Rebar Calculator
Work out how much rebar a slab or footing grid needs. Enter the slab size, bar spacing and size to get total linear feet, 20 ft stock bars and estimated weight.
Rebar needed
Assumes a grid tied in both directions, standard stock bars, and a lap-splice overlap at joins.
How to calculate rebar for a slab
A reinforcing grid runs bars in both directions, spaced evenly, tied where they cross. To find how many bars run across the width, divide the width (in inches) by the spacing and add one for the starting bar; each of those bars runs the full length of the slab. Do the same the other way: divide the length by the spacing and add one, with each of those bars running the full width. Add the two totals together for the overall linear feet of rebar, then add a percentage for lap splices where bars overlap at the ends, since very few slabs are shorter than a single stock length.
The formula is linear feet = (bars across width × slab length) + (bars across length × slab width), then scaled by your waste percentage. The calculator above does this automatically and also converts the total to 20 ft stock bars and an estimated weight for the bar size you choose.
Choosing a grid spacing
Spacing is driven by the load the slab carries and local code, not a single universal number. A 12 inch grid is a common, conservative default for driveways and slabs that will see vehicle loads. Patios, walkways and light-duty slabs are often reinforced on a 16 to 18 inch grid. Footings and foundation work usually follow an engineer's spec rather than a rule of thumb. When in doubt, check with your local building department or a structural engineer before finalizing the spacing.
Bar size and weight
Rebar is sized by its diameter in eighths of an inch: a #4 bar is 4/8 (1/2) inch and weighs about 0.668 lb per foot, a #3 bar is 3/8 inch at 0.376 lb per foot, and a #5 bar is 5/8 inch at 1.043 lb per foot. #4 is the most common size for residential slabs. Multiply the total linear feet by the weight per foot to estimate how much steel to order, which is also how many suppliers price it.
Waste and stock lengths
Rebar ships in 20 ft stock lengths (10 and 40 ft are also available from some suppliers). Since a cut length still consumes a whole stick unless the offcut is reused elsewhere, round up to whole bars. A 10 percent allowance covers lap splices, where overlapping bars at a joint need roughly 12 to 18 bar diameters of overlap, plus the odd cutting mistake. Once the grid is in place, size the concrete itself with the concrete calculator.