About this tool
The Rebar & Tie Wire Calculator works out how much reinforcing steel a rectangular slab or footing needs when it is gridded with rebar at a set spacing. Enter the slab length and width in feet, the grid spacing in inches on-centre, the bar size, your concrete cover, the stock bar length you buy and how often you tie the intersections. It returns the bar count, total linear feet, weight and tie wire — all computed locally in your browser.
For a slab L × W with spacing s and cover c (all worked in inches), the number of bars running lengthwise is floor((W×12 − 2c) / s) + 1 and each is (L×12 − 2c) long; the number running widthwise is floor((L×12 − 2c) / s) + 1 and each is (W×12 − 2c) long. Total linear feet sums both directions. When a single bar is longer than your stock length, lap splices are added at 40 × bar diameter each (a #4 bar is 0.5", so a 20" lap), and the number of bars to buy is the total linear feet divided by stock length, rounded up.
Every grid intersection can be tied, or every other intersection to save wire — the tool counts intersections as bars-lengthwise × bars-widthwise and applies your choice. Tie wire assumes about 8" per tie plus 10% waste. Steel weight uses standard unit weights: #3 = 0.376, #4 = 0.668, #5 = 1.043 and #6 = 1.502 lb per foot, so you can order by weight as well as by length.