# Rebar Hook Length Calculator

Added length for three hook types, with the seismic tail check.

- Page: https://jianzhujisuan.com/en/rebar-hook-length
- Category: Rebar (https://jianzhujisuan.com/en/c/rebar)
- Chinese version: https://jianzhujisuan.com/zh/rebar-hook-length.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Bar diameter 8 mm, Hook type 135° stirrup hook, Seismic detailing Seismic (tail ≥10d and ≥75 mm), Straight tail for 90° hook 80 mm): **Added length per hook 95.2 mm**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Bar diameter | number | 8 | mm |  |
| Hook type | select | 135° stirrup hook | — |  |
| Seismic detailing | select | Seismic (tail ≥10d and ≥75 mm) | — |  |
| Straight tail for 90° hook | number | 80 | mm |  |

## 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

## Worked example

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.

## Questions

### 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.

## Related tools

- [Rebar Weight Calculator](https://jianzhujisuan.com/en/rebar-weight): Pick a bar diameter, enter length and quantity to get weight per metre, per bar and total tonnage.
- [Rebar Anchorage Length Calculator](https://jianzhujisuan.com/en/rebar-anchorage): Basic and seismic anchorage length from bar grade, diameter and concrete class.
- [Rebar Lap Length Calculator](https://jianzhujisuan.com/en/rebar-lap): Lap length ll / llE by splice percentage, plus the permitted number of splices.
- [Rebar Cut Length Calculator](https://jianzhujisuan.com/en/rebar-cut-length): Cut length and bar weight with standard hook allowances.
- [Rebar Spacing and Bar Count Calculator](https://jianzhujisuan.com/en/rebar-spacing): Bars and actual spacing from run length and design spacing, with total weight.
- [Stirrup Length and Count Calculator](https://jianzhujisuan.com/en/rebar-stirrup): Stirrup length, count and weight with the 11.9d seismic hook allowance.

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