’s often the tipping point where a $200-plus fee flips the recommendation back toward bags.
Getting a real quote
This calculator uses national-ballpark defaults. Prices vary widely by region, mix design, delivery distance, and time of year. Once you have your yardage figure, call two or three local producers for actual quotes — most will give you a firm price over the phone in a few minutes.