BS 8666 Shape Code 51 Explained: The Standard Stirrup, Formula and Worked Example

Shape code 51 is probably the single most-used shape in any bar bending schedule — it's the standard closed stirrup, and almost every beam and column on a project needs dozens of them. This guide covers exactly what shape 51 is, its cutting-length formula, a worked example, and the mistakes that most often trip people up when scheduling it.

What shape code 51 is

BS 8666:2020 shape code 51

Shape 51 is a closed rectangular link: the bar runs around a rectangle defined by dimensions A and B, then finishes with a short diagonal tail (dimension C) that overlaps back across a corner rather than butting the two ends together. That overlap is what makes it a genuinely closed link — structurally, it needs to hold the main bars in place and resist shear even if one leg is knocked, so the ends have to properly engage rather than just meet.

The formula

Here, A and B are the two leg dimensions of the rectangle, C is the end projection (tail) length, r is the bend radius for the bar size and grade in use, and d is the bar diameter. The −2.5r−5d term is the bend deduction — it accounts for the four 90° corners of the rectangle plus the bend into the tail, each of which shortens the bar's true length compared to simply summing the straight legs.

Working out A, B and C

• A and B are the beam or column's width and depth, less the concrete cover on each side — the stirrup sits just inside the cover zone, not flush with the outer face of the concrete, so both dimensions are reduced by twice the cover from the member's overall size.
• C is the end projection or "tail" of the stirrup beyond the last bend. BS 8666:2020 sets a minimum end projection for links — commonly the greater of 10d or 90mm for a typical stirrup hook angle — so C needs to meet or exceed that minimum for the bar size in use.

Worked example

A 300mm × 500mm beam, 25mm cover, 10mm stirrup bar.

1. Sum the legs. 2(A + B + C) = 2(450 + 250 + 100) = 1,600mm.
2. Apply the deduction. Subtract 2.5r + 5d = (2.5 × 20) + (5 × 10) = 50 + 50 = 100mm.
3. Cutting length. 1,600 − 100 = 1,500mm per stirrup.
4. Total length and weight. Multiply by the number of stirrups on the beam, then apply the weight-per-metre for a 10mm bar (0.617 kg/m) to get the total steel weight for that bar mark.

This matches the same logic used in our BBS for a beam guide — shape 51 is the shape behind every stirrup bar mark in that worked example.

Common mistakes with shape 51

• Using the outer beam/column dimensions instead of the cover-reduced ones. Forgetting to subtract cover on both sides of A and B produces a stirrup that's too large for the section.
• Using too short an end projection. A tail shorter than the minimum for the bar size doesn't give the stirrup enough anchorage, and won't pass inspection even if the rest of the schedule is correct.
• Applying the wrong bend radius for the steel grade. The minimum radius depends on both bar size and grade — see our steel grades guide — so using a generic radius across different grades can under- or over-state the deduction.
• Confusing shape 51 with a similar closed-link variant. Codes like 52 and 56 look similar on a small sketch but use different corner or tail details — check against the full shape codes guide before assuming 51 is the right one.

Frequently asked questions

What is BS 8666 shape code 51?

Shape code 51 is the standard closed rectangular link (stirrup) in BS 8666:2020 — a bar bent into a rectangle with a short overlapping tail, used in beams and columns to resist shear and hold the main reinforcing bars in position.

What is the formula for shape code 51?

The BS 8666:2020 formula for shape code 51 is L = 2(A + B + C) − 2.5r − 5d, where A and B are the two rectangle leg dimensions, C is the end projection (tail) length, r is the bend radius, and d is the bar diameter.

How do you work out dimensions A and B for a shape 51 stirrup?

Dimensions A and B for a shape 51 stirrup are the beam or column's width and depth, less the concrete cover on each side, since the stirrup sits just inside the cover zone rather than at the outer face of the concrete.

This is an illustrative worked example, not a substitute for a checked, engineer-approved bending schedule. Always take cover, end projection and bend radius from the actual structural design and BS 8666:2020 for the bar size and grade in use.