What Size Rebar for a Concrete Slab?
Complete diameter, spacing, and quantity guidance for ground-bearing and suspended concrete slabs — DIN 488 / EN 10080 B500B reinforcing steel, export-ready worldwide.
What Size Rebar Does a Concrete Slab Need?
The right rebar size for a concrete slab depends on three primary variables: slab thickness, applied load, and exposure conditions. As a structural overview, most ground-bearing slabs in residential and light industrial construction use 10 mm or 12 mm B500B deformed bar at 150–200 mm centre-to-centre in both directions. Heavier industrial floors or transfer slabs may require 16 mm, 20 mm, or larger.
All figures below reference B500B (min yield 500 MPa, Agt ≥ 5.0 %, k ≥ 1.08) per DIN 488 / EN 10080 — the standard European structural bar grade. Steel Rebar Germany supplies this grade in diameters 8–40 mm in stock lengths of 6–18 m and as factory cut-and-bent elements.
Rebar Size Guide by Slab Type
Typical bar diameters, spacings, and steel areas for common concrete slab applications.
| Slab Type | Thickness | Bar Dia | Spacing c/c | As (mm²/m) | Weight kg/m² |
|---|---|---|---|---|---|
| Residential ground floor slab | 100–150 mm | 10 mm | 200 mm | 393 | ~3.09 |
| Garage / light industrial floor | 150–200 mm | 12 mm | 200 mm | 565 | ~4.44 |
| Warehouse / logistics floor | 200–250 mm | 16 mm | 200 mm | 1005 | ~7.90 |
| Suspended floor / transfer slab | 200–350 mm | 20 mm | 150 mm | 2094 | ~16.47 |
| Heavy industrial / raft | 300–500 mm | 25 mm | 150 mm | 3272 | ~25.67 |
As = steel area per metre width (one direction). Two-way reinforcement doubles the steel. Indicative values only — confirm with structural engineer.
Bar Weight and Cross-Section Reference
The EN 10080 weight formula is: kg/m = d²(mm) × 0.00617. Key values for slab reinforcement:
| Dia (mm) | Weight (kg/m) | Section (mm²) |
|---|---|---|
| 10 | 0.617 | 78.5 |
| 12 | 0.888 | 113 |
| 16 | 1.58 | 201 |
| 20 | 2.47 | 314 |
| 25 | 3.85 | 491 |
Multiply the weight per metre by total bar length to estimate tonnage for your order. Add 5–10 % for laps and waste.
Ground-Bearing vs Suspended Slabs
Ground-bearing slabs sit on a prepared sub-base and transfer loads directly to the ground. Reinforcement primarily controls shrinkage and thermal cracking. A single two-way mat of 10 mm or 12 mm bar is usually sufficient, positioned at mid-depth or in the upper third of the slab.
Suspended slabs (e.g. upper floors, precast filigree elements with lattice girders, transfer plates) span between supports and require designed top and bottom reinforcement with precise positioning. Bar diameters of 16–25 mm are common, and the design is strictly engineer-led. For precast concrete applications, see our precast concrete reinforcement guide.
The transition between the two is a critical design decision — if in doubt, treat the slab as suspended and engage a structural engineer.
Mesh vs Bar for Concrete Slabs
Factory-welded reinforcing mesh (B500A, DIN 488-4) in standard 6.0 × 2.3 m panels is widely used for ground-bearing residential and commercial slabs where the loading is uniform and the geometry is regular. Common mesh types:
- Q188: 6 mm wire at 150 mm c/c, As ≈ 188 mm²/m — light residential slabs
- Q257: 7 mm wire at 150 mm c/c, As ≈ 257 mm²/m — standard residential/patio
- Q335: 8 mm wire at 150 mm c/c, As ≈ 335 mm²/m — garage/workshop floors
- Q524: 10 mm wire at 150 mm c/c, As ≈ 524 mm²/m — light industrial floors
For heavier slabs, suspended structures, or non-rectangular areas, loose B500B bar cut-and-bent to shape is more economical and flexible. We supply both — request a quote with your slab dimensions and load category.
Frequently Asked Questions
What size rebar is standard for a concrete slab?
How do I calculate rebar for a concrete slab?
Should rebar go top, bottom, or middle of a slab?
Can I use mesh instead of rebar for a concrete slab?
What concrete cover is needed over rebar in a slab?
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