Count reinforcing bars for a slab grid: bars each way and total length, from slab dimensions and spacing. Grid layout only, not a structural design tool.
| Input | Result |
|---|---|
| 20 × 20 ft slab, 18 in spacing | 14 + 14 bars · 560 ft total |
| 30 × 12 ft slab, 12 in spacing | 13 + 31 bars · 762 ft total |
| 10 × 10 ft slab, 16 in spacing | 8 + 8 bars · 160 ft total |
It turns a slab outline and a bar spacing into a material count: how many bars run each way, and what their combined length is. That is a take-off, not a design. Bar size, grade, concrete cover, chair height and lap splices all come from the structural drawings and none of them can be derived from length, width and spacing.
The +1 in each direction trips people up. Dividing the width by the spacing counts the gaps between bars, not the bars themselves, a 20 ft width at 18 in spacing has 13 gaps but 14 bars, because there is one at each edge. Getting this wrong is a bar short on every row.
Spacing on slabs commonly runs 12, 16 or 18 inches on centre, but which one is correct depends on thickness, loads and the engineer’s design. For anything structural, work from stamped drawings and confirm with your building department.
Common slab spacings are 12, 16 or 18 inches on centre, but the correct spacing depends on the slab thickness, loads and the engineer's design. Use the spacing your plans specify, this calculator turns a spacing into a quantity, it does not choose one for you.
Because a grid with bars spaced across a width needs a bar at each edge as well. Dividing the width by the spacing gives the number of gaps, not the number of bars.
No. Bar size (#3, #4, #5 and so on), grade, cover and chair height all come from the structural design. This page only counts bars and totals their length once you know the spacing.
No. Reinforcement is engineered. For anything load-bearing, follow stamped drawings and your local code; treat this as a material take-off for a grid you have already specified.