How to Calculate Rebar Quantity for a Concrete Slab
To calculate rebar for a slab, divide the slab width by bar spacing to get bars running in one direction, and the slab length by spacing for the other direction. Add 1 to each count for the edge bar. Multiply by the number of layers (single or double).
Worked example: 6m × 4m slab, 12mm bars at 150mm spacing, double layer. Bars in X = ceil(4/0.15) + 1 = 28. Bars in Y = ceil(6/0.15) + 1 = 41. Total = (28 + 41) × 2 = 138 bars. With 500mm lap, total length ≈ 138 × avg 5.5m × 1.05 wastage ≈ 798m. Weight = 798 × 0.888 = 709 kg.
Standard Rebar Sizes and Weight Per Metre
| Dia (mm) | Weight (kg/m) | Common Use |
|---|---|---|
| 6 | 0.222 | Stirrups, light mesh |
| 8 | 0.395 | Light slabs, links |
| 10 | 0.617 | Residential slabs |
| 12 | 0.888 | Standard slabs, beams |
| 16 | 1.578 | Beams, columns |
| 20 | 2.466 | Heavy structural |
| 25 | 3.854 | Large columns |
| 32 | 6.313 | Bridge structures |
Minimum Concrete Cover Requirements
Cover protects rebar from corrosion and provides fire resistance. Insufficient cover is one of the most common causes of structural deterioration.
| Location | Min Cover |
|---|---|
| Internal floor slab | 25mm |
| External slab / wall | 40mm |
| Foundation (ground contact) | 50mm |
| Exposed to seawater | 75mm |
What Is Lap Length and Why Does It Matter?
Reinforcing bars come in stock lengths of 6m or 12m. Where two bars must be joined, they overlap by a defined lap length to transfer stress. Standard lap = 40 × bar diameter. For 12mm bar: 40 × 12 = 480mm, rounded to 500mm. Seismic zones may require 50×d or more. Laps should be staggered so all bars don’t join at the same section.
Y Bar vs R Bar — Which Should You Use?
Y bars (deformed/ribbed) have surface ridges for superior concrete bond. They are used for all structural reinforcement. R bars (round/plain) are smooth and used for stirrups, links and light mesh where bending is required. In Pakistan, Y10, Y12 and Y16 are the most commonly stocked sizes. Always specify grade 60 (420 MPa) for structural work.
Bar Bending Schedule
A bar bending schedule (BBS) is a detailed list of every bar mark, diameter, length, shape, and bend in a structure. This calculator provides quantity estimates; the actual BBS must come from structural drawings prepared by a qualified engineer. Never substitute a calculator estimate for a proper BBS on a real project.