Rebar Calculator
Counts the bars. Does not design the slab.
Work out Rebar. Counts the bars. Does not design the slab. Answers in the units the job actually uses.
Reinforcement weight
174 kg
45 bars · 196.4 linear metres
Reinforcement carries tension, which concrete cannot. Concrete is strong in compression and roughly a tenth as strong in tension, so steel goes wherever the slab will try to stretch — near the bottom in the middle of a span, and near the top over supports. Cover is what protects the steel from corrosion and fire, and it is the dimension most often lost on site. Too little and the bars rust, expand and spall the concrete off; too much and the steel sits too close to the neutral axis to do its job. The tie count assumes every intersection is tied, which is normal for a mat that has to hold its position during the pour. Alternate-intersection tying is common on large slabs and roughly halves both the wire and the labour.
How the Rebar Calculator works
Bar count, length and weight for a two-way mat at a stated size and spacing, with the tying-wire intersections. This counts a layout you specify — it does not design the reinforcement.
Also known as: how much rebar for a slab · rebar weight per metre · reinforcement bar spacing · steel for a concrete base
Frequently asked questions
How much rebar does a slab need?
That is a structural design question rather than a quantity one. Bar size, spacing and cover come from an engineer working to the applicable code, and this page counts what you tell it.
What does rebar weigh?
12 mm bar is 0.888 kg per metre and 16 mm is 1.579 — mass goes with the square of the diameter. A 5 × 4 m slab at 200 mm centres in 12 mm bar comes to roughly 160 kg.
Why does concrete need reinforcement?
Concrete is strong in compression and roughly a tenth as strong in tension. Steel goes wherever the slab will try to stretch — near the bottom mid-span, near the top over supports.
What is cover and why does it matter?
The concrete between bar and surface, protecting the steel from corrosion and fire. Too little and the bars rust, expand and spall the concrete off; too much and the steel sits too close to the neutral axis to work.
Do I need to tie every intersection?
For a mat that must hold position during a pour, usually yes. Alternate-intersection tying is common on large slabs and roughly halves both the wire and the labour.
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