Calculate the number of rebar pieces, total length, and weight for slabs, footings, and walls.
Step 1: Enter the slab/footing length and width.
Step 2: Set the bar spacing (center to center). Common spacing is 12" (300 mm) for slabs.
Step 3: Select the rebar size. #4 (13M) is common for residential slabs; #5 (16M) for footings.
Step 4: Toggle two-way reinforcement for slabs. One-way is used for strip footings.
The calculator accounts for lap splices where bars need to overlap at joins.
#4 (13M) rebar at 12" (300 mm) spacing is standard for 4" residential slabs. Thicker slabs or higher loads may require #5 (16M) bars or tighter spacing.
The standard lap splice is 40 bar diameters. For #4 bar that's 20" (500 mm). Many codes specify a minimum of 24" (600 mm).
Two-way rebar runs in both directions (a grid) and is used for slabs. One-way rebar runs in one direction only, used for strip footings or walls.
Divide the slab length and width by the bar spacing, then add bars for both directions if reinforcing two ways. The calculator does this for you.
Rebar needs enough concrete around it to resist corrosion — typically 40–50 mm in foundations and about 25 mm in internal slabs. Keep bars off the ground on spacers.
