Counting a grid is arithmetic. What people leave out is lap splice — every joint in a run overlaps by dozens of bar diameters, and it is real steel you have to buy.
Cover is subtracted from every end. Steel closer than the specified cover to the surface rusts, spalls the concrete off and stops working.
Footings usually run continuous bars the long way with stirrups or transverse bars at intervals; a slab is a true grid both ways.
Rebar comes in stock lengths. Any run longer than a stock bar needs a splice, and a splice is not a butt joint — the two bars overlap so the load transfers through the concrete between them.
The overlap is expressed in bar diameters. At the common 40d figure, a #4 bar laps 20″, a #5 laps 25″ and a #6 laps 30″. On a slab with a lot of long runs that is a meaningful percentage of the total steel, and it is the line most calculators skip entirely.
Class B splices at 60d are required in some conditions, which raises those figures by half again.
Bar number is the diameter in eighths of an inch: a #4 is 4/8 = ½″, a #8 is one inch. Weight follows from that, and matters because rebar is frequently priced by weight even when ordered by the foot.
| Bar | Diameter | lb / ft | Lap at 40d |
|---|---|---|---|
| #3 | 3/8″ | 0.376 | 15″ |
| #4 | 1/2″ | 0.668 | 20″ |
| #5 | 5/8″ | 1.043 | 25″ |
| #6 | 3/4″ | 1.502 | 30″ |
| #7 | 7/8″ | 2.044 | 35″ |
Cover is the concrete between the steel and the surface, and it is what stops the bar rusting. Rust expands, and expanding steel spalls concrete off in sheets.
Three inches is the usual figure for concrete cast against earth, two inches for formed surfaces exposed to weather, and less for interior work. Cover comes off every end of every bar, which is why the steel is always shorter than the slab.
It also has to be maintained during the pour. Steel lying on the subgrade is doing nothing at all — chairs or bolsters are not optional.
Ties hold the grid in position while the concrete goes in; they carry no structural load. Every intersection is common on small residential slabs. On bigger pours every second or third intersection, staggered, is normal.
A pound of 16 gauge tie wire gives roughly 100 to 120 ties.
Welded wire mesh is common in light slabs, but it is thin, easy to walk flat during the pour, and mostly controls cracking rather than adding capacity. Where drawings call for bar, use bar.
Generally yes. Splicing every bar in a run at the same station creates a weak plane. Stagger them by at least a lap length.
Corner bars are usually specified as bent bars or separate corner pieces lapping into both runs, not two straight bars butting. That detail comes from the drawings.