Calculate bars, total length, weight and cost for a concrete slab rebar grid.
Total bars
40
Weight (lb)
534.4 lb
Calculator
Total rebar length
800 ft
Total bars
40
Weight (lb)
534.4 lb
Weight (kg)
242.4 kg
Bars along length
20
Bars along width
20
Total length (ft)
800 ft
Weight (lb)
534.4 lb
Weight (kg)
242.4 kg
Slab grid diagram
How to calculate rebar for a slab
First determine the bar count in each direction: divide the slab span minus twice the edge clearance by the bar spacing, take the floor, and add one. Bars running across the length span the full width; bars running across the width span the full length. Add both sets to get the total bar count.
Multiply the bar count in each direction by the corresponding slab dimension to get the total linear feet of rebar. Then multiply that by the ASTM A615 weight factor for your bar size — for example, #4 bar weighs 0.668 lb per foot. Divide pounds by 2.205 to convert to kilograms.
What bar size should I use for a slab?
#4 (half-inch) bar is the most common choice for residential slabs. #3 suits light-duty flatwork; #5 and above are used for heavier structural slabs or walls. Always consult your local building code.
What spacing and clearance are typical?
Residential slabs commonly use 12-inch (1 ft) spacing with 1.5-inch (0.125 ft) edge clearance on each side. Closer spacing (6–8 inches) is used for driveways or higher loads. Check the structural drawings for your project.
How do I convert the weight to tonnes?
Divide pounds by 2,204.6 for metric tonnes, or divide by 2,000 for short tons. The calculator shows both pounds and kilograms so you can work with your supplier's pricing unit.
Results are estimates. Verify with a professional for important decisions.
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.
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.
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.
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