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Rebar Spacing Calculator

Work out rebar spacing instantly with clear inputs, formula shown and shareable results.

About Rebar Spacing Calculator

Spacing is set by how much steel area one bar contributes: 1000 mm of width divided by the required area per metre times the area of a single bar. The result is rounded down to a practical 25 mm increment and capped at the maximum spacing the code permits for crack control.

Formulas

Spacing

s = 1000 x area of one bar / required steel area per metre

Provided steel

Ast provided = (1000 / s) x area of one bar

Worked example

With the default values below, Rebar Spacing Calculator returns Bar spacing to use of 125. Change any input above and every figure here is recalculated in your browser.

Inputs used

Steel area requiredminimum 1 mm² per m
800 mm² per m
Bar diameterminimum 4 mm
12 mm
Maximum permitted spacingminimum 25 mm
300 mm

Results

Bar spacing to use
125 mm
Bars per metre width
8.00
Steel area provided
904.8 mm² per m
Area of one bar
113.10 mm²

Step by step

  1. 1Values used
  2. 2Spacing
  3. 3Provided steel
  4. 4Bar spacing to use= 125 mm
  5. 5Bars per metre width= 8.00
  6. 6Steel area provided= 904.8 mm² per m
  7. 7Area of one bar= 113.10 mm²

Assumptions and limits

  • The result is derived only from the 3 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 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Reinforcement detailing must be designed and checked by a qualified structural engineer against the governing code.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Reinforcement detailing must be designed and checked by a qualified structural engineer against the governing code.

Frequently Asked Questions

How is the Rebar Spacing figure worked out?

s = 1000 x area of one bar / required steel area per metre. Spacing is set by how much steel area one bar contributes: 1000 mm of width divided by the required area per metre times the area of a single bar.

Why cap the spacing?

Widely spaced bars leave the concrete between them to crack unrestrained. Codes limit spacing to about three times the slab thickness or 300 mm, whichever is less, for crack width control.

What values do I need to enter?

This calculator takes 3 inputs: Steel area required, Bar diameter, Maximum permitted spacing. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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Rebar Spacing Calculator – Calculate Bar spacing to use