How this instrument works
Rebar — steel reinforcing bar — is typically laid inside a concrete slab as a two-way grid: bars running one direction, and a second set running perpendicular to them, evenly spaced across the whole slab. This instrument counts bars in each direction, sums their total length, and converts that to a weight using the standard bar-weight-per-foot figure for whichever rebar size is selected.
Bar count in each direction comes from floor(the perpendicular dimension in inches ÷ spacing) + 1, the same 'evenly spaced, one at each edge' pattern used throughout this site's layout calculators. Total linear feet sums bars-times-their-own-length across both directions, and weight multiplies that by a bar size's lb/ft figure — #3 through #8 bar weigh 0.376, 0.668, 1.043, 1.502, 2.044 and 2.670 lb/ft respectively, the standard, uncontested ASTM A615/CRSI values used industry-wide.
The grid spacing used here — a disclosed, typical 12 to 18 inch on-center range — is a commonly used residential figure, not a code-mandated or load-derived spacing pulled from a structural calculation. Actual required spacing, bar size, and whether reinforcement is even needed at all depends on the slab's intended load, soil conditions, slab thickness, and the applicable local code. This is a preliminary quantity and layout estimate only, not a structural design — always consult a licensed structural engineer or your local building department before finalizing a reinforcement plan for anything load-bearing, such as a foundation, driveway apron, or structural slab.
- Enter Slab length (ft) and Slab width (ft) — the rectangular slab's footprint dimensions.
- Enter Grid spacing (in) — a typical 12 to 18 inch on-center figure; adjust based on your slab's actual design or an engineer's specification.
- Select Rebar size — #3 through #8, each carrying its own standard lb/ft weight.
- Read Bars running the length direction and Bars running the width direction, plus Total linear feet and Total rebar weight (lb).
- For any load-bearing slab, verify spacing and bar size against a licensed structural engineer's design or your local building code before ordering material or pouring.
Worked example — a 20 × 15 ft slab on a 16 in grid
Enter 20 into Slab length (ft), 15 into Slab width (ft), 16 into Grid spacing (in), and select #4 rebar (0.668 lb/ft). Bars running the length direction, spaced across the 15 ft (180 in) width: floor(180 ÷ 16) + 1 = 11 + 1 = 12 bars, each 20 ft long, for 240 ft. Bars running the width direction, spaced across the 20 ft (240 in) length: floor(240 ÷ 16) + 1 = 15 + 1 = 16 bars, each 15 ft long, for another 240 ft.
Total linear feet reads 480.0 ft, combining both directions. At #4 rebar's standard 0.668 lb/ft, Total rebar weight (lb) reads 320.64 lb — the material quantity to price out and order, still subject to a structural engineer's actual spacing and bar-size specification for any load-bearing application.
Questions
What grid spacing should I actually use?
The 12 to 18 inch on-center range built into this calculator's default is a commonly used residential figure for light-duty slabs like patios and walkways, not a code-mandated or load-derived spacing. The correct spacing for any load-bearing slab — a foundation, garage floor, or driveway apron — depends on the specific load, soil, and slab design, and should come from a licensed structural engineer or your local building code, not a general-purpose default.
What do rebar size numbers like #3 or #4 mean?
US rebar sizes are numbered by their diameter in eighths of an inch — a #4 bar is roughly 4/8, or 1/2 inch, in diameter, while a #3 bar is about 3/8 inch. Each size carries a standard weight per linear foot (0.376 lb/ft for #3, 0.668 for #4, 1.043 for #5, and so on), set by ASTM A615 and used industry-wide regardless of who manufactures the bar.
Does this replace an engineer's structural design?
No. This is a quantity and layout estimator for a simple two-way rectangular grid — it does not analyze the slab's actual load, soil bearing capacity, or determine whether the chosen spacing and bar size are structurally adequate. For any load-bearing application, a licensed structural engineer's design (or your local building department's requirements) should govern the actual spacing and bar size used, not this calculator's typical defaults.
Why does the calculator report two different bar counts?
Because a two-way rebar grid has bars running in both directions, and each set is spaced across the OPPOSITE dimension of the slab — bars running the length direction are spaced out across the slab's width, and bars running the width direction are spaced out across its length. Reporting both separately (and their combined linear footage) reflects how a real two-way grid is actually built.
Is this only for concrete slabs, or does it work for footings too?
The same rectangular-grid math applies to any flat, rectangular reinforced concrete element — a slab, a footing, or a pad — as long as you're laying out a simple evenly-spaced two-way grid. It doesn't account for footing-specific details like edge cover, dowels, or a different top-and-bottom mat, which a footing design frequently needs and an engineer should specify.