Question 1

Why is the cutting length of a bent bar shorter than its outer dimensions?

Accepted Answer

When a bar is bent, steel on the inside of the bend gets compressed and steel on the outside gets stretched, but the bar's actual center-line (the neutral axis that doesn't change length during bending) is shorter than the sum of the outer straight measurements you'd get by simply adding up the legs of the shape as drawn. If you cut a bar to the exact sum of the outer leg lengths and then bend it, the finished shape comes out larger than intended because the bend consumes less material along the neutral axis than the outer corner-to-corner measurement suggests. The bend deduction is the correction subtracted from the sum of outer leg lengths to get the actual straight-bar cutting length needed before bending, so the finished bent shape matches the drawing dimensions.

Question 2

What bend deduction factor should I use for different bend angles?

Accepted Answer

The deduction scales with how sharp the bend is, expressed as a multiple of the bar diameter (d): a 45° bend commonly uses a deduction of 1d, a 90° bend uses 2d, and a 135° bend (or sharper, including the bend used at most stirrup corners) uses 3d. These factors are subtracted once for every individual bend in the shape — a rectangular stirrup with four 90° corner bends deducts 4 × 2d = 8d in total from the sum of its four outer leg lengths (plus the center-line correction for wrapping around the bar diameter itself). Different national standards and detailing references publish slightly different exact multipliers, so always confirm the factor your project specification or local code expects before finalizing a schedule for structural work.

Question 3

What is a hook, and how much extra length does it add?

Accepted Answer

A hook is a short bent extension at the end of a bar — typically a 90°, 135°, or semi-circular 180° bend — added purely for anchorage, most commonly at the ends of stirrups and at the free ends of main bars in some detailing conditions, to grip the surrounding concrete and resist the bar pulling straight under load. Hook length is expressed as a multiple of bar diameter, commonly in the range of 6d to 9d beyond the bend depending on the hook angle and the specification referenced (a tighter 135° seismic hook typically needs a longer extension than a plain 90° hook). A stirrup with a hook at each end adds two hook allowances to its total cutting length — using a 9d hook factor on each end of a rectangular stirrup adds 18d in total, which is a meaningful fraction of a small stirrup's total length and is easy to forget when estimating by eye.

Question 4

How is lap length different from bend deduction, and when do I need it?

Accepted Answer

Lap length and bend deduction solve two unrelated problems. Bend deduction corrects for the geometry of bending a single continuous bar. Lap length is the extra straight length added when two separate bar pieces are overlapped and tied together to act as one continuous bar — needed whenever a required bar run is longer than the stock length available, or wherever a construction joint interrupts a continuous reinforcement run. Lap length is commonly specified as a multiple of bar diameter (a frequently used general figure is around 40d to 50d for tension laps in normal-strength concrete, though the exact multiplier depends on concrete grade, bar stress condition, and whether the lap sits in a tension or compression zone), and must be added once per lap location — not once per bar — since only the bars that actually need splicing carry the extra length.

Question 5

How is the weight of a bar calculated once the cutting length is known?

Accepted Answer

The standard approximate formula for the unit weight of a steel reinforcement bar is d² ÷ 162 kilograms per meter, where d is the bar diameter in millimeters — this comes from the bar's cross-sectional area combined with steel's typical density, simplified into a single constant that's accurate enough for quantity takeoff. Multiplying this unit weight by a bar's total cutting length gives that bar's individual weight, and multiplying by the total number of identical bars (count per member × number of members) gives the total weight for that schedule row. Summing every row's total weight, usually grouped by diameter, gives the grand total steel weight for the BBS — this is the number used for ordering and for cost estimation against a rate per kilogram or per tonne.

Question 6

Why does a BBS reduce wastage compared to ordering steel by rough estimate?

Accepted Answer

Ordering steel by a rough overall estimate (like a flat percentage of concrete volume) has no way to account for the actual mix of bar diameters, shapes, and cut lengths on a real job — it tends to over-order some diameters and under-order others, and gives no guidance to the site team on how to actually cut the delivered stock length bars with minimal offcut waste. A proper BBS lists every bar's exact cutting length and quantity, which lets the site team plan cutting patterns against standard stock lengths (commonly 12m bars) to minimize offcut, and lets procurement order close to the actual requirement per diameter rather than a single blended estimate. This is also why a BBS is normally prepared per structural element (or per floor) rather than for a whole building at once — it keeps the schedule a manageable, checkable size while still capturing the real bar-by-bar detail.

Question 7

Do stirrups and main bars use the same bend deduction convention?

Accepted Answer

The underlying bend deduction factors (1d/2d/3d by angle) apply the same way to any bent shape, whether it's a stirrup, an L-bend, or a cranked bar, but different shapes accumulate deductions differently because they have different numbers and angles of bends. A rectangular stirrup deducts for four 90° corner bends plus adds hook allowance at both free ends if hooked; a simple L-bend (like a bent-up bar at a beam end) deducts for just one bend at whatever angle it's bent to; a cranked bar (which steps up or down between two straight sections, common in slabs and beams to move a bar from bottom to top reinforcement) deducts for two bend angles, one at each end of the inclined section. Always check which shape a schedule row represents before assuming a deduction count — using a stirrup's four-bend deduction on a single L-bend row (or vice versa) is a common transcription mistake when building a schedule by hand.

Bend Angle	Deduction per Bend	Typical Use
45° or sharper acute bend	1 × d	Occasional acute bends in cranked or splayed bars
90° bend	2 × d	Stirrup corners, L-bends, most bent-up bars
135° bend or sharper (including most hook bends)	3 × d	Sharp corner bends and the bend portion of hooked ends

Hook Type	Typical Extra Length	Notes
90° hook	~6d–8d beyond the bend	Simpler anchorage, less resistance to straightening under load
135° hook (common seismic/stirrup hook)	~8d–10d beyond the bend	Widely used for stirrup ends; resists opening under cyclic or reversing load
180° semi-circular hook	~9d beyond the bend (varies by reference)	Strongest anchorage for a given length; common at discontinuous main-bar ends

Shape	Bends to Deduct	Notes
Straight bar	0	No deduction — cutting length equals the straight length required
L-bend (single bent-up bar, dowel, foundation starter bar)	1	One deduction at the single bend angle used
Rectangular stirrup / tie	4 (all 90°, unless detailed otherwise)	One deduction per corner, plus hook allowance at both free ends if hooked
Circular stirrup / spiral tie	0 (continuous curve, no discrete bend)	Cutting length is closer to the ring circumference plus hook allowance
Cranked bar (stepped between two levels)	2	One deduction at each end of the inclined transition segment

Step	Formula / Substitution	Result
Rectangle perimeter	2 × (0.230 + 0.450)	1.360 m
Center-line correction (4 corners × 1d)	4 × 8mm = 32mm	0.032 m
Bend deduction (4 × 90° corners)	4 × 2 × 8mm = 64mm	0.064 m
Hook allowance (2 ends × 9d)	2 × 9 × 8mm = 144mm	0.144 m
Cutting length	1.360 + 0.032 − 0.064 + 0.144	1.472 m per stirrup
Unit weight (d² ÷ 162)	8² ÷ 162	0.395 kg/m
Weight per stirrup	1.472 × 0.395	~0.582 kg

Step	Formula / Substitution	Result
Inclined length (rise ÷ sin 45°)	0.150 ÷ sin(45°)	0.212 m
Sum of straight + inclined segments	1.2 + 0.212 + 1.5	2.912 m
Bend deduction (2 × 45° bends)	2 × 1 × 12mm = 24mm	0.024 m
Cutting length	2.912 − 0.024	2.888 m per bar

Bar Bending Schedule (BBS) Guide: Cutting Length, Bend Deduction, and Hooks

Bend Deduction by Angle

Hook Allowance at Free Ends

Bend Count by Shape

Worked Examples

Example 1 — Rectangular Stirrup

Example 2 — Cranked Bar

Common Mistakes

Using Outer Leg Dimensions as the Cutting Length With No Deduction

Forgetting Hook Allowance on Stirrups

Applying the Same Bend Deduction Factor Regardless of Angle

Confusing Lap Length With Bend Deduction

Not Grouping the Schedule by Diameter Before Ordering

Not Planning Cutting Patterns Against Standard Stock Length

Relevant Standards and References

Final Verdict

Related calculators

Related resources

FAQ

Region	Relevant Standards
United States	ACI detailing practice (ACI 315/318) covers standard hooks and bend requirements for reinforcement detailing
Europe / UK	BS 8666 specifies standard bar shapes, bend allowances, and scheduling codes used across UK and many Commonwealth-influenced practices
India	IS 2502 covers bending and bar bending schedule practice; IS 456 covers hook and anchorage detailing requirements
Australia / New Zealand	AS 3600 covers reinforced concrete detailing including standard hooks, cogs, and bend requirements
General guidance	Bend deduction and hook factors vary slightly between these references — always confirm the exact multiplier your project specification or local code expects before finalizing a schedule for structural (not just estimating) purposes