Work out how many reinforcing bars a slab needs in each direction, the total linear feet, the weight of steel and how many standard sticks to order, including lap splices and edge cover.
Enter the slab's real dimensions, then pick the spacing and bar size from your drawing. For anything structural, the drawing governs, this is a take-off tool, not a design.
People ask "how many feet of rebar" and get an answer they cannot use, because the material is bought as sticks laid in two directions. So count the pieces first: the number of bars running one way is the perpendicular span divided by the spacing, rounded up, plus one for the bar at the far end.
A 12 × 12 ft slab with a 16 in grid works out at 9 bars each way plus one, so 10 bars each way and 20 in total. Each is 12 ft long, giving 240 linear feet before any allowance.
The two adjustments that follow are not optional. Cover pulls the outermost bars in from the edge, so on anything but a tiny slab you lose one bar per direction compared with a naive count. Laps are where a run is longer than one stick: the bars have to overlap so the load transfers through the concrete, and the overlap in a residential slab is conventionally about 40 bar diameters.
In the US a bar's number is its diameter in eighths of an inch: a #4 bar is 4/8 in, or half an inch. The nominal weights come from ASTM A615.
| Bar | Diameter | Weight | Lap at 40 diameters | Typical use |
|---|---|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft | 15 in | Light flatwork, mesh replacement |
| #4 | 1/2 in | 0.668 lb/ft | 20 in | Standard residential slab, footings |
| #5 | 5/8 in | 1.043 lb/ft | 25 in | Driveways, structural slabs, footings |
| #6 | 3/4 in | 1.502 lb/ft | 30 in | Heavy foundations |
| #7 | 7/8 in | 2.044 lb/ft | 35 in | Commercial |
| #8 | 1 in | 2.670 lb/ft | 40 in | Heavy structures |
Weight climbs faster than you expect: a #5 bar is 56% heavier per foot than a #4, and a #6 is more than double. Getting the specified size right matters for the steel order as much as for the slab.
| Job | Typical bar and spacing |
|---|---|
| Patio, walkway, shed floor | #3 or #4 on 18 in centres |
| Residential slab, garage floor | #4 on 12–16 in centres |
| Driveway | #4 or #5 on 12 in centres, or a tighter grid |
| Footing, stem wall | Engineer or code specified, often continuous bars |
Tightening the grid from 18 in to 12 in roughly doubles both the bar count and the weight of steel. The grid and the bar size together are what an engineer sizes; on a drawing, follow the callout and do not improvise reinforcement on a load-bearing element.
Rebar works only where it bonds with concrete, near the surface it rusts and cracks the slab from the inside out, and lying loose on the ground it does nothing structural at all. It has to be held in the lower third of the slab on wire chairs, plastic supports or dobies, with the cover the code requires.
The cover figures that matter most in residential work are 3 in for concrete cast against and permanently exposed to earth (which is why slab-on-grade reinforcement sits on chairs rather than on the sub-base), and about 1.5–2 in for surfaces exposed to weather. In a 4 in slab that leaves little room, which is exactly why 4 in slabs get mesh or #3 rather than #5.
The other practical rule: an insufficent lap is worse than no lap, because it looks finished and fails later. Overlap about 40 bar diameters and tie the intersection.
Nominal bar diameters and unit weights are the ASTM A615 values for grade 60 deformed bar, which is the standard reinforcement for residential construction in North America. The grid-count method, span divided by spacing, rounded up, plus one bar per direction, is the standard estimating method used in rebar take-offs, and lap splices at approximately 40 bar diameters are the conventional field rule for residential work. Cover requirements follow ACI 318 (3 in against earth, less for protected surfaces). Spacing guidance follows ordinary residential practice. Footings, retaining walls and any structural slab must be designed, the spacing and bar size on your drawing override everything on this page.
Divide each span by the spacing, round up and add one bar per direction. A 12 × 12 ft slab on a 16 in grid works out at 10 bars each way, so 20 bars and about 240 linear feet. With a 10% allowance for laps and cutting waste, order roughly 264 ft, or fourteen 20 ft sticks.
For most residential flatwork #4 (1/2 in) is the standard bar, often on a 12 to 18 in grid. Patios and walkways can use #3 on 18 in centres; driveways and garage floors usually tighten to #4 or #5 on 12 in centres. Structural slabs and footings follow the drawing.
Nominal weights are 0.376 lb/ft for #3, 0.668 lb/ft for #4, 1.043 lb/ft for #5 and 1.502 lb/ft for #6. So 240 linear feet of #4 is about 160 lb of steel, and 820 ft of #4 would be roughly 548 lb.
The conventional field rule is to lap by about 40 bar diameters, roughly 20 in for a #4 bar and 25 in for a #5. Bars must overlap, never butt: the lap is what carries the load from one bar to the next through the concrete.
Yes. Reinforcement has to be encased in concrete to bond, and ACI 318 requires 3 in of cover for concrete cast against and permanently exposed to earth. Set the grid on wire chairs, plastic supports or dobies in the lower third of the slab. Rebar lying on the sub-base provides no structural benefit.
20 ft is the standard stock length, with 40 ft and 60 ft also commonly stocked. Any dimension longer than a single stick needs two bars and a splice, which is why the lap allowance matters so much on large slabs.