Prepare a bar bending schedule (BBS) for an RCC slab in a minute. Enter the slab size, thickness, cover, bar sizes and spacing, and the calculator lists every bar with its number, cutting length and weight, for one-way or two-way slabs. Part of our BBS calculators.
How to use the slab BBS calculator
- Choose one-way or two-way slab (two-way when the long side is up to twice the short side).
- Enter the overall slab size, from the outer face of one supporting beam or wall to the other, and the thickness.
- Pick the cover, the bar diameters, spacings and shapes from the drawing. The schedule updates straight away.
- Edit any row or add your own (for example extra top bars at more edges); the totals update as you type.
- Download the schedule as CSV or copy a share link that opens the same schedule.
Slab BBS Calculator
One-way & two-way RCC slab · IS 2502 hooks · Cut length + steel kg · India
Overall slab size, from the outer face of one supporting beam or wall to the other. Bars run full length minus the side cover at each end. In two-way mode the shorter side is taken as Lx.
| Mark | Description | φ | Qty | Shape | A | B | Hook | Cut L | kg/m | Row kg | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Total | — | — | |||||||||
Schedule with cut-length breakdown
| Mark | Description | φ | Qty | Shape | Cut L (mm) | kg/m | Row kg |
|---|
Constants & formula reference (IS 2502 / IS 456)
Bar count: n = floor(clear perpendicular / spacing) + 1
Hooks (IS 2502): None · 90° = 4φ · 135° = 6.25φ · 180° = 9φ per hooked end
Bend deduction: 2φ per 90° bend (IS 2502 Table 1)
L-bar: A + B + 1 hook − 2φ bend
U-bar: A + 2B + 2 hooks − 4φ bends
Crank (45°): +0.42 × (t − 2×slab cover) per crank end
Weight: φ² ÷ 162.2 × L_cut(m) × qty
Extra top: L/4 from the support at one continuous edge (add a row for each further edge)
Lap (optional): added only where a bar is longer than the 12 m stock length — one lap per extra 12 m piece; 50φ Fe415 · 60φ Fe500 tension · 40φ compression · 41φ development
Educational quantity estimate — verify cover, laps and bar marks on structural drawing.
Frequently asked questions
What is the difference between one-way and two-way slab reinforcement?
A one-way slab spans primarily in one direction (aspect ratio Ly/Lx > 2). Main bars run in the span direction; distribution bars run perpendicular at wider spacing. A two-way slab carries load in both directions (Ly/Lx ≤ 2). Steel is provided in both directions, with more steel in the shorter span direction. This calculator auto-generates rows for both cases from slab dimensions and bar spacing.
What hook length is used for slab bars per IS 2502?
Per IS 2502:1963, standard hook allowances added to straight length are: 90° bend = 4 times bar diameter (4φ); 135° seismic hook = 6.25φ; 180° semi-circular hook = 9φ. For a U-bar with two 180° hooks, add 18φ total. Simply supported slab bottom bars are often straight (no end hook) when development length is satisfied by straight embedment — select Hook: None for that case.
How is crank bar cutting length calculated in slab BBS?
Cranked (bent-up) bars at continuous supports add approximately 0.42 × d_eff per crank, where d_eff = slab thickness minus top and bottom cover. Total cutting length = clear span + 2 × (0.42 × d_eff) for bars cranked at both supports. This matches common Indian site BBS practice and CPWD-style worked examples.
What cover is used for RCC slabs in India?
IS 456:2000 Table 16 gives nominal cover for durability: 20 mm typical for internal slabs (M20/M25, moderate exposure); 25 mm for external slabs or moderate exposure; 40 mm for severe exposure. Side cover to beam face at slab support is often 25 mm in structural drawings. Always verify cover on the structural drawing — this tool uses editable preset chips.
How is steel weight calculated in a slab BBS?
Weight per metre W = φ² ÷ 162 kg/m (φ in mm). Row weight = W × (cut length in m) × quantity. Total schedule weight is the sum of all rows. This calculator uses divisor 162.2 internally for precision. Fe 415, Fe 500 and Fe 500D have the same unit weight per IS 1786:2008.
What is the L/4 rule for extra top bars in slabs?
At continuous supports, extra top reinforcement is often detailed for a length of L/4 from the center of support on each side, where L is the center-to-center span. This calculator optional toggle adds top bar rows with cutting length = span/4 for two-way and one-way continuous slabs.
How is this different from the Speak Arch steel weight calculator?
The steel weight calculator multiplies lengths you enter by D²/162. This slab BBS calculator computes number of bars from spacing, cutting length from clear span plus IS 2502 hooks, cranks and bend deductions, then weight — producing a full bar schedule for slab reinforcement quantity estimate, not structural design.
How to calculate a slab bar bending schedule — with formulas
A bar bending schedule (BBS) lists every steel bar in an RCC slab — its mark, diameter, shape, cutting length and weight — so you can order the right quantity of steel before casting. On Indian sites this is prepared from the structural drawing using IS 2502 and IS 456. The calculator above automates the full calculation; the sections below explain the method so you can verify the numbers by hand.
1. Is your slab one-way or two-way?
Measure the longer span (Ly) and shorter span (Lx). The ratio decides how the steel is laid:
- One-way slab: Ly/Lx > 2. Main bars run along the shorter span; lighter distribution bars run along the longer span.
- Two-way slab: Ly/Lx ≤ 2. Main bars run in both directions, with the shorter span usually taking equal or marginally heavier steel.
2. Counting the bars from spacing
Bars are placed at a fixed centre-to-centre spacing read off the drawing. The number of bars in a direction is:
n = floor[(slab dimension across the bars − 2 × side cover) ÷ spacing] + 1
Drop the remainder of the division and add one bar for the starting edge. For example 4950 ÷ 150 = 33, so 34 bars. The calculator does this automatically.
3. Cutting length formulas (IS 2502)
The cutting length is the length of steel actually cut for each bar, after adding hooks and cranks and subtracting bend allowances:
| Bar shape | Cutting length formula |
|---|---|
| Straight (no hook) | Clear span |
| Straight + 2 hooks | Clear span + 2 × hook allowance |
| Cranked / bent-up | Clear span + 2 × 0.42 × (t − 2 × cover) |
| L-bar | A + B + 1 hook − 1 bend deduction |
| U-bar | A + 2B + 2 hooks − 2 bend deductions |
Per IS 2502 a standard 180° semi-circular hook adds 9φ, a 90° hook adds 4φ, and each 90° bend in a U-bar/stirrup deducts 2φ (φ = bar diameter). Slab bottom steel is usually left straight — when the bar extends far enough into the support the straight embedment already meets the development length, so a hook is unnecessary. Keep the default Hook: None unless your drawing calls for one.
4. Cranked bars at continuous supports
At a continuous support the bottom steel is sometimes cranked (bent up) to also serve as top steel over the support. For a 45° crank each bent end adds 0.42 × effective depth, where effective depth = slab thickness − top cover − bottom cover. This is why the calculator adds 2 × 0.42 × (t − 2 × cover) to the clear span for a cranked bar.
5. Extra top steel — the L/4 rule
At a continuous support, top steel resisting negative moment is detailed for a length of L/4 measured from the centre of the support on each side. Toggle “Extra top bars” in the calculator to add a top-bar row with this length automatically.
6. Steel weight from diameter
Unit weight of a circular steel bar is derived from the volume of a cylinder at a density of 7850 kg/m³:
W = φ² ÷ 162 kg/m (φ in mm)
Row weight = unit weight × cutting length (m) × number of bars. The calculator divides by 162.2 internally for slightly better precision; the result is the same for practical purposes. Fe 415, Fe 500 and Fe 500D bars (IS 1786) all share the same unit weight — only strength differs.
7. Worked example — one-way slab (4 m × 5 m, 150 mm thick)
Slab 4000 × 5000 × 150 mm, clear cover 20 mm, side cover 25 mm, main bars 10 mm @ 150 c/c along the 4 m span, distribution 8 mm @ 200 c/c.
- Clear span (main) = 4000 − 2 × 25 = 3950 mm
- Number of main bars = (5000 − 2 × 25) ÷ 150 + 1 = 4950 ÷ 150 + 1 = 33 + 1 = 34 bars
- Main cutting length (straight, no hook) = 3950 mm = 3.95 m
- Main steel weight = (10² ÷ 162) × 3.95 × 34 = 0.617 × 3.95 × 34 = 82.9 kg
- Distribution bars = floor(3950 ÷ 200) + 1 = 20 bars at 4950 mm each
This matches what the calculator returns for these inputs — switch to two-way mode to see both-direction steel and the optional L/4 top bars.
IS codes referenced
- IS 456:2000 — Plain and Reinforced Concrete (cover, development length, slabs)
- IS 2502:1963 — Bending and Fixing of Concrete Reinforcement (hooks, bends, cutting length)
- IS 1786:2008 — High-strength deformed steel bars (Fe 415 / Fe 500 / Fe 500D)
- SP 34 — Handbook on Concrete Reinforcement and Detailing
More reinforcement tools
TMT bar weight calculator
Weight of any bar list by diameter and length (D²/162).
Concrete volume calculator
Concrete, cement, sand and aggregate for the same slab.
One-way vs two-way slab
How to tell them apart and how their steel differs.
Beam BBS calculator
Main bars, extra bars and stirrups for beams and lintels.
Column BBS calculator
Vertical bars, laps and ties from footing to roof.
Footing BBS calculator
Bottom and top mesh for isolated footings and rafts.
Retaining wall BBS calculator
Base slab and stem bars for cantilever RCC walls.
Disclaimer: Online educational publication only. Not a substitute for structural design or detailing by a licensed engineer. Full disclaimer · contact@speakarch.com
Last updated: 8 November 2026