Steel for a slab can be estimated with a tape measure, the drawing's bar sizes and spacings, and the weight chart. The method is the same whatever the room size.
What You Need From the Drawing
- Slab dimensions (clear spans between beams).
- Main bar diameter and spacing (across the short span).
- Distribution bar diameter and spacing (across the long span).
- Top bars over supports — size, spacing, length.
- Cover and anchorage into beams.
Worked Example: A 3.6 m × 4.0 m Slab
Assume: short span Lx = 3.6 m, long span Ly = 4.0 m; main bars 10 mm @ 150 mm; distribution bars 8 mm @ 200 mm; 150 mm anchorage into beams at each end; 25 mm deducted at each edge for cover.
Main bars (10 mm, spanning the 3.6 m direction, spread along 4.0 m):
- Number = (4,000 − 50) ÷ 150 + 1 = 26.3 + 1 → 27 bars
- Length each = 3.6 + 2 × 0.15 = 3.9 m
- Weight = 27 × 3.9 × 0.617 = 65 kg
Distribution bars (8 mm, spanning 4.0 m, spread along 3.6 m):
- Number = (3,600 − 50) ÷ 200 + 1 = 17.75 + 1 → 19 bars
- Length each = 4.0 + 2 × 0.15 = 4.3 m
- Weight = 19 × 4.3 × 0.395 = 32 kg
Bottom mesh total: about 97 kg. Top bars over supports and cranks typically add 10–20 percent, bringing the slab to roughly 110 kg — about 7.6 kg per m², or 0.7 kg per sq ft.
IS 456 Spacing Limits
| Bars | Maximum spacing |
|---|---|
| Main bars | 3 × effective depth or 300 mm, whichever is smaller |
| Distribution bars | 5 × effective depth or 450 mm, whichever is smaller |
IS 456 also sets a minimum amount of steel in slabs — 0.12 percent of the gross cross-section for high-strength deformed bars — which the distribution steel usually satisfies.
From One Slab to the Whole Floor
Repeat for each panel, then add:
- Laps where bars are joined (about 50d — see lap length).
- Extra bars around openings such as stair or duct cut-outs.
- Cutting waste of 2–5 percent.
Sum the weights by diameter to build the steel order for the slab.
The Short Version
Bars = dimension ÷ spacing + 1; length = span + anchorage; weight = bars × length × kg/m. Do it for each direction, add 10–20 percent for top bars and laps, and a few percent for waste. A typical house slab comes to about 0.7–1 kg of steel per sq ft.
