UK Rebar Sizes — BS 4449 Grade B500B Table

Reinforcing steel bars for UK concrete construction follow BS 4449:2005+A3:2016 and EN 1992-1-1. Bar diameters, cross-sectional areas, and mechanical properties including yield strength, elongation, and bendability are standardized.

UK rebar designations include bar size identifiers that relate to nominal diameter. Standard grades provide yield strengths suitable for EN 1992 concrete design.

Code Reference: BS 4449:2005+A3:2016 and EN 1992-1-1

BS 4449:2005+A3:2016 specifies dimensions, tolerances, mechanical properties, and testing requirements for reinforcing steel. EN 1992-1-1 gives design rules for reinforced concrete structures using these bars.

BS 4449 Standard Bar Sizes and Properties

Bar Size (mm) Cross-Sectional Area (mm²) Mass (kg/m) Perimeter (mm) Standard Length
8 50.3 0.395 25.1 12 m
10 78.5 0.617 31.4 12 m
12 113 0.888 37.7 12 m
16 201 1.579 50.3 12 m
20 314 2.466 62.8 12 m
25 491 3.854 78.5 12 m
32 804 6.313 100.5 12 m
40 1257 9.864 125.7 12 m

Reinforcement Area per Metre Width (mm²/m)

Bar Size Spacing 100mm Spacing 125mm Spacing 150mm Spacing 200mm Spacing 250mm Spacing 300mm
10 785 628 523 393 314 262
12 1131 905 754 565 452 377
16 2011 1609 1340 1005 804 670
20 3142 2513 2094 1571 1257 1047
25 4909 3927 3273 2454 1964 1636
32 8043 6434 5362 4021 3217 2681

BS 4449 Grade B500B and B500C Mechanical Properties

Property Grade B500B Grade B500C Test Standard
Yield strength fy (MPa) ≥ 500 ≥ 500 BS EN ISO 6892-1
Tensile strength ft (MPa) ≥ 575 ≥ 575 BS EN ISO 6892-1
ft/fy ratio (k) ≥ 1.08 ≥ 1.15 < 1.35 BS EN ISO 6892-1
Total elongation at max force Agt (%) ≥ 5.0 ≥ 7.5 BS EN ISO 6892-1
Bend/rebend test 3 × nominal diameter 4 × nominal diameter BS EN ISO 7438
Fatigue strength (2×10⁶ cycles) ≥ 150 MPa ≥ 150 MPa BS EN ISO 15630-1
Rib geometry (relative area) ≥ 0.05 ≥ 0.05 BS 4449 Annex A

Maximum Bar Spacing Requirements (EN 1992-1-1)

For crack control (Clause 7.3.3):

Steel Stress σs (MPa) Max Bar Spacing — Crack width wk = 0.3mm Max Bar Spacing — Crack width wk = 0.4mm
160 300 mm 300 mm
200 250 mm 300 mm
240 200 mm 250 mm
280 150 mm 200 mm
320 100 mm 150 mm
360 50 mm 100 mm

For minimum reinforcement: As,min = 0.26 × (fctm/fyk) × bt × d ≥ 0.0013 bt × d (EN 1992-1-1 Clause 9.2.1.1)

Worked Example — Simply Supported RC Beam

Given:

Step 1 — Design concrete strength: fcd = αcc fck / γc = 0.85 × 30 / 1.5 = 17.0 N/mm²

Step 2 — Required steel area: K = MEd / (b d² fck) = 250 × 10⁶ / (300 × 500² × 30) = 0.111

z = d × 0.5 × [1 + √(1 - 3.53K)] = 500 × 0.5 × [1 + √(1 - 3.53 × 0.111)] = 500 × 0.891 = 445 mm

As,req = MEd / (0.87 fyk z) = 250 × 10⁶ / (0.87 × 500 × 445) = 1292 mm²

Step 3 — Select bars: Use 5 H20 bars (As = 1571 mm²) — Satisfactory (As,prov > As,req)

Step 4 — Check minimum spacing: Bar spacing = (300 - 2 × 40 cover - 2 × 10 links - 5 × 20) / 4 = 40 mm EN 1992-1-1 requires spacing ≥ max(20 mm, 1.2 × aggregate size, bar diameter) = 20 mm — OK

Standard Fabric Mesh (BS 4483)

Mesh Type Longitudinal Wire (mm) Transverse Wire (mm) Mesh Size (mm) Mass (kg/m²)
A142 6 6 200 × 200 2.22
A193 7 7 200 × 200 3.02
A252 8 8 200 × 200 3.95
A393 10 10 200 × 200 6.16
B503 8 6 100 × 200 4.53
B785 10 6 100 × 200 6.72
C503 8 6 100 × 100 5.76
C785 10 8 100 × 100 8.76

Weldability and Bendability

Per BS 4449, all B500B and B500C bars are weldable, with carbon equivalent value (CEV) ≤ 0.50% (BS 4449 Clause 7.3). Standard mandrel diameters for bend tests:

Design Resources

Frequently Asked Questions

What rebar sizes are standard in the UK?

BS 4449 standard sizes: 8, 10, 12, 16, 20, 25, 32, 40 mm diameter. Grade B500B (fy = 500 MPa) is standard for UK building structures and is used for beams, columns, slabs, and foundations. B500C is required for seismic applications per EN 1998-1 and for critical structural elements where higher ductility is needed. Bars are supplied in standard 12 m lengths, cut and bent to schedule by the reinforcement supplier.

What is the ductility class B vs C in BS 4449?

Grade B500B has Agt ≥ 5.0% (medium ductility). Grade B500C has Agt ≥ 7.5% (high ductility). B500C is specified for seismic detailing per EN 1998-1 where energy dissipation through plastic hinging is required. The key difference is that B500C provides greater elongation capacity after yield, which is essential for structures designed with ductility classes DCM or DCH. In practice, B500C also has tighter ft/fy ratio limits (1.15 ≤ k ≤ 1.35) to ensure predictable overstrength.

What is the minimum concrete cover for UK reinforced concrete?

EN 1992-1-1 Clause 4.4.1 specifies minimum cover based on exposure class and structural class. For UK building interiors (XC1): minimum cover = 15 mm (slabs), 20 mm (beams), 25 mm (columns). For external (XC3/XC4): minimum cover = 25 mm (slabs), 30 mm (beams), 35 mm (columns). Cover must be increased by 10 mm for concrete cast against ground (XF1). The UK NA does not modify these values.

What lap lengths are required for UK rebar?

Lap length l₀ = α₁ α₂ α₃ α₄ α₅ α₆ l₀,reqd ≥ l₀,min per EN 1992-1-1 Clause 8.7.3. For B500B in C30/37: basic anchorage length lb,rqd ≈ 40-50 × bar diameter depending on bond conditions. For a 20 mm bar in good bond conditions: lb,rqd ≈ 800 mm. Compression laps are typically 30-40 × diameter. UK practice typically uses 40-50 × diameter for tension laps in slabs and beams.

How is reinforcement area calculated for slabs?

For UK slabs designed to EN 1992-1-1, the required reinforcement area per metre width As = MEd / (0.87 fyk z). Bar spacing governs the selection: for 12 mm bars at 150 mm centres, As = 754 mm²/m. Maximum spacing is typically 3h or 400 mm (whichever smaller) per Clause 9.3.1.1(3). Minimum reinforcement: As,min = 0.26 (fctm/fyk) bt d ≥ 0.0013 bt d for tension reinforcement.

Reference only. Verify all values against the current edition of BS 4449:2005+A3:2016 and EN 1992-1-1. This information does not constitute professional engineering advice.