Rebar Calculator
A slab grid needs a bar every so many inches in each direction, plus 10% for laps and waste. Enter your slab size and spacing to get bars, linear feet, weight, and cost.
How to estimate rebar for a slab
Work out the bars in each direction by dividing the slab dimension by your grid spacing and adding one for the closing bar. Multiply each set by the length it spans, total the two, then add 10% for the overlaps where bars are tied together and for offcuts.
Spacing and bar size
A typical residential slab uses #4 bar on a 12 to 18 inch grid. Tighter grids and larger bars (#5 and up) go into driveways, footings, and anything carrying heavy loads. Match the weight-per-foot input to your bar size for an accurate tonnage.
Pair it with concrete
Rebar sits inside the pour, so size the concrete volume with the cubic yard calculator. For masonry reinforcement, see the brick calculator.
Frequently asked
How do I calculate rebar for a slab?
Count the bars in each direction by dividing the slab dimension by the grid spacing and adding one, multiply each set by its bar length, total the linear feet, then add 10% for laps and waste.
What spacing should rebar be?
Residential slabs commonly use a 12 to 18 inch grid with #4 bar. Closer spacing and larger bars are used for driveways and heavier loads.
How much does rebar weigh?
Weight depends on bar size: #3 is 0.376 lb/ft, #4 is 0.668 lb/ft, and #5 is 1.043 lb/ft. The calculator multiplies your total length by the weight per foot.