Rebar Calculator
Calculate linear feet of rebar, number of bars and total weight for any concrete slab or footing.
The rebar calculator returns linear feet, bar count and weight for any concrete slab or footing. Enter the slab dimensions and your rebar grid spacing (16 inches on-center is the standard for most residential slabs, 12 inches for heavy loads, 24 inches for light pads and most footings). #4 rebar (1/2 inch diameter) is standard for slabs; bump to #5 or #6 for driveways or commercial work. The calculator adds 5% for lap splices where bars overlap (industry standard is 40 bar diameters of overlap) and counts 20 ft sticks - the most common length sold at lumberyards.
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Frequently asked questions
A 20 ft x 10 ft slab with #4 rebar at 16 in on-center needs 9 bars running lengthwise (20 ft each) and 16 bars running widthwise (10 ft each), totaling 340 linear feet. Add 5% for lap splices = 357 linear feet. That is 18 sticks of 20 ft #4 rebar weighing about 239 lbs total.
#4 rebar (1/2 inch) is standard for residential slabs, patios and walkways. #3 rebar (3/8 inch) for light-duty pads under 4 inches thick. #5 rebar (5/8 inch) for driveways and any slab carrying vehicle weight. #6 rebar (3/4 inch) for heavy commercial slabs and footings under load-bearing walls.
16 inches on-center in both directions for most residential slabs, creating a 16x16 inch grid. 12 inches OC for slabs under heavy loads, garage floors and driveways with truck traffic. 18-24 inches OC for light-duty pads, footings and shed slabs. Some engineers specify different spacing in each direction - the long direction tighter, the short direction looser.
#3 rebar: 0.376 lbs per linear foot. #4: 0.668 lbs/ft. #5: 1.043 lbs/ft. #6: 1.502 lbs/ft. A 20 ft stick of #4 rebar weighs 13.4 lbs, #5 weighs 20.9 lbs. Total weight matters when ordering - 1 ton of #4 rebar is about 1,500 linear feet, enough for a 30x30 ft slab at 16 in OC.
Yes for any slab that will carry load - patios with furniture, driveways, walkways. The minimum is #3 or #4 rebar on a 16-24 in grid placed in the middle of the slab thickness (use chairs or dobies to hold it up off the ground). Welded wire mesh is a cheaper alternative for light-duty pads but underperforms rebar over time. Skip reinforcement only on decorative pads under 2 inches thick.