Slab rebar calculator (bar grid)
Enter the slab size, bar spacing and concrete cover to see the steel in both directions.
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How to use it
- Enter slab length, slab width, bar spacing, edge cover, bar diameter and mesh layers.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Bars along the length = (width − 2 × cover) ÷ spacing (down) + 1, each length − 2 × cover long, and the other way for the cross bars. Weight = total length × π ÷ 4 × d² × 7850 ÷ 1,000,000 × layers.
Example
Inputs: Slab length = 6 m, Slab width = 4 m, Bar spacing = 20 cm, Edge cover = 5 cm, Bar diameter = 12 mm. Result: Steel weight: 208.6 kg; Total bar length: 235 m; Bars along the length: 20 pcs.
Limits and caveats
- Laps, hooks and chairs for the top mesh are not added.
- Spacing and diameter come from the structural drawing; this is for take-off, not design.
Questions about this tool
How much steel for a 6 × 4 m slab at 20 cm spacing?
With 12 mm bars, 5 cm cover and one mesh: 50 bars, 235 m in total, about 209 kg.
Why add 1 to the bar count?
There is always one more bar than gaps, so 4 m at 20 cm spacing has 20 gaps and 21 bars.
What cover is usual for slabs?
Often 2 to 5 cm depending on exposure to moisture and soil; the structural drawing sets the binding figure.