Stirrup Length and Count Calculator
Stirrup length, count and weight with the 11.9d seismic hook allowance.
Inputs
Result
Formula
| Outer dimensions | b' = b − 2c, h' = h − 2c |
| Perimeter | U = 2 × (b' + h') |
| Length per stirrup | L = U + 2 × 11.9d (seismic 135° hooks) |
| Count | n = floor(member length ÷ spacing) + 1 |
| Unit weight | 0.00617 × d² kg/m |
A 300 × 600 beam with 25 mm cover, Φ8 stirrups at 200 mm over 6000 mm: 1600 mm perimeter, 1790 mm per stirrup, 31 stirrups, 55.5 m total and 21.9 kg.
FAQ
Where does 11.9d come from?
The developed length of a 135° hook is a 10d straight tail plus the arc around a 2.5d bend, less the bend deduction — 11.9d in total. The 10d tail is mandatory in seismic detailing; non-seismic work uses 5d, hence the 5.5d figure. Short hooks are a classic earthquake failure.
How do confined and non-confined zones differ?
They are calculated separately. Beam confinement extends 2h and at least 500 mm for class I, at min(h/4, 8d, 100 mm) spacing, with non-confined spacing typically double. Column confinement takes the greater of clear height/6, the long section dimension and 500 mm. This calculator uses one spacing throughout.
Can stirrup diameter be chosen freely?
No. The code requires at least d/4 of the largest longitudinal bar and not less than 6 mm, and 8 mm where the beam is wider than 800 mm, plus a minimum shear reinforcement ratio — seismic volumetric ratios are tabulated by axial load ratio and class. Undersized stirrups cause shear failure before flexural yield, which is brittle.
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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.