How many rebar bars — the “+1” grid formula (subtract cover, divide by spacing, add one)
Count bars with the “+1” rule: subtract cover from both edges, divide the clear span by the spacing, then add one for the far-edge bar. A 20×20 ft slab, #4 @ 12″, 3″ cover → ⌊(240−6)÷12⌋+1 = 20 each way. Keep spacing ≤ 18″.
What this diagram shows
A slab plan showing how to count the bars in a two-way rebar grid. Edge cover is subtracted from both sides of each dimension to get the clear span, the clear span is divided by the on-center spacing, and one is added for the bar that lands on the far edge — the bar most estimating tools forget. The formula is bars equals the floor of the clear span divided by spacing, plus one. Worked for a 20-by-20-foot slab with #4 bars at 12 inches on center and 3 inches of cover: (240 minus 6) divided by 12 is 19.5, floored to 19, plus 1, equals 20 bars each way. Spacing for temperature and shrinkage steel must not exceed 18 inches per ACI 318-19.
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Rebar Calculator
Free rebar calculator: exact bar count, linear feet, weight, and stock sticks to buy for slabs, driveways, footings and round pads. ASTM A615 & ACI 318.
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