Rebar Spacing for Concrete: Rules, Tables, Drawing Notation

Why Does Rebar Spacing Control the Strength of Concrete?

Concrete resists compression but fails in tension. Steel rebar handles the tension. The amount of steel per metre of width — measured in mm²/m — determines how much tension force the section can carry. Spacing controls that amount: T16 at 200 mm centres gives 1,005 mm²/m. Reduce the spacing to 150 mm and the same bar gives 1,340 mm²/m — a 33% increase in steel area without changing bar size.

The engineer calculates the bending moment and shear force at each point in the structure, then selects a bar size and spacing that provides enough steel area to resist those forces. A ground-bearing slab with low loading might need T12 at 200 mm (565 mm²/m). A suspended slab spanning 5 m with heavy loading might need T20 at 150 mm (2,094 mm²/m). The spacing is never arbitrary — it comes directly from the structural calculation.

What Are the Minimum Spacing Rules for Rebar?

Eurocode 2 (BS EN 1992-1-1, Clause 8.2) sets three minimum clear spacing rules. The clear gap between adjacent bars must be at least: the diameter of the largest bar, 25 mm, or the maximum aggregate size plus 5 mm. Whichever of these three values is largest governs.

For T16 bars with 20 mm aggregate: bar diameter = 16 mm, absolute minimum = 25 mm, aggregate + 5 = 25 mm. The minimum clear gap is 25 mm. Centre-to-centre spacing = 25 mm gap + 16 mm bar = 41 mm minimum. In practice, engineers rarely specify closer than 75 mm centres for T16.

For T25 bars with 20 mm aggregate: bar diameter = 25 mm, absolute minimum = 25 mm, aggregate + 5 = 25 mm. The minimum clear gap is 25 mm. But if you use 40 mm aggregate (common in mass concrete), the minimum gap becomes 45 mm. Centre-to-centre spacing = 45 + 25 = 70 mm minimum. Tight spacings in congested areas need smaller aggregate or a self-compacting mix.

What Are the Maximum Spacing Limits?

Maximum spacing prevents wide gaps between bars where concrete cracks without restraint. For slabs, Eurocode 2 Clause 9.3.1.1 limits main bar spacing to the lesser of 3h (three times the slab depth) or 400 mm. A 150 mm slab: 3 × 150 = 450 mm, but 400 mm governs — maximum spacing is 400 mm. A 100 mm slab: 3 × 100 = 300 mm governs.

Secondary (distribution) bars in slabs have a slightly wider limit: 3.5h or 450 mm. For beam shear links, the maximum is 0.75d (d = effective depth) or 600 mm. A 450 mm deep beam with 40 mm cover and T32 main bars has d = 394 mm, so 0.75 × 394 = 296 mm maximum link spacing.

Column links have the tightest rules: the lesser of 20 times the longitudinal bar diameter, the smallest column dimension, or 400 mm. A 300 mm column with T20 bars: 20 × 20 = 400, but the column width of 300 mm governs. Links at 300 mm maximum.

How Do You Read Spacing from Reinforcement Drawings?

Rebar spacing is written on drawings as “T16-200”: bar size (T16 = 16 mm diameter), dash, centre-to-centre spacing (200 mm). A suffix indicates position: B1 = bottom layer 1, T1 = top layer 1, B2 = bottom layer 2, EF = each face. “T16-200 B1” means T16 bars at 200 mm centres in the bottom layer.

Beam reinforcement uses a different format. “2T25 + 2T20” means 2 bars of T25 and 2 bars of T20 in one layer. The engineer shows the cross-section with each bar marked. Links are noted as “T8-200 links” — T8 stirrups at 200 mm centres along the beam length.

When the drawing says T12-150, measure 150 mm from the centre of one bar to the centre of the next. Not 150 mm gap between bars — that would give 162 mm centres. Use a spacer bar cut to 150 mm or mark the formwork at 150 mm intervals. Accuracy matters: a 10 mm error over 20 bars shifts the last bar by 200 mm.

How Accurate Does Spacing Need to Be on Site?

BS 8666:2020 and the National Structural Concrete Specification allow a spacing tolerance of ±15 mm or ±5% (whichever is greater) for bars placed on site. T16-200 means each bar should be at 200 mm ±10 mm centres. If Building Control measures 230 mm between two bars, that exceeds tolerance and they may fail the inspection.

Mark the spacing on the formwork or blinding before laying bars. For a slab with T12-150 B1, mark every 150 mm along two opposite edges of the slab. Lay bars to the marks and tie at every second intersection. This keeps spacing consistent across the full slab width.

Where bars converge at laps and intersections, spacing can tighten. Two bars lapped at the same point effectively halve the spacing locally. The engineer accounts for this in the design, but check that the concrete can still flow between the congested bars. If the gap drops below 25 mm at a lap, use a smaller aggregate or vibrate the concrete more thoroughly at that location. NextDaySteel supplies T8 to T40 rebar to BS 4449:2005+A3:2016, cut and bent to BS 8666:2020.

Important: This guide is for general information only. All structural reinforcement must be specified by a qualified structural engineer. Always follow the engineer's specification and relevant British Standards for your project. NextDaySteel supplies materials only.