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Only your own figure is used — InstaBOQ adds no rate.
A slab bar bending schedule lists the main bars that run across the short span, the distribution bars that run the long way, and any top bars, cranked bars or corner torsion bars. Enter the clear spans, thickness, cover and bar spacing to get each bar's cutting length, count and weight. The editor opens with the worked example below (unless you already have a schedule on this device) — change any figure, or add more members. Need it in Excel with formulas? Download the free bar bending schedule Excel template.
Nothing here yet. Add your first member. Each member is one beam, column, slab (and so on) with its bars; if you have six identical beams, add one and set "identical members" to 6.
The example is a beam, a column and a footing.
Assumptions used
- Bend deductions (× bar diameter, per bend)
- 45° = 1d, 90° = 2d, 135° = 3d (SP 34). If you give a bend radius, 45° and 90° bends use the exact geometric deduction 2(r + d)·tan(θ/2) − θ(r + d/2) instead; hooks always use the table.
- Hook length (× bar diameter)
- 90° = 12d, 135° = 10d
- Minimum hook length (IS 13920)
- 75mm floor, applied to the 135° hook only
- Unit weight (IS 1786)
- d² / 162 kg per metre
- Cutting lengths
- rounded half-up to the nearest 10mm, once
- Development length
- Ld = 0.87 fy × d / (4 × 1.6 × τbd), IS 456 Cl 26.2.1, used exactly — not rounded to a whole multiple of d, which can fall short of the code by up to half a diameter.
- Tie leg arrangements resolved
- 2 legs = 1 × closed_rect; 3 legs = 1 × closed_rect, 1 × single_leg; 4 legs = 1 × closed_rect, 2 × single_leg
- Stock length for splicing
- 12000mm. A bar longer than this is split into stock-length pieces and the lap steel at each splice is ADDED to the total length and weight — so for a spliced bar, total length is more than cutting length × bar count, by design.
- Splice lap length
- the greater of Ld and 30d (IS 456 Cl 26.2.5.1(c), flexural tension). If a grade pair has no tabulated bond stress, a bar needing a splice is reported as uncomputable rather than spliced with a zero lap.
- Closer stirrups / ties near the ends
- If you give an end-zone length and spacing, the run is counted as end zone + middle + end zone without counting the boundary bars twice. Zone lengths are never assumed.
- Bent-up (cranked) bars
- two 45° cranks, each adding 0.42 × the centreline offset D' (member depth − 2 × cover − bar diameter), on top of the anchored straight length. Centreline basis, so no extra bend deduction. Not yet hand-verified.
- Continuous beams
- Bottom bars per span: bent down into end supports, Ld/3 past the face at interior supports (IS 456 Cl 26.2.3.3(a)). Top bars continuous, bent down into both end supports, spliced with billed laps when over stock length. Not yet hand-verified.
- Column bars
- Floor height + lap carried up; a crank into the column above adds the slope length at 1 in 6; ground-floor starter bars add the footing depth less cover and an L-foot, with the 90° bend deducted. Circular columns: hoops are π × centreline diameter plus hooks; a helix adds 1.5 turns at each end.
- Lintel, staircase, pile cap
- Lintel bars run opening + bearing each side less cover; the stair is a one-flight waist slab with main bars along the slope (√(run² + rise²)); the pile cap is a plain block with a bottom mat and an optional straight top mat. Step concrete, pile heads and side-face bars are not modelled.
- Steel density ranges (kg/m³)
- beams 100–200, columns 150–280; other members are shown without a verdict — no published range has been checked for them, and a guessed range would be worse than none
A calculation aid, not a design. It applies standard IS conventions to the numbers you enter and does not check that they satisfy any code requirement. A qualified engineer must check and sign off the schedule before any bar is cut.
Worked example — 4000 × 3000 mm slab panel
A 4000 × 3000 mm clear panel, 125 mm thick, 20 mm cover, with 10 mm main bars at 150 mm and 8 mm distribution bars at 200 mm, straight, M25 / Fe500. These figures are computed by the same code the editor above runs — they are not typed in.
| Bar mark | Bar | Shape | Dia | Dimensions (mm) | Cutting length | Total bars | Weight |
|---|---|---|---|---|---|---|---|
| S1/1 | Main bar (short span) | Straight | 10 mm | A 3000 | 3000 mm | 27 | 50.00 kg |
| S1/2 | Distribution bar (long span) | Straight | 8 mm | A 4000 | 4000 mm | 16 | 25.28 kg |
Total steel 75.28 kg in 1.708 m³ of concrete.
- Short span direction
- 3000mm (the clear width). Main bars run the short way and are counted across the perpendicular 4000mm dimension; distribution bars run the long way and are counted across the 3000mm dimension.
- Concrete volume basis
- Gross: clear dimensions plus the 230mm support width both ways (4230 × 3230mm).
How does the calculator work out each slab bar?
- Which way the bars run
- Main bars run across the short span and are spaced along the long span; distribution bars run the long way and are spaced along the short span.
- Number of bars
- floor(spacing extent ÷ spacing) + 1 for each layer — so 4000 mm at 150 mm is 26 spaces and 27 bars.
- Straight bars
- cut to the clear span you enter. Choose "bent up at both ends" to add a leg at each end; the two 90° bends are then deducted at 2d each (SP 34).
- Top bars, cranked bars and torsion bars
- optional groups. Top bars over the supports need their diameter, spacing, extent and direction; cranked (alternate) main bars add 0.42 × the crank height per 45° crank (a convention not yet hand-checked, which the calculator says when you use it); corner torsion bars follow IS 456 Annex D-1.8. Each is entered in full or not at all — nothing is assumed.
- Weight
- total length × d² ÷ 162 kg per metre, d in mm.
Bend deductions, hook lengths and rounding are listed in full under "Assumptions used" above.
Where can you learn the slab calculation step by step?
- Turn spacing into a steel quantity — a lesson in the free bar bending schedule course.
Frequently asked questions
- How do you calculate the number of bars in a slab?
- Divide the distance the bars are spread over by the spacing, round down, and add one. Main bars that run across a 3000 mm short span are spread over the 4000 mm long span, so at 150 mm that is floor(4000 / 150) + 1 = 27 bars.
- Which bars in a slab are main bars and which are distribution bars?
- In a one-way or two-way slab the main bars run across the shorter span, because that is the direction that carries most of the bending. Distribution bars run the other way, tie the main bars together and control cracking.
- Does the calculator add extra top bars over the supports?
- Only if you ask for them. Tick top bars and give their diameter, spacing, how far they extend from the support face and which direction they run. The calculator never invents a top bar the drawing does not show.
- Is the steel weight in this example a thumb rule?
- No. It is the sum of every bar's length times its unit weight, d squared over 162 kg per metre. Thumb rules like kg per square metre are only a check on the result.
Bar bending schedule for other members: beam · column · footing · every member type.