Lay out a rebar grid on paper before you’re standing in the dirt: bars each direction, total footage with lap splice allowance, and how many 20-ft sticks to throw on the trailer.
Slab & grid
ft
ft
in
Total rebar (with 10% laps/waste)
—
Bars running the length—
Bars running the width—
20-ft sticks to buy—
Chairs/supports (~1 per 4 sq ft)—
Overlap splices where bars meet end-to-end: 24 in is a safe field lap for #4 bar (spec is typically 40× bar diameter). Keep bars in the middle of the slab on chairs — rebar lying on the dirt does nothing.
How a rebar grid is counted
Bars running the slab's length are spaced across its width, and vice versa. Take the dimension, subtract the edge clearance on both sides, divide by the spacing, add one for the starting bar:
bars = floor((span − 2 × inset) ÷ spacing) + 1 · total ft = (bars × their length) both ways × 1.10
Worked example
A 20 × 12 ft slab on a 16-in grid with 3-in edges: 9 bars run the length (9 × 20 = 180 ft) and 15 run the width (15 × 12 = 180 ft). With 10% for laps and cuts, that's about 396 linear ft — 20 sticks of 20-ft bar, plus roughly 60 chairs to hold it mid-slab.
Rebar vs. wire mesh
For 4-in residential flatwork, #3 or #4 bar on 16–24 in centers or 6×6 wire mesh are both common; bar wins where soil is questionable or loads are real (driveways, shop floors). Whichever you use, the steel belongs in the middle third of the slab thickness — laid on the ground, it's just expensive gravel.
Frequently asked questions
What spacing should rebar be in a concrete slab?
Residential slabs commonly run #3 or #4 bar at 16 to 24 inches on center each way; driveways and loaded slabs tighten to 12–16 inches. Local code and soil conditions rule, so verify with your inspector.
How much should rebar overlap at splices?
The typical spec is 40 bar diameters — about 20 inches for #4 (1/2") bar, so 24 inches is a safe field number. Tie laps in at least two places so they don’t drift during the pour.
Does rebar need to be lifted off the ground?
Yes — steel only works when it’s embedded in the concrete, ideally the middle third of the slab. Use chairs or dobies every few feet; pulling the grid up with a rake hook mid-pour is unreliable at best.