Bars overlap where they join, and that steel is used twice. Calculators that divide the area by the spacing miss the laps entirely and leave you short — usually on the last bar of the last row.
Enter the slab, the spacing and your bar length. We'll count both directions, add the laps, and tell you how many bars to order and how many intersections you'll be tying.
Works in metric and US imperial — you choose at the start and can switch at any time. Every figure this calculator uses is published with its source, and the result comes with a printable order sheet. No account, no email, and nothing you type leaves your browser.
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Bars overlap where they join, and that overlapping length is steel doing one job while being paid for twice. Calculators that simply divide the area by the spacing miss the laps entirely and leave you short — usually on the last bar of the last row, which is a trip to the yard for one piece. This page counts both directions, adds the laps, and tells you how many intersections you will be tying.
Bar chairs are not optional: steel lying on the ground does nothing at all, because it has to sit at the right depth within the slab to be in the part that goes into tension. Allow roughly four chairs per square metre. Cover matters as much as spacing — steel too close to the surface rusts, expands and spalls the concrete off the face, which is a repair far more expensive than the chairs would have been.
Stagger the laps: all the joints landing in one line creates a weak plane through the slab, so offset them between adjacent bars. And consider whether you need loose bar at all — for a domestic slab, welded mesh sheets are often faster, cheaper and much harder to get wrong. Ask what your supplier stocks before committing. Either way, order a couple of spare bars, because cutting rebar accurately on site is harder than it looks and a short bar cannot be lengthened.
Yes, and most calculators do not. Bars overlap where they join, so the total steel is more than the area divided by the spacing. Missing the laps typically leaves you one bar short at the end of the job.
Yes. Steel lying on the sub-base does nothing — it has to sit at the correct depth inside the slab to work. Allow roughly four chairs per square metre.
For a domestic slab, welded mesh sheets are often faster, cheaper and considerably harder to get wrong than loose bar. Ask your supplier what they stock before deciding.
Because steel too close to the surface rusts, and rusting steel expands and spalls the concrete off the face. Cover protects the bar, and it matters as much as getting the spacing right.