Rebar Spacing Calculator

In short Laying out reinforcement at a given spacing is counting, and the count has a trap in it that costs one bar per direction every single time. Bars at a spacing across a clear width are FENCE POSTS, NOT FENCE PANELS: the number is the clear width divided by the spacing, rounded down, PLUS ONE. Five gaps need six bars. Divide and stop there and you are short by exactly one, in every direction, on every slab. An 8 by 5 metre slab at 40 mm cover has 7.92 by 4.92 metres of clear width for the steel, and the naive division gives 84 bars where the real answer is 86 or 88 depending on what the spacing means. THE COVER COMES OFF TWICE, once at each end of every direction, which is the second thing people drop. AND A MAXIMUM AND A PITCH ARE DIFFERENT QUESTIONS WITH DIFFERENT ANSWERS. A drawing reading 150 mm normally means AT MOST 150 mm. Honour that across 4.92 metres and you need 34 bars, which land at 149.1 mm — at or under the figure. Set out at a 150 mm pitch from one edge instead and you get 33 bars whose even spread comes to 153.8 mm, which is OVER what the drawing asked for. This page reports the never-exceed count as its answer, because ordering the smaller number against a maximum is how a slab ends up under-reinforced, and it shows the pitch count alongside it: 88 bars against 86 on this slab. Laps are the third quantity, and they are steel you pay for and never see: a bar longer than the stock length has to be spliced, each splice overlapping by a multiple of the bar diameter, and on this slab that is 34 laps and 16.3 metres of extra steel — about 3% on top of what the drawing measures. Weight comes from the site rule, diameter squared over 162, which gives 0.889 kg per metre for a 12 mm bar against an exact 0.888 from the area and a density of 7,850 kg per cubic metre. THE LIMIT IS THE IMPORTANT PART. This lays out and weighs a mat from the spacing, cover, bar size and lap length you give it. It does not choose any of them, and it has no opinion on whether they are adequate. Those numbers come from your drawing or your code.