What Size Rebar for a Concrete Beam?
A practical guide to selecting bar diameters, spacing, and cover for reinforced concrete beams — using B500B rebar to DIN 488 / EN 10080. Always confirm final sizing with a qualified structural engineer.
Why Bar Size Matters in Beam Design
Selecting the correct rebar size for a concrete beam is one of the most consequential decisions in structural design. Beams carry bending moments and shear forces transferred from slabs and walls, so under-reinforcing a beam can result in brittle, catastrophic failure. Over-reinforcing wastes material and can make the section impractical to pour.
The dominant reinforcing steel grade for beams in Germany and across the EU is B500B — high-ductility, hot-rolled, 500 MPa minimum yield strength, with a strain at maximum force (Agt) ≥ 5.0% and a characteristic strength ratio k ≥ 1.08. These ductility properties matter enormously in beams because they ensure ductile flexural failure rather than sudden fracture, a requirement of Eurocode 2 (EN 1992-1-1).
The information below reflects common practice and indicative figures used in preliminary sizing. All structural calculations must be performed and verified by a licensed structural engineer in accordance with the applicable national annex.
Typical Bar Diameters for Concrete Beams
Most residential and light-commercial beams use longitudinal (flexural) bars in the range of 12 mm to 25 mm. Heavy-transfer beams in high-rise or industrial structures may reach 32 mm or 40 mm. Shear links (stirrups) are typically 8 mm or 10 mm, spaced at 150–200 mm centres in mid-span zones and closer near supports.
| Beam Type | Typical Long. Bar Dia. | Min. Bars (tension) | Stirrup Dia. |
|---|---|---|---|
| Residential floor beam (light loads) | 12–16 mm | 2–3 bars | 8 mm @ 150–200 mm |
| Medium-span commercial beam | 16–20 mm | 3–4 bars | 10 mm @ 150 mm |
| Heavy transfer / long-span beam | 25–32 mm | 4–6 bars | 10–12 mm @ 100–150 mm |
| Deep coupling beam (seismic zone) | 16–25 mm diagonal | 4 diagonal bars + links | 8–10 mm @ 100 mm |
Bar Weight and Cross-Section Reference
Use the formula kg/m = d²(mm) × 0.00617 to convert diameter to linear weight. Below are the most-used diameters for beam reinforcement:
| Diameter (mm) | Weight (kg/m) | Cross-section (mm²) |
|---|---|---|
| 10 | 0.617 | 78.5 |
| 12 | 0.888 | 113 |
| 16 | 1.58 | 201 |
| 20 | 2.47 | 314 |
| 25 | 3.85 | 491 |
| 32 | 6.31 | 804 |
Concrete Cover Requirements
Eurocode 2 specifies minimum cover (cmin) based on exposure class. For a typical interior beam (exposure class XC1), minimum cover to the stirrups is 15 mm; adding the recommended deviation Δcdev of 10 mm gives a nominal cover of 25 mm. Exterior or below-grade beams in exposure class XC3/XC4 typically require 30–35 mm nominal cover. Cover is measured to the outer face of the stirrup, not the longitudinal bar.
Indicative Worked Estimate (Preliminary Only)
Consider a simply-supported rectangular beam: span 6 m, breadth 300 mm, effective depth 500 mm, carrying a factored bending moment of approximately 180 kN·m. Using a simplified approach (fyk = 500 MPa, partial factor γs = 1.15, lever arm z ≈ 0.9d = 450 mm):
- Required As = MEd / (fyd × z) = 180×10⁶ / (435 × 450) ≈ 919 mm²
- 3 × ø20 bars provide 3 × 314 = 942 mm² — satisfactory
- Linear weight: 3 bars × 2.47 kg/m = 7.41 kg per metre of beam
- Total rebar for a 6 m beam (longitudinal only): ≈ 44 kg
This ignores compression steel, shear design, anchorage, and bar laps — all of which are required in a full design. We supply B500B straight bars, cut-and-bend stirrups, and standard mesh panels to any project specification.
Ordering DIN 488-Compliant Beam Reinforcement
Steel Rebar Germany supplies B500B bars in diameters from 8 mm to 40 mm, stock lengths 6–12 m, or cut-and-bent to your bending schedule (BS 8666 / DIN 488 shape codes). Each consignment ships with an EN 10204 3.1 Mill Test Certificate. For projects requiring pre-bent stirrups, spacers, and tying wire as a coordinated package, request a combined quote. We export worldwide — see our export and delivery page for container and break-bulk options.
Frequently Asked Questions — Rebar for Concrete Beams
What is the most common rebar size used in concrete beams?
Can I use B500A coil rebar in a beam?
How do I calculate the weight of rebar in a beam?
What concrete cover is required for beam reinforcement?
Does Steel Rebar Germany supply pre-bent stirrups for beams?
Source German-standard rebar with full export documentation
Tell us your beam schedule, destination port, and required standards — we’ll respond with a detailed quotation including MTC and CE documentation.