Calculate rebar length, bar count and weight for a slab or footing grid.
Bar counts add one because both edges need a bar, and the d² over 162 rule gives steel mass directly from diameter. Laps are added as a percentage because splice positions depend on the bar lengths actually delivered. Rebar is ordered by weight but placed by spacing, so the conversion is where quantity errors appear, and a 10 per cent lap allowance is the usual minimum on a gridded slab.
Rebar
Weight per metre = d² ÷ 162 for steel; bars each way = span ÷ spacing + 1
Weight per metre = d² ÷ 162 for steel; bars each way = span ÷ spacing + 1 Bar counts add one because both edges need a bar, and the d² over 162 rule gives steel mass directly from diameter. Laps are added as a percentage because splice positions depend on the bar lengths actually delivered.
Rebar is ordered by weight but placed by spacing, so the conversion is where quantity errors appear, and a 10 per cent lap allowance is the usual minimum on a gridded slab.
This calculator takes 5 inputs: Slab length, Slab width, Bar spacing, both directions, Bar diameter, Lap and waste allowance. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.