Rebar Hook Length Calculator
Added length for three hook types, with the seismic tail check.
Inputs
Result
Formula
| 180° hook | add 6.25d (includes a 3d tail) |
| 90° hook | tail + 0.5d |
| 135° stirrup hook | seismic max(10d, 75) + 0.5d; else max(5d, 75) + 0.5d |
| Seismic tail | at least 10d and 75 mm |
Φ8 stirrup with seismic detailing: tail max(10×8, 75) = 80 mm plus 1.9×8 = 15.2 mm of arc, so 95.2 mm per hook (the familiar 11.9d). A closed stirrup has two hooks, adding 190.4 mm.
FAQ
Why do plain bars need hooks but ribbed bars do not?
Different bond mechanics. Plain bars are smooth and rely on a 180° hook to mechanically anchor; ribbed bars develop anchorage through rib interlock, so hooks can be omitted. The hook on a plain bar is structural - make it too short and the bar pulls out at the joint.
Why both 10d and 75 mm?
They address different failure modes. 10d scales the tail with bar diameter; 75 mm is an absolute floor so that small bars (Φ6 gives only 60 mm at 10d) cannot slip out once concrete crushes. Taking the larger value means 75 mm governs for small bars and 10d for large ones.
Do hook allowance and bend deduction double-count?
No, but order matters: subtract bend deductions from the out-to-out total to get centreline length, then add the hook allowance. They act at different places - deductions at intermediate bends, allowance at the end. For a stirrup with four 90° bends and one 135° hook: out-to-out minus 4×deduction plus hook allowance.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.