Rebar Calculator
Calculate rebar length, bar count and weight for a slab or footing grid.
About Rebar Calculator
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.
Formula
Rebar
Weight per metre = d² ÷ 162 for steel; bars each way = span ÷ spacing + 1
- spacing
- Centre-to-centre distance between parallel bars
- d² ÷ 162
- Standard shortcut for the mass of round steel bar in kilograms per metre
Worked example
With the default values below, Rebar Calculator returns Total Rebar Weight of 403.82. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Slab lengthminimum 0 m
- 8 m
- Slab widthminimum 0 m
- 5 m
- Bar spacing, both directionsminimum 10 mm
- 200 mm
- Bar diameteraccepts 4–50 mm
- 12 mm
- Lap and waste allowanceaccepts 0–100 %
- 10 %
Results
- Total Rebar Weight
- 403.82 kg
- Total Bar Length
- 454.30 m
- Bars Running Lengthwise
- 26
- Bars Running Widthwise
- 41
- Weight per Metre
- 0.8889 kg/m
Step by step
- 1Values used
- 2Rebar
- 3Total Rebar Weight= 403.82 kg
- 4Total Bar Length= 454.30 m
- 5Bars Running Lengthwise= 26
- 6Bars Running Widthwise= 41
- 7Weight per Metre= 0.8889 kg/m
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Rebar calculated?
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.
Why does Rebar matter?
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.
What values do I need to enter?
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.