Rebar Spacing Calculator

How much rebar a slab takes: enter the slab size, the spacing on center, the edge cover and the bar size, and read the bars each way, their lengths, the total feet and the weight. Counted the way RebarCalc lays the mat out.

Slab length ft in

Slab width ft in

Spacing, on center Edge cover Bar size Bar count

34 bars

889′ of #4 · 593.9 lb before laps

15 bars × 29′-6″ running the length, at 18″ o.c. across the width — edge bars 18″ in from the slab edge

19 bars × 23′-6″ running the width, at 18″ o.c. along the length — edge bars 18″ in from the slab edge

Bar lengths here are slab size less cover; a run longer than the stock length is spliced, and each splice adds a lap. RebarCalc does that splitting and puts the laps on the cut sheet.

What “on center” means

Rebar spacing is measured center to center — “#4 at 18″ o.c. each way” puts the centerline of every bar 18″ from the next in both directions, making a grid of 18″ squares. The cover is the distance from the slab edge to the nearest steel, on every edge: the bars that end at an edge stop that far short of it, and the bars that run parallel to it sit at least that far in. Everything between the covers is the band the bars are spaced across.

Bars across a width

How many bars run across a given width at a given spacing, with 3″ of cover on each side. Count = the clear band divided by the spacing, rounded down; the remainder is split to the two edges, which is why the edge bars sit a little farther in than the cover.

Bars across a slab width, 3″ cover each side (RebarCalc’s neutral count)
Width	12″ o.c.	16″ o.c.	18″ o.c.	24″ o.c.
8′	7	5	5	3
10′	9	7	6	4
12′	11	8	7	5
16′	15	11	10	7
20′	19	14	13	9
24′	23	17	15	11
30′	29	22	19	14
40′	39	29	26	19

When the band divides evenly — 24′ at 18″ leaves a 30″ remainder, but 30′ less cover at 24″ leaves 18″ — the edge bars land close to the cover. When it does not, choose: the neutral count keeps every bar at full spacing and lets the edges take up the slack; “more” adds a bar and tightens the edges; “fewer” drops one. RebarCalc offers the same three choices per direction.

Common spacings

Spacing comes off the drawings, and where a slab has an engineer the drawings say what it is. Where they do not, what is usually poured: residential slabs on grade — patios, garage and shop floors — with #3 or #4 at 18″ to 24″ each way; driveways and anything that sees a truck at 12″ to 18″; structural slabs and slabs on piers by design, commonly #4 or #5 at 12″. Tighter than 12″ is rare in a slab, and wider than 24″ stops doing much for crack control.

From count to steel

Total feet is each bar’s length times the count, both directions added; weight is total feet times the size’s pounds per foot — 889′ of #4 at 0.668 lb/ft is about 594 lb. That is the steel in the slab, not the steel to buy: runs longer than the stock length are cut into pieces that overlap by a lap, and the laps, the cut waste and the stock lengths decide the order. A cut list does that arithmetic; a spacing calculator stops here.