Spacing is on-center, cover comes off twice
A 12 in grid in a 20 ft bay is not the same count as a 16 in grid. Subtract cover from both edges before you divide by spacing, then add one bar.
Laps belong on the schedule. If you do not have one, keep lap as an explicit allowance rather than a hidden extra 10%.
Weight is a check, not a design
Bar size changes kg/m or lb/ft. Do not mix imperial bar numbers with metric weights on the same line. This grid is still not a PE stamp.
Do it in order
- Enter slab length and width and the cover you will actually pour to.
- Set long and transverse spacing independently.
- Keep lap visible, then carry weight into the estimate as its own line.
Worked example — 20 ft × 12 ft slab, 12 in O.C., 2 in cover
Clear span in the long direction is 20 ft minus 4 in of cover. At 12 in O.C. that is a 19-bar count in that direction before lap. Repeat for the 12 ft side. Add lap only if the schedule says so.
Planning boundary
This is a planning grid. It does not set bar size, development length or a stamped drawing.
Does this replace a structural schedule?
No. Bar size, spacing and cover come from the responsible professional.
Can I use mesh instead?
Wire mesh is a different calculator. Do not convert a grid into a roll by eye.
What about a thickened edge?
That is extra bars on a footing or edge line, not a fatter slab grid.