Standard bend deductions and hook allowance (SP-34)
A bar's cutting length is never the same as the straight-line distance it covers — bending it changes how much steel is consumed. SP-34:1987 (Handbook on Concrete Reinforcement and Detailing) gives the standard deductions and allowances this calculator uses:
The four core formulas
- Straight main bar (anchored): cutting length = clear span + 2 × (development length factor × dia), when the bar terminates into a support.
- Stirrup / tie: cutting length = 2 × (a + b) + 12 × dia, where a, b = member width/depth minus 2 × clear cover.
- Bar count from spacing: count = ⌊span ÷ spacing⌋ + 1, applied across the dimension perpendicular to the bar's run.
- Steel weight: total weight (kg) = total length (m) × (dia² ÷ 162).
Worked example — a typical residential beam
A 4000 mm clear-span beam, 230 × 450 mm, 25 mm cover, with 4–16 mm bottom bars, 2–12 mm top bars and 8 mm stirrups at 150 mm c/c, both main bar groups anchored with a 50d development length:
- Bottom bars: 4000 + 2×50×16 = 5600 mm × 4 nos = 22.4 m × 1.580 kg/m ≈ 35.4 kg
- Top bars: 4000 + 2×50×12 = 5200 mm × 2 nos = 10.4 m × 0.889 kg/m ≈ 9.2 kg
- Stirrups: a = 180, b = 400 → 2×(580) + 12×8 = 1256 mm; count = ⌊4000÷150⌋+1 = 27 → 33.9 m × 0.395 kg/m ≈ 13.4 kg
- Total ≈ 58.0 kg of steel for this one beam — enter the same numbers into the calculator above to verify.
How stirrup zones are counted
Every beam on a real drawing tightens its stirrups near the supports, where shear is greatest, and opens out through the middle. The schedule has to follow that, and the counting rule is specific:
- Each zone contributes its intervals — ⌊zone length ÷ spacing⌋ — not its bar count.
- One closing bar is added for the span as a whole.
- Two adjacent zones therefore share the stirrup that sits on their boundary, because only one bar gets tied there.
Worked through on a 4,686 mm (15'-4½") span with 8⌀ stirrups @4" c/c over 3' at each end and @6" c/c in the middle: 9 intervals at each end, 18 across the 2,857 mm middle, plus one closing bar — 37 stirrups. Assuming a single @6" spacing gives 31, which is about 16% light. Zones apply symmetrically at both ends; if the lengths you enter don't fit the span, the tool blocks saving rather than quietly rescaling a dimension you took off a drawing.
What this tool doesn't do
This is a quantity/geometry calculator, not a structural design tool. It won't derive development or lap length from your concrete and steel grade (that's IS 456 Table 26, a structural decision) — instead you enter the multiplier your drawing specifies. It also does model mixed-diameter bar groups, variable stirrup zones, extra top bars curtailed over supports, side face bars, per-level column lifts, inner/cross ties and slab top steel — but only from the length or size the drawing states, never derived from a moment envelope. It doesn't model cranked/bent-up beam bars, multi-span continuous beams, or independently designed two-way slab steel. For anything beyond a straightforward single member, always verify against your structural engineer's BBS before ordering steel.
Frequently asked questions
How is stirrup cutting length calculated?
Cutting length = 2 × (a + b) + 2 × (9 × dia) − 3 × (2 × dia), which simplifies to 2×(a+b) + 12×dia. Here a and b are the clear internal dimensions of the stirrup — member width/depth minus twice the clear cover — the "2×9d" is the allowance for two 135° hooks, and the "3×2d" deducts the three effective 90° corner bends in a closed rectangular link. This is the standard SP-34:1987 convention.
What development length or lap length should I use?
Development and lap length depend on the concrete grade, steel grade and bar diameter (IS 456 Table 26 / bond stress) — a structural design decision, not a geometry one. This calculator exposes it as an editable "× dia" factor (50d by default, a common site rule of thumb) so you can enter the exact value from your structural drawing instead of it being guessed for you.
Does this replace a structural engineer’s bar bending schedule?
No. It converts reinforcement details you already have — spans, cross-sections, bar diameters, spacing — into cutting lengths, bar counts and weight, exactly like a site engineer would tabulate by hand. It doesn’t design the reinforcement itself (bar sizes, spacing or development length are structural decisions), so always cross-check against the approved structural drawing before ordering steel.
Why isn’t development length auto-calculated from the concrete and steel grade?
Because that number varies with bond stress, which depends on the concrete grade, steel grade, bar type and exposure condition — getting it wrong could mean under-ordering steel or an unsafe splice. Rather than silently assume a grade combination, the calculator lets you enter the multiplier your structural drawing specifies.
What is the formula for the unit weight of a TMT bar?
Unit weight (kg/m) = dia² ÷ 162, where dia is the bar diameter in millimetres. For example, a 16 mm bar weighs 16² ÷ 162 ≈ 1.58 kg per running metre. This is the standard IS 1786 approximation.
Can I calculate a bar bending schedule for a slab in both directions?
This calculator covers one-way slab behaviour (main steel one way, distribution/temperature steel the other) in v1. True two-way slabs — with independently designed main steel in both directions — aren’t modelled yet; treat the "two-way" option as a label for now and verify both-direction steel against your structural drawing.
Can I enter dimensions in feet and inches, the way the drawing states them?
Yes — switch the unit toggle to feet-inches and type what the drawing says: 15'-4½" for a span, 9 for a 9" width, 4 for @4" c/c spacing. Half and quarter inches, decimal feet (3.5'), plain inches (42") and the 9''x15" style all parse. Bar diameters and clear cover stay in millimetres in both modes, exactly as Indian structural drawings state them. Every length field shows the converted value underneath, which is the quickest way to catch a slip like a 9" section typed as 9 mm.
Why does a single stirrup spacing under-count the steel?
Because real beams are not stirruped at one spacing. A typical drawing calls up @4" c/c over about 3' at each end (where shear is highest) and @6" c/c through the middle. On a 4,686 mm span that works out to 37 stirrups, against 31 if you assume @6" throughout — roughly 16% of the stirrup steel missing. Enter each tightened run as a zone and the schedule counts every interval, sharing the bar where two zones meet rather than double-counting it.
Can I enter a bar group that mixes diameters, like 2-20Ø + 1-16Ø?
Yes — add a row per diameter. Each becomes its own line in the schedule, which is how a real bar bending schedule tabulates it, and development length is added per diameter rather than once for the whole group. This matters: forcing that example into a single diameter is out by +15% (all 20Ø) or −31% (all 16Ø).
Can I build a schedule for a whole floor or building, not just one member?
Yes — after calculating any beam, column, slab or footing, click "Add to Project" to save it into a running project list, then keep switching tabs and adding more members. The tool shows a grand total, a diameter-wise steel summary (exactly how many kg of 8mm, 10mm, 12mm… to order) and lets you edit or remove any member. Your project is saved in your browser (localStorage) so it survives a reload — export it as CSV or PDF any time as a backup.
How much extra steel should I order for wastage?
Site practice typically adds 2–5% over the theoretical calculated weight to cover cutting waste, bending losses and site handling — the same range used elsewhere on this site for concrete and material ordering. The project schedule defaults to 3% and is fully editable, so you can match your own site’s typical wastage.
Also try the free concrete calculator for the cement, sand and aggregate this member needs, and the construction cost calculator for your full project budget.
