Rebar Calculator

How to Count Rebar for a Concrete Slab

Count the bars running in each direction, then add those two counts. Multiplying them gives the number of grid intersections, not the number of bars. A rebar calculator for slab quantities also needs the specified spacing and edge positions; square footage alone does not define the grid.

The extra one includes a bar at the other outer centerline. This model distributes the intervals evenly between both boundaries, at or below the maximum spacing you enter. It counts a rectangular grid; it does not approve a reinforcement layout or choose the spacing.

Example: one 20 × 10 ft mat

Suppose the drawing specifies an outer-bar centerline offset of 3¼ in, 18 in maximum spacing both ways and 3 in clearance at straight bar ends. The count is 8 lengthwise runs and 14 widthwise runs. Their straight cut lengths are 19.5 ft and 9.5 ft, giving 8 × 19.5 + 14 × 9.5 = 289 linear ft before any laps.

Two identical mats double those counts and lengths. If the top and bottom mats have different spacing or bar sizes, calculate them separately. The example measurements are supplied inputs, not recommendations for a slab.

Centerline Offset, End Clearance and Lap Length

Clear cover measures to the steel surface. Grid spacing measures between bar centers. The edge-to-centerline input therefore differs from the clearance at a straight bar's end; copy the appropriate dimensions from the placing drawing rather than using one cover number for both automatically.

Straight cut length is the slab dimension minus the clearance at both ends. Hooks, bends, thickened edges, openings and additional reinforcing around penetrations need their own bar schedule. This rectangular-grid model does not subtract an opening and assume the remaining reinforcement is sufficient.

For specified equal-length lap splices, add number of splices × overlap length to the straight total. Ten specified 2 ft overlaps add 20 ft of steel; this arithmetic does not mean a 2 ft lap is suitable for any particular bar. Splice locations and lengths come from the project drawings.

Why total length is not a stock-bar order

Ten 12 ft straight pieces contain 120 linear ft. Dividing by a 20 ft stock length suggests six bars, but without permitted splices each 20 ft bar produces only one 12 ft piece, so that cutting arrangement needs ten stock bars. This calculator reports bar runs, scheduled pieces when laps are entered, and steel length. A fabricator's cutting schedule determines stock purchases and offcut reuse.

Rebar Weight from the Calculated Length

The rebar weight calculator option multiplies the steel length, including entered overlaps, by the selected nominal weight per length. Use a custom supplier value for another bar system or product. A metric designation is not automatically interchangeable with a rounded US diameter.

For the 289 ft example, selecting US #4 gives 289 × 0.668 = 193.05 lb of nominal steel weight before laps. Purchase offcuts, tie wire, chairs and accessories are outside that weight. Use the concrete calculator separately for the slab's concrete volume; this steel rebar calculator does not infer concrete thickness or reinforcement requirements.