Rebar Calculator
Works out rebar for a slab or footing from the spacing on the drawing — bar counts each way, total length, weight, stock bars, ties and chairs. The spacing and size are inputs because they are the design, and the design is not something a quantity calculator should guess.
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The formula
bars each way = floor((dimension − 2 × cover) ÷ spacing) + 1 total length = Σ bars × run length (+ laps) × layers × (1 + waste) weight = length × kg per metre
A rebar grid is two sets of parallel bars. The number running one way is set by the slab’s other dimension divided by the spacing. Weight per metre is fixed by the bar diameter — 12 mm bar is 0.888 kg/m, and #4 is almost identical at 0.996 — so weight follows directly from length.
What to watch out for
Things that go wrong on site rather than on paper:
- Cover. Bars too close to the surface rust and spall the concrete. Chairs hold the mesh at the right height — about one per square metre, more under foot traffic during the pour.
- Laps. Where bars join, they overlap by a length the drawing specifies, often 40 diameters. Those laps are extra length, and they are included here.
- Mesh may be simpler. For a plain ground-bearing slab, welded mesh sheets (A142, A193 and so on) are quicker to lay than loose bar at the same reinforcement.
Frequently Asked Questions
How much rebar do I need for a slab?
Bars each way = (dimension less cover) ÷ spacing + 1. A 6 × 4 m slab at 200 mm centres with 50 mm cover is 20 bars one way and 30 the other, about 247 m of bar with 5% waste — around 220 kg in 12 mm.
How much does rebar weigh per metre?
10 mm bar is 0.617 kg/m, 12 mm is 0.888, 16 mm is 1.578 and 20 mm is 2.466. In US sizes, #3 is 0.560 kg/m, #4 is 0.996 and #5 is 1.552.
What spacing should rebar be?
Whatever the drawing or engineer specifies — 150 to 300 mm is the common range for slabs. The spacing is part of the structural design and depends on the load, the slab thickness and the ground, so it is an input here rather than an answer.