Use this rebar calculator after the dimensions, spacing, cover, and bar size have been specified for the work. It counts a two-way slab grid or a wall and footing grid, totals every run, applies only the cut-and-waste allowance you enter, and reports length, theoretical stock-length equivalents, and approximate weight.
How the slab grid is counted
The calculator keeps the spacing at or below the entered maximum, then totals the clear run of every bar in both directions.
Worked example
- For a 10 × 10 slab with 3 cover, #4 bars at 12 on center, and a 5% allowance, the clear grid is 9.5 × 9.5 .
- The engine counts 11 bars each way, totals 219.45 after allowance, and reports 11 theoretical 20 equivalents weighing about 146.59 .
Rebar sizes and weights
The selected bar designation supplies the published FHWA M 22-24 unit weight. The engine multiplies that pounds-per-foot value by the takeoff length, then also reports the metric equivalent.
What this takeoff does not design
The tool does not choose reinforcement or model individual cuts, reusable offcuts, hooks, development length, or lap locations. Its stock-length result is only the aggregate takeoff divided by stock length, so use the drawings and a cutting plan to determine what to buy.
Frequently asked questions
How do I calculate rebar for a slab?
Use the specified cover to find the clear grid, divide each clear dimension by the specified spacing, round up and add one, then multiply each directional count by the perpendicular clear run. This calculator performs those steps in both directions.
How many pieces of rebar do I need?
The result divides total takeoff length after your cut-and-waste allowance by the entered stock-bar length and rounds up. Check the actual cutting and splice plan because usable offcuts determine whether that theoretical minimum can be achieved.
How much does rebar weigh?
Multiply total bar length by the published unit weight for the selected bar designation. For example, the FHWA table lists #4 at 0.668 per foot; the calculator reports both pounds and kilograms.