The hook is the extra length left at the 4‌th corner of a stirrup so that the stirrup retains its shape. Generally, hook length is taken as 9d for one side. Where d = Diameter of the bar. The total length of stirrups = Total length of the bar + 2 × hook length (for two hooks) = L + 2 × 9d = L + 18d. Where L = length of the bar for stirrup.