TradesHow this works
For a rectangular slab, the tool lays a grid of bars both directions at your spacing, counting a bar at each edge. It multiplies the bar counts by the run lengths, adds a lap splice wherever a run is longer than a 20-foot stick, applies a cut-waste allowance, and gives you the total bars, linear feet, and sticks to buy.
It also lays the common spacings side by side so you can see the tradeoff: tighter spacing means more steel and better crack control at more cost and tying labor; wider spacing saves both but allows bigger cracks. The structural drawings always govern the real size, spacing, and lap.
Common questions
How many bars do I need?
For a grid, count the bars across each span at your on-center spacing, plus one for the far edge, in both directions. This tool does that for the length and the width, then adds them up and converts to linear feet and 20-foot sticks.
What about lap splices?
When a run is longer than a 20-foot stick, the bars overlap where they join. A common planning lap is about 40 bar diameters, so roughly 1.7 feet for a #4 bar. The tool adds a lap each time a run exceeds the stock length.
What spacing should I use?
Slab-on-grade often runs #4 bar at 16 to 18 inches on center, but the right size, spacing, and lap come from the structural drawings for your project. Tighter spacing gives better crack control at more cost; wider spacing saves steel and labor.
Is this a structural design?
No. This is a material takeoff for planning and ordering. It does not design reinforcement or certify that a slab is adequate. For anything structural, follow the engineer’s drawings and your local building code.
Sources: Rebar geometry: bars each way at on-center spacing, 20 ft sticks, ~40-diameter lap splices · Common slab-on-grade spacings (12/16/18/24 in) — drawings govern the real design. Engine version 1.0.0. Risk tier 1.