Guide
Thickened slab edges
The slab calculator assumes one thickness. A garage pad or a patio that turns down at the walls is two thicknesses. If you enter only the thin field, the yardage is short. If you enter the thick edge for the whole rectangle, the yardage is high.
Field plus band
Run the slab calculator on the full length and width at the field thickness. That is the thin part. Then run the footing calculator on the extra depth around the edge: the length of the band, the width of the thickening, and only the inches that sit below the field. Add the two yardages.
Example talk, not a design: a 20×20 slab at 4 inches is the field. If a 12-inch-wide band drops another 8 inches along two walls, that band is a trench on those two walls, not a second 20×20 at 12 inches.
Do not average the depths
Averaging 4 inches and 12 inches “to be safe” treats the whole pad as 8 inches. That invents concrete in the middle that is not there, and it can still miss a long thickened wall. Two honest rectangles beat one fuzzy average.
Corners on a closed loop get counted twice if you add every wall at full length. Measure the centerline of the band once, or subtract the overlaps. The footing page says the same thing.
What this is not
A thickened edge can be a frost or bearing detail. Depth, width, and steel are not quantity questions. This page only keeps you from using the wrong thickness in the wrong area. Confirm the detail on the plan or with the office that has authority over the lot.
Pour Plan does not size a frost wall. Add the two volumes, then treat the sum as a planning figure, not a mix order.