Flat slab rebar quantity calculator
This calculator estimates the concrete and steel of a flat slab: slab concrete = area × thickness, plus the drop panels (count × length × width × extra depth) if any; then steel = total concrete × a typical ratio of 110 kg per m³. Use it for preliminary estimates; the exact quantity comes from the structural drawings.
Calculator
- Reinforcing steel
- 12,238 kg (12.24 t)
- Total concrete
- 111.25 m³
- Slab / drop panel concrete
- 100 / 11.25 m³
Method, step by step
- Multiply the slab area by its thickness in metres to get the slab concrete.
- For each drop panel: length × width × extra depth below the slab, then multiply by the number of drop panels.
- Add the two volumes for the total concrete in m³.
- Steel weight = total concrete × the steel ratio in kg per m³; divide by 1000 for tonnes.
Worked example
Inputs
- Slab area (m²): 400
- Slab thickness (m): 0.25
- Number of drop panels: 12
- Drop panel length (m): 2.5
- Drop panel width (m): 2.5
- Drop panel extra depth (m): 0.15
- Slab concrete = area × thickness = 400 × 0.25 = 100 m³.
- Drop panel concrete = count × length × width × extra depth = 12 × 2.5 × 2.5 × 0.15 = 11.25 m³.
- Total concrete = 100 + 11.25 = 111.25 m³.
- Steel weight = 111.25 × 110 = 12,238 kg (12.24 t).
Result
- Reinforcing steel
- 12,238 kg (12.24 t)
- Total concrete
- 111.25 m³
- Slab / drop panel concrete
- 100 / 11.25 m³
Assumptions and default ratios
| Ratio | Default value |
|---|---|
| Flat slab steel ratio (typical) | 110 kg per m³ of concrete |
- The steel ratio of 110 kg per m³ is an editable typical assumption for preliminary estimates, within the 95 to 135 kg/m³ range that international quantity-surveying references give for flat slabs; it is not a design value; the structural drawings and the bar bending schedule govern.
- Column capitals, edge beams and openings are not included; deduct openings from the area and add the other elements separately.
- The drop panel extra depth is only the part below the slab, since the part within the slab thickness is already counted in the slab area.
Frequently asked questions
How does a flat slab differ from a hollow-block slab in the calculation?
A flat slab is solid and sits directly on the columns without beams, so its concrete is area × thickness plus drop panels. In a hollow-block slab the blocks replace part of the concrete; it has its own calculator.
How do I calculate drop panel concrete?
Multiply the drop panel length by its width and by its extra depth below the slab, then by the number of panels. In the worked example: 12 × 2.5 × 2.5 × 0.15 = 11.25 m³.
How many kg of steel are there in one m³ of flat slab?
The default is 110 kg per m³, a typical value for preliminary estimates, not a design value. The real ratio depends on spans, loads and punching reinforcement; adjust it to your drawings.
Does this replace the bar bending schedule?
No. The result is for early pricing and comparison; steel orders and payment certificates rely on the actual lengths in the schedule.
Add the result to your project BOQ in Ta3mir
In Ta3mir the same calculators work inside your project: send a result to a BOQ line in one click and save your own default ratios for every calculation.