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Rebar Calculator

Calculate how many bars and what total length of rebar you need for any concrete slab, footing or retaining wall. Get bar count, weight and cost estimate instantly. Metric and imperial.

Quick answer: Rebar spacing of 200mm in both directions on a 5m × 4m slab: (5÷0.2+1) × 4m + (4÷0.2+1) × 5m = 26×4 + 21×5 = 104m + 105m = 209m of rebar total.

🔨 Rebar Calculator

Typical slab: 200mm. Footing: 300mm.
Total Rebar Length
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📐 How It Works

Rebar quantity is calculated by counting the number of bars in each direction and multiplying by the span in that direction. Bars in the length direction run the full width; bars in the width direction run the full length. Add 10% for laps, offcuts and waste.

Bars in L-direction = (Length ÷ Spacing) + 1 Bars in W-direction = (Width ÷ Spacing) + 1 Total Length = (BarsL × Width + BarsW × Length) × Layers × 1.10

📋 Worked Example

5m × 4m slab, N12 at 200mm, two layers: BarsL = 26, BarsW = 21. Length = (26×4 + 21×5) × 2 = (104+105) × 2 = 418m net. With 10% waste = 460m. Weight = 460 × 0.888 = 408kg.

❓ Frequently Asked Questions

Most residential slabs use N12 (12mm diameter) bars. N10 is used for light slabs and pathways. N16 and above are for heavily loaded slabs, footings and structural elements. Always follow your engineer's specification — slab reinforcement requirements depend on soil conditions, loading and slab thickness.

Standard residential slab: 200mm × 200mm grid (SL82 mesh equivalent). Heavier loads or poor soil: 150mm spacing or engineer-specified. Footings: typically 300mm spacing with additional ties. Always verify with your structural engineer's specification.

Mesh (SL72, SL82, RL918 etc.) is faster to install and commonly used for residential slabs. Rebar is used where specific bar sizes, spacing or lapping is required by the engineer. Both are structurally equivalent when specified correctly. Cost is similar — mesh saves labour, rebar has less waste.

Minimum concrete cover to rebar: residential slab on ground 40mm (bottom), walls 20–40mm, suspended slabs 20mm (top) and 25mm (bottom). Exposed to weather: 45–50mm. Near coast: 65mm. Check AS 3600 for your specific exposure classification.