How it is calculated
For a mesh the calculator subtracts the concrete cover at the edges, divides the sizes by the spacing and gets the number of bars each way. If a bar is longer than the bars sold, a lap of 40 diameters is added. Weight uses 0.00617 × d² kg per metre. Bars to buy are chosen to keep offcuts small.
Bars along the length = (width − 2 × cover) ÷ spacing + 1Weight of 1 m = 0.00617 × d² (d in mm)Lap at a joint = 40 × dExample
A 6 × 4 m slab with two meshes at 200 mm in Ø12 rebar has 100 cut bars, 470 m in total and 418 kg. With 6 m bars you need to buy 100 bars and about 5.3 kg of tie wire.
Tips from practice
- Lift the bottom mesh 40–50 mm off the base on spacers and hold the top mesh on chairs.
- Stagger the laps of neighbouring bars so they do not line up.
- Rebar is often sold by the tonne: one tonne of 12 mm bar is 1,126 m.
Questions and answers
How much does a metre of 12 mm rebar weigh?
0.888 kg. A metre of 10 mm is 0.617 kg, 8 mm is 0.395 kg and 14 mm is 1.21 kg.
What mesh spacing should a slab have?
Raft slabs usually have two meshes at 150–200 mm; a ground slab for a garage 150–200 mm as well.
How much tie wire do I need?
Each tie uses about 0.5 m of 1.2 mm wire. The calculator counts the ties and converts them to kilograms.
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