Rebar Hook Length Calculator — ACI 318-19 §25.3
Standard hooks transfer development length without long straight extensions. ACI 318-19 §25.3 spells out the geometry; this calculator gives you total hook length per bar, including bend allowance.
Formulas
| Quantity | Formula |
|---|---|
| Standard 90° hook extension | 12·d_b past the bend |
| Standard 135° / 180° hook extension | max(6·d_b, 65 mm) past the bend (180°: 4·d_b end of bar) |
| Seismic hook (135°, on stirrups & ties) | ≥6·d_b extension |
| Inside diameter of bend, db ≤ 19 mm | 6·d_b for stirrups/ties; 4·d_b for primary |
How to use it
- Enter bar diameter d_b in mm.
- Pick hook angle: 90°, 135°, or 180°.
- Pick hook type: standard or seismic.
- Read total hook length from end of bar.
Frequently Asked Questions
Where do I use seismic vs. standard hooks?
Seismic hooks (135°, ≥6·d_b extension) are required on hoops and crossties of seismic-resisting frames per ACI 318-19 Ch. 18.
What about hooked-bar development length ℓ_dh?
Use the Rebar Development Length calculator's 'hooked' mode. The straight extension is part of ℓ_dh, not in addition to it.