Rebar Calculator

Enter the slab length and width, choose the bar size, select a standard or custom spacing, and provide the lap allowance and waste percentage from your plans. The result separates the bars along each direction, total grid bars, footage, 20-ft sticks, weight, and your editable cost input.

The spacing helper shows how the same slab changes at 12, 18, and 24 inches. It is a quantity comparison, not a structural prescription. Confirm the design before ordering.

Tighter spacing puts more steel into the slab and increases both weight and purchase length. Twelve-inch spacing is a denser layout often considered for heavier loading; 18 inches is a common planning comparison for residential work; 24 inches may suit lighter applications only when the design allows it.

Bars per direction use floor(dimension in inches ÷ spacing) + 1, so the grid has a bar at each boundary. Edge cover, chairs, openings, thickened edges, and construction joints can change the field layout.

#3, #4, and #5 identify nominal bar size, not the spacing. The calculator uses standard planning weights of 0.376, 0.668, and 1.043 lb per linear foot. Heavier bar raises weight and cost, but selecting it should follow the plan, soil, loads, and local practice rather than a generic rule.

A 4-in slab may use #3 or #4 in some residential designs, while driveways and heavier loads may call for different reinforcement. Treat that guidance as TBD until confirmed against the structural requirements for the job.

For a 20×20 slab at 18-in spacing, the helper shows 14 bars along each direction. With a 24-in lap input and 5% waste, the calculator produces 616 ft before waste, 647 ft after waste, about 432 lb of #4 steel, and 33 twenty-ft sticks.

A 10×10 or 30×40 slab follows the same logic: calculate each direction separately, add laps, add waste, then convert to weight and sticks. Use the helper before deciding whether a tighter grid materially changes the order.

Short bars may need overlap when a run is longer than the stock length. This tool applies the entered lap length once per grid bar as a planning allowance, then adds the selected waste percentage. A supplier may cut or bundle differently, so compare total weight and stick count with the quote.

Support the reinforcement on chairs or dobies so it remains in the middle third of the slab during placement. Steel lying on the subgrade does not provide the intended placement.

Steel pricing can be quoted by pound, piece, bundle, or 20-ft stick. This page leaves the price editable and marked TBD because local supplier, freight, and market conditions move quickly. Ask whether chairs, tie wire, cuts, delivery, and tax are included.

A weight-based comparison is usually cleaner than comparing only stick prices when bar sizes or laps change.

The calculator models one rectangular mat with boundary bars in both directions, entered lap footage, standard bar weights, and waste. It does not design reinforcement, account for openings, hooks, dowels, beams, thickened edges, or engineering requirements. Confirm the final layout with the project plans.