Bar Bending Schedule Calculator

A bar bending schedule — a BBS — is the reinforcement equivalent of a cutting list. Before a single bar is cut on site, the schedule sets out, for every bar mark on the drawing, the bar diameter, the shape it is bent to, the dimensions of each leg, the cut length, the number of bars, and the total weight. It is the bridge between the structural drawing (which shows where steel goes) and the steel yard (which needs a list of straight lengths to cut and bend).

The single most important number the schedule produces is the cutting length: how long each straight bar must be before it is bent, so that after bending it fits the member with the correct cover and anchorage. Get the cutting length wrong and every bar of that mark is either short (scrap, or an unsafe lap) or long (wasted steel that still gets billed). Multiply a small per-bar error across the thousands of bars in a house and the money is real.

Steel is not paper. When a bar is bent around a pin, the material on the outside of the bend stretches and the material on the inside compresses. The bar follows a curve at the corner rather than a sharp point, so the true length of steel needed is less than the sum of the leg dimensions you measure to the outside faces of the member.

IS 2502 accounts for this with a fixed bend deduction per bend, expressed as a multiple of the bar diameter (d):

• 45° bend → deduct 1d
• 90° bend → deduct 2d
• 135° bend → deduct 3d

The sharper the bend, the more the bar “short-cuts” the corner, so the larger the deduction. You add up the leg dimensions, subtract one deduction for every bend in the bar, add back the hook allowances, and that is your cutting length. This tool does exactly that — the breakdown panel shows each term so you can trace where the number comes from.

Bars that need to be anchored — stirrup ends, the free ends of many main bars — finish in a hook. IS 2502 gives a standard hook allowance per hook of the greater of 9d or 75 mm. For a small bar the 75 mm floor governs; for 10 mm and above the 9d term takes over. The hook adds length, so it is added to the cutting length (the bend at the hook itself is the 135° deduction that goes the other way).

In seismic zones III to V, IS 13920:2016 requires stirrups and ties to close with 135° hooks rather than 90° hooks, because a 90° hook can open up and lose its grip when the concrete cover spalls during an earthquake. That is the only difference between shape 07 and shape 08 in the library above — the geometry is identical; the detailing standard is stricter.

• Forgetting the bend deduction entirely. The most frequent mistake — adding the legs and stopping there. On a rectangular stirrup with five bends that alone over-states the length by several diameters, and the error repeats on every stirrup in every column and beam.
• Applying the wrong deduction for the angle. Treating a 135° seismic hook as if it were a 90° bend (2d instead of 3d), or vice-versa, is easy to do by hand and silently wrong.
• Using 9d for the hook when 75 mm governs. For 6 and 8 mm stirrup steel, 9d is only 54–72 mm, but the code floor is 75 mm. Hand calculations that always use 9d under-cut small-diameter hooks.
• Missing the crank addition. A 45° crank adds 0.42 × the crank depth for the inclined portion. Bars cranked at a support are routinely scheduled as if they were straight, and come up short.
• Wrong unit weight. The d²/162 rule (kg per metre) is simple, but rounding 8 mm to “0.4” or 12 mm to “0.9” instead of the true 0.395 and 0.888 quietly shifts every weight — and steel is billed by weight.

A bar is a cylinder of steel. Its weight per metre is its cross-sectional area times the density of steel (7850 kg/m³ per IS 1786:2008). Working the area through in millimetres and simplifying the constants collapses to the well-known site rule:

unit weight (kg/m) = d² ÷ 162

So an 8 mm bar is 64 ÷ 162 = 0.395 kg/m and a 12 mm bar is 144 ÷ 162 = 0.888 kg/m. Multiply the cutting length in metres by this figure and you have the weight of one bar; multiply by the bar count for the mark and you have the schedule weight the yard will invoice.

Take shape 07, a rectangular stirrup in 8 mm steel with legs A = 180 mm and B = 400 mm (centre-line dimensions). The perimeter is 2 × (180 + 400) = 1160 mm. It carries two hooks, each the greater of 9 × 8 = 72 mm or 75 mm, so 2 × 75 = 150 mm. Its bends are three 90° corners (3 × 2d = 48 mm) and two 135° hooks (2 × 3d = 48 mm), a total deduction of 96 mm.

1160 + 150 − 96 = 1214 mm → 1.214 m × 0.395 kg/m = 0.4796 kg

That is exactly what the calculator returns when you load shape 07 with those inputs — the default values above. Change the diameter to a seismic 500D bar or the shape to 08 and you can watch each term in the breakdown move.