Estimate the rebar grid for a slab — number of bars each way, total linear feet, and weight — from your slab size, grid spacing, and bar size.
bars = floor((side_in − 2×cover) ÷ spacing) + 1 per direction
Bars are spaced evenly each way with concrete cover at the edges. A 12″ or 18″ grid is typical for residential slabs.
Weights follow ASTM A615 nominal values — a #4 bar weighs 0.668 lb per foot, a #5 weighs 1.043 lb/ft.
Rebar in the US is sized in eighths of an inch, which is why the size number maps to diameter: a #4 bar is 4/8 (half) an inch, a #5 is 5/8 inch, and so on. As the size goes up, weight per foot climbs steeply — a #5 bar is 56% heavier than a #4, and a #6 is more than double. That's why getting the specified size right matters for both strength and your steel order.
Spacing is the other lever. Residential slabs commonly use #4 bar on a 12 to 18-inch grid. Tightening the grid from 18 to 12 inches roughly doubles the number of bars and the weight of steel. The spacing and size together are usually called out on a drawing or by an engineer for anything structural — don't improvise reinforcement on a load-bearing element.
Two practical details push your real rebar order above the simple grid count. The first is lap splices: rebar comes in standard stock lengths (commonly 20 feet), so any dimension longer than a single bar requires two bars to overlap. Code typically requires a lap of around 40 bar diameters — for a #4 bar that's about 20 inches of overlap at each splice, which adds up across a large slab.
The second is concrete cover: ACI requires 3 inches of cover for concrete cast against and permanently exposed to earth, which is why slab-on-grade reinforcement sits up on chairs rather than on the ground. The cover slightly reduces how far the grid extends, but more importantly it's a durability requirement — steel too close to the surface rusts, and rusting steel cracks concrete from the inside.
For a 12 x 16 ft slab reinforced with #4 bar on a 12-inch grid and 3 inches of edge cover: along the 16 ft direction you place bars across the 12 ft width, so (12 × 12 − 6) ÷ 12 + 1 = about 12 bars, each 16 ft long. Along the 12 ft direction, (16 × 12 − 6) ÷ 12 + 1 = about 16 bars, each 12 ft long.
That's 12 × 16 + 16 × 12 = 384 linear feet before laps and waste. A #4 bar weighs 0.668 lb per foot, so roughly 256 lb of steel. Add lap splices where bars meet (typically 40 bar diameters of overlap) and a 5% cut-waste factor, and you'd order closer to 280-300 lb.
Bar diameters and per-foot weights are ASTM A615 nominal values. Lap-splice and bend allowances vary by project — confirm against your drawings.
#4 (1/2″) bar on a 12″ grid is common for residential slabs; check your local code and engineer.
Number 4 (half-inch) bar on a 12 to 18-inch grid is typical for residential slabs and driveways. Heavier loads or engineered slabs may call for #5 bar or tighter spacing — follow your drawings.
Per foot: #3 is 0.376 lb, #4 is 0.668 lb, #5 is 1.043 lb, #6 is 1.502 lb. These are ASTM A615 nominal weights. Multiply by your total linear feet to get the order weight.
A common rule is 40 times the bar diameter for a lap splice — about 20 inches for a #4 bar — but the exact requirement depends on the concrete strength and code. Check your specifications for structural work.