Rebar Bend Deduction Calculator
How much to deduct per bend when converting out-to-out to centreline length.
With the default inputs: Bar diameter 20 · Bend angle 90° · Bends per bar 2 处 etc.Deduction per bend 40.0 mm
Edit the inputs below to recalculate.Inputs
Result
| Coefficient k | 2.00d |
| Deduction per bar | 80.0mm |
| Total deduction | 8.00m |
| Unit weight | 2.468kg/m |
| Weight equivalent | 19.7kg |
Cutting lists measure out-to-out, while the bent bar follows an arc along its centreline, so the out-to-out figure is longer and must be reduced. The coefficients above are the values used in standard practice and the 16G101 series; they depend on the bend diameter (typically 4d, 2.5d for plain bars) - larger bend diameter, larger deduction. Refer to the project drawings for settlement-grade accuracy.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Deduction = k × d | k = 0.35 / 0.50 / 0.85 / 2.00 / 2.50 by angle |
| Per bar = deduction × bends | |
| Total = per bar × number of bars | |
| Unit weight = 0.00617 × d² | kg/m, d in mm |
How to work it out
- The bend deduction is the sum of outside dimensions minus the centreline length, i.e. what must be subtracted at each bend.
- Values by angle: 0.35d at 30°, 0.50d at 45°, 0.85d at 60°, 2.00d at 90°, 2.50d at 135°.
- Deduction per bar = number of bends × the value for that angle.
- Cut length = sum of outside dimensions − total deduction.
- Total deduction = per-bar deduction × bar count, for reconciling the overall quantity.
Where it goes wrong
- Cutting to the outside dimensions — those measure the outer corner while the bar's centreline cuts the corner on an arc, so every bar comes out long.
- Using the angle written on the drawing — benders only have 30/45/60/90/135 settings, so a 40° callout is made at 45° and the deduction will not match.
- Applying this deduction to stirrups — stirrups are measured as inside perimeter plus hooks, so mixing conventions double-deducts.
Material and cost
A wrong deduction makes bars too long or too short. Too long means cutting to waste; too short scraps the bar entirely. A 6 m Φ20 bar costs about 148, and a project with thousands of bars turns a 1% error into tens of thousands.
On site
- Bend and measure the first piece; different machines and mandrel diameters shift the deduction, so table values are only a baseline.
- Bars elongate slightly when bent through cold work, especially high-grade steel — correct the cut length from the measured result.
- Retest yield strength and elongation per batch; steel that fails will crack when bent.
Basis
- GB 50666-2011 Code for construction of concrete structures, Section 5.3 (bar fabrication)
- GB 50204-2015 Code for acceptance of concrete structures, Section 5.3 (fabrication tolerances)
- 16G101-1 Standard drawings for flat-slab detailing
FAQ
Why 2d for a 90° bend?
How do I handle a 135° stirrup hook?
Does it really matter on site?
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Rebar Bend Deduction Calculator. 2026-10-25. https://jianzhujisuan.com/en/rebar-bend-deduction