Rebar Calculator

About this calculator

This calculator tells you how many rebar bars a concrete slab needs, how many linear feet of steel that comes to, and how much it all weighs. Use it when planning a driveway, patio, foundation, or any other flat concrete pour that requires a reinforcing grid.

How to read your results

The headline figure is the total linear feet of rebar required. Beneath it, a summary card breaks the result into bars running along the length, bars running along the width, combined bar count, weight in pounds, and weight in kilograms. The SVG grid diagram below the card draws the actual bar layout to scale so you can spot-check the spacing visually before ordering material.

How it's calculated

Bar count in each direction uses the formula: floor((span − 2 × clearance) / spacing) + 1, where span is the slab dimension in feet, clearance is the edge cover in feet, and spacing is the center-to-center bar spacing in feet. This formula is described in the Inch Calculator rebar guide. Total linear footage equals (bars along length × slab width) + (bars along width × slab length), because each set of bars must span the full perpendicular dimension. Weight is computed by multiplying total linear feet by the ASTM A615 nominal weight per linear foot for the chosen bar size, sourced from the Southern Rebar reference chart: #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft, #7 = 2.044 lb/ft, #8 = 2.670 lb/ft.

Worked example

A 20 ft by 15 ft slab, #4 bar, 1 ft spacing, 3 in (0.25 ft) edge clearance.

20 bars run along the length direction and 15 bars run along the width direction, for 35 bars total. Total rebar run is 600 ft. At 0.668 lb per foot for #4 bar, that comes to 400.8 lb (181.8 kg).

Frequently asked questions

What does "bars along length" mean versus "bars along width"?

Bars along the length direction are spaced across the slab's length dimension and run the full width of the slab. Bars along the width direction are spaced across the width dimension and run the full length. Together they form the grid.

How is the bar count calculated?

The formula is: bars = floor((span − 2 × clearance) / spacing) + 1. Clearance is the gap left at each edge before the first and last bar. This matches standard construction practice for maintaining cover at slab edges.

Which rebar sizes does this cover?

The calculator supports ASTM A615 standard sizes #3 through #8. The weight per linear foot for each size comes from the Southern Rebar ASTM A615 reference chart.

Can I use this for a non-square slab?

Yes. Enter the actual length and width separately. The calculator handles rectangular slabs; the bar counts for each direction are computed independently based on that dimension's span.

Does the optional price field affect the bar count or weight?

No. Entering a price per pound only adds an estimated material cost line to the results. All structural outputs — bar count, total length, and weight — remain unchanged.

Sources

• www.southernrebar.com/reference/rebar-weight-per-linear-foot
• www.inchcalculator.com/rebar-weight-calculator

Reviewed by the YouCalc Team · Last reviewedJune 12, 2026