How Much Rebar Do I Need for a Slab?

Choosing bar spacing

Residential slabs commonly use 300-400mm spacing for light-duty slabs on grade, tightening to 150-200mm for slabs carrying structural loads or requiring crack control. Tighter spacing means more bars and more steel, but better distributes shrinkage cracking and load - follow your structural drawings when they're available rather than guessing.

Choosing bar diameter

Common slab reinforcement uses 10mm or 12mm bars for typical residential and light commercial work, moving up to 16-20mm for heavier structural slabs. Diameter and spacing trade off against each other - a structural engineer might specify wider spacing with a larger diameter bar to hit the same steel area per meter more economically.

Single mat vs. double mat

A single mat (one layer, usually placed on chairs near the mid-depth or lower third of the slab) suits slabs on grade with light, uniformly distributed loads. A double mat (top and bottom layers) is used where the slab experiences both positive and negative bending - elevated slabs, slabs over voids, or structural slabs designed by an engineer.

Converting the grid count to a material order

Once you know bars-per-direction from your slab dimensions and spacing, multiply by unit length and the bar's D²/162 weight-per-meter formula to get total steel weight - then add 5-10% for lap splices, since bars longer than standard stock lengths (commonly 6m or 12m) need to be overlapped and tied together, consuming extra length beyond the bare grid calculation.

Frequently Asked Questions