Rebar Calculator
Find how many reinforcement bars a slab, beam or footing needs, the total length, weight and cost.
Inputs
Formula
- Bars = (span ÷ spacing) + 1
- Bar length = perpendicular span − 2 × cover + development length
- Weight = D² ÷ 162 × total length
Reinforcement is laid at a fixed spacing across each direction. Count the bars along one side, take each bar's length across the other side, then convert to weight with the D²/162 rule. A 10 mm bar weighs 0.617 kg per metre. Hooks and laps are covered by adding 10%.
Step-by-step calculation
- 1
Bar count
span ÷ spacing + 1
41 and 27
- 2
Total length
count × bar length + 10%
372.6 m
- 3
Weight
D²/162 × length = 0.616 × 372.6
229.6 kg
Worked example
- 1A 6 m × 4 m slab with 10 mm bars at 150 mm both ways.
- 2Along the 6 m side: 6000/150 + 1 = 41 bars of about 4 m.
- 3Along the 4 m side: 4000/150 + 1 = 28 bars of about 6 m → total 332 m ≈ 205 kg.
Tips
- Order 12 m bars where possible — fewer laps mean less waste.
- Keep the cover exactly as designed; too little cover is the main cause of rusting slabs.
Frequently asked questions
What spacing should I use for a house slab?
8 mm or 10 mm bars at 125–150 mm centres is common for a 4–5 m residential span, but always follow your structural drawing.
How much extra steel for laps?
The calculator adds 10% for laps, hooks and wastage, which matches normal site practice.
How do I convert to bar counts?
Divide the total length by the bar length you buy — usually 12 m.
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