Rebar bending schedule calculator
Cutting lengths, bar weights, mandrel diameters, and lap lengths for reinforcement take-offs. Built around BS 8666:2020 shape codes for UK steel fixers, RC subbies, and design engineers.
A planning tool, not a fabrication drawing
This calculator gives indicative cutting lengths and bar weights for material take-offs and on-site sanity checks. Schedules issued for fabrication must be prepared to BS 8666:2020 by a competent scheduler, with bar marks, shape codes, and dimensions verified against the structural drawings.
Cutting lengths here use the listed shape-code formulas for the supported BS 8666:2020 shapes. They are useful for checks and take-offs, but not a substitute for a checked fabrication schedule.
Cutting length & rebar weight calculator
Pick a shape code, enter the dimensions in millimetres, and get cutting length, mass per bar, and total mass for N bars. Use it as a bar bending schedule (BBS) check before ordering reinforcement.
Cutting length per bar
1175mm
L = A + (B) - 0.5r - d
Mass per bar
1.04kg
at 0.888 kg/m for 12mm
Total mass (N bars)
20.9kg
20 × 1.18m × 0.888 kg/m
How this is calculated
Mass per metre uses the steel density of 7850 kg/m3: m = π/4 × d2 × 7850 / 106 kg/m. The "d² ÷ 162" rule of thumb gives the same result.
Cutting length uses the displayed BS 8666:2020 shape formula for the selected shape code. Parenthesised dimensions in the standard are included as labelled inputs for take-off checks.
BS 8666:2020 minimum bend radius is taken as half the minimum mandrel diameter: 2d for bars 6-16mm and 3.5d for bars 20mm and above. Switch to "Custom internal radius" if your fabricator's formers are larger.
Bar mass per metre
Standard masses for ribbed reinforcement bars to BS 4449:2005+A3:2016, the mainstream UK rebar specification.
| Bar size | Cross-section area | Mass per metre | Mass per 12m bar | Bars per tonne (12m) |
|---|---|---|---|---|
| 6mm | 28.3 mm² | 0.222 kg/m | 2.66 kg | 375 |
| 8mm | 50.3 mm² | 0.395 kg/m | 4.74 kg | 211 |
| 10mm | 78.5 mm² | 0.617 kg/m | 7.40 kg | 135 |
| 12mm | 113.1 mm² | 0.888 kg/m | 10.66 kg | 94 |
| 16mm | 201.1 mm² | 1.578 kg/m | 18.94 kg | 53 |
| 20mm | 314.2 mm² | 2.466 kg/m | 29.59 kg | 34 |
| 25mm | 490.9 mm² | 3.853 kg/m | 46.24 kg | 22 |
| 32mm | 804.2 mm² | 6.313 kg/m | 75.76 kg | 13 |
| 40mm | 1256.6 mm² | 9.864 kg/m | 118.37 kg | 8 |
Masses derived from steel density of 7850 kg/m³. The shorthand m = d²/162 (with d in mm, m in kg/m) gives the same values. UK merchants commonly stock 6m and 12m straight lengths; 6mm bar is also sold in coils.
Minimum mandrel diameters
Per BS 8666:2020 Table 2. Smaller formers risk fracturing the bar at the bend. Always check the fabricator's actual former sizes; many shops use larger mandrels than the published minimum.
| Bar size | Min. mandrel diameter | Min. internal bend radius | Notes |
|---|---|---|---|
| 6mm | 24mm (4d) | 12mm (2d) | Links and stirrups |
| 8mm | 32mm (4d) | 16mm (2d) | Links and stirrups |
| 10mm | 40mm (4d) | 20mm (2d) | Light main bars, links |
| 12mm | 48mm (4d) | 24mm (2d) | Most common slab/stirrup size |
| 16mm | 64mm (4d) | 32mm (2d) | Foundations, beams, columns |
| 20mm | 140mm (7d) | 70mm (3.5d) | Step up at 20mm |
| 25mm | 175mm (7d) | 87.5mm (3.5d) | Heavy beams, transfer slabs |
| 32mm | 224mm (7d) | 112mm (3.5d) | Pile caps, large columns |
| 40mm | 280mm (7d) | 140mm (3.5d) | Specialist heavy structures |
The 4d / 7d minimums apply to ribbed Grade B500 bar to BS 4449. Tighter bends require special detailing and are usually rejected on schedules. Confirm any non-standard mandrel with the engineer before fabrication.
Common BS 8666:2020 shape codes
A short selection of the shapes that turn up most often on UK groundworks and superstructure schedules. The full set is published in BS 8666:2020 with cutting length formulas.
Straight bar
No bends. Cut to length.
Used for slab top and bottom mats, column verticals between starters, and lap-bar pieces. Cutting length L = A.
L-bar (one 90° bend)
Most common starter / kicker bar.
Foundation-to-column starter bars, slab kickers off retaining walls, edge-of-slab L-bars. Two outside legs A and B with a single 90° bend at the corner.
U-bar (two 90° bends, same way)
Edge closer for slabs and walls.
Three outside dimensions A, B, C with two parallel legs. Used as a top-and-bottom edge closer where slab or wall reinforcement turns back on itself.
Cranked bar / dog-leg
Two 90° bends in opposite directions.
Three legs A, B, C with bends going opposite ways, creating an offset. Used where a bar moves from one face to another, e.g. column-to-column lap offsets.
Z-bar (bends at both ends)
90° bend at each end.
Three legs with bends at the two ends pointing the same way. Common as anchorage bars in slab edges and as splice bars between adjoining elements.
Four-leg bent bar
Shape using A, B, C, and D dimensions.
A common multi-leg bend for anchorage and continuity details. The calculator uses L = A + B + 0.57C + (D) - 0.5r - 2.6d.
Multi-bend shape
Standard BS 8666 shape using A, B, and C.
Shape 33 is not the closed link shape. Its published BS 8666:2020 length formula is L = 2A + 1.7B + 2(C) - 4d, so it needs its own calculator option.
Closed link / stirrup
Beam and column shear reinforcement.
Closed rectangle with link return dimension C. The mandrel diameter and hook detail control the buildable inside dimension; check beam cover before scheduling.
Special / custom shape
Anything not covered by 00-77.
Used when the bar shape doesn't match a standard code. Requires a dimensioned sketch on the schedule. Fabricators often charge a premium for shape 99 bars and lead times can run longer.
Lap length quick reference
Indicative tension lap lengths for Grade B500 ribbed bar in normal-weight concrete, derived from BS EN 1992-1-1 (Eurocode 2). These are starting points for take-off only; the project structural drawings give the binding values.
| Bar size | Tension lap (good bond, C30/37) | Tension lap (poor bond) | Common rule of thumb |
|---|---|---|---|
| 10mm | ~400mm (40d) | ~575mm (58d) | 40d common; 50d safe |
| 12mm | ~480mm (40d) | ~690mm (58d) | 40d common; 50d safe |
| 16mm | ~640mm (40d) | ~920mm (58d) | 40d common; 50d safe |
| 20mm | ~800mm (40d) | ~1150mm (58d) | 50d for safety in pile caps |
| 25mm | ~1000mm (40d) | ~1450mm (58d) | Often 50-55d on drawings |
| 32mm | ~1280mm (40d) | ~1855mm (58d) | Always defer to engineer |
| 40mm | ~1600mm (40d) | ~2320mm (58d) | Couplers usually preferred |
"Good bond" applies to bars more than 250mm from the top of a pour, or in the bottom of pours less than 250mm deep. Top bars in deeper pours sit in poor bond conditions and need the larger lap. Eurocode 2 lap lengths also depend on lap percentage, cover, transverse reinforcement, and concrete class; the values shown assume up to 50% of bars lapped at the section in C30/37 with normal cover.
Practical tips from the bar bending shop
Things that aren't in BS 8666 but every steel fixer and scheduler picks up after a few jobs.
Schedule in 100mm increments where you can
Fabricators bend off shape templates and batch-cut to nominal lengths. Specifying a leg as 825mm rather than 800mm saves nothing on site but slows the schedule because the cutter has to single-set for every bar. Round to 25mm at minimum, 100mm where the dimension allows. The drawings carry tolerance; the bench saw doesn't care about your last 5mm.
Order 6m bars where 12m won't fit on the wagon
12m bars are cheaper per tonne but need an artic with a long bed. Sites with tight access (terraced street fronts, roof slabs craned in stages) get less waste from 6m bars even at the higher unit cost. Tell the merchant the access constraints when you order; they'll bundle the cuts to suit.
Watch the inside of the bend on tight stirrups
A stirrup with internal width less than the minimum mandrel diameter plus two bar diameters cannot be bent. A 12mm closed link with 48mm minimum mandrel needs at least 48 + 2×12 = 72mm clear inside dimension, plus cover allowances. Beams designed without checking this end up with stirrups that cannot be fabricated, and the schedule comes back from the bender rejected.
Check the deliverable bar mark count, not just tonnage
A schedule of 8 tonnes of mixed shapes is straightforward; the same 8 tonnes split across 240 bar marks is a fabrication scheduling problem. Fabricators charge per cut and per bend, not just per tonne. Reducing the number of distinct bar marks (by standardising shapes and lap dimensions) cuts both cost and lead time.
Use couplers, not laps, for 32mm and 40mm
A 50d lap on a 40mm bar is two metres of bar overlap at every splice. For pile caps and transfer beams with large bars, mechanical couplers (threaded or swaged) save weight, simplify rebar congestion, and reduce concrete volume. CARES-approved couplers are widely accepted on UK projects; check the engineer's specification first.
Sources
- BS 8666:2020 Scheduling, dimensioning, bending and cutting of steel reinforcement for concrete - Specification (BSI)
- BS 8666:2020 (BSI Shop) Standard purchase page
- BS 8666:2020 shape code chart Public shape-code formula reference from Collins Reinforcements
- Approved Document A: Structure for the regulatory framework that adopts Eurocode 2 in England
- UK CARES Certification body for reinforcement steel and reinforcement-related products in the UK
- BS 4449:2005+A3:2016 Steel for the reinforcement of concrete - Weldable reinforcing steel - the source of the standard bar mass values
- BS EN 1992-1-1:2004+A1:2014 (Eurocode 2) for tension lap and anchorage requirements
Built by Rospower Projects, a specialist groundworks and civil engineering contractor. 35+ years on site.
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