Rebar Spacing Calculator
Bars go both ways — this is the count for one axis.
Work out Rebar Spacing. Bars go both ways — this is the count for one axis. Names the misconception directly.
Set by the span table for your load, timber grade and section
Across 5 m at 200 mm centres
26 bars
25 gaps, so 26 members
Reinforcement goes in both directions, so the total is this count for each axis. Bars need a lap where they join — typically 40 times the bar diameter — and cover from the concrete surface, usually 40 to 50 mm for external work, because reinforcement too close to the surface rusts and spalls the concrete off. Reinforcement is specified by an engineer for anything structural.
How the Rebar Spacing Calculator works
Reinforcement bars across a slab at a centre spacing, for one direction. Reinforcement usually runs both ways, so the total is this count for each axis plus laps at any joins.
Also known as: how much rebar for a slab · reinforcement bar spacing calculator · rebar quantity calculator · slab reinforcement layout
This counts one direction
Reinforcement in a slab normally runs both ways, forming a mat. The count here is for one axis — the total is this figure for each direction, plus additional length for laps where bars join.
A lap is the overlap that transfers force from one bar to the next, commonly around 40 bar diameters and set precisely by code and concrete grade. Bars simply butted end to end transfer nothing and the reinforcement is discontinuous at that point.
Welded mesh is the alternative for many slabs — faster to place with spacing fixed by manufacture. Loose bars allow size and spacing to vary across a slab where the structure needs them to, which mesh cannot.
Cover and position decide whether it works
Reinforcement needs cover — typically 40 to 50 mm from the concrete surface for external work. Steel too close to the surface rusts, expands as it does, and spalls the concrete off. That is how most reinforced concrete structures eventually fail.
Position within the depth matters as much. Reinforcement resists tension, so it belongs where the tension is — usually the lower third of a ground-bearing slab, and varying across the span in a suspended one.
Chairs and spacers hold it there during the pour. Reinforcement lying on the ground or on the membrane does nothing useful, and it is one of the most common site failures because it looks correct until the concrete covers it.
What reinforcement is and is not for
Concrete is strong in compression and weak in tension. Reinforcement supplies the tensile capacity, which is why its position matters so much — steel in the compression zone contributes very little.
Fibre reinforcement mixed into the concrete addresses shrinkage cracking, which is a different problem. It does not replace structural reinforcement, and marketing that blurs the two has caused real failures in slabs that needed bars.
For anything structural, the size, spacing and position come from an engineer rather than a rule of thumb. A garden path generally needs none; anything carrying vehicle loads or spanning ground that may move is a design problem rather than an arithmetic one.
Where to go next
The Rebar Spacing question rarely arrives on its own. These are the ones that usually come with it:
- Concrete Slab Calculator — Dry materials occupy 54% more than the wet mix.
- Joist Spacing Calculator — Depth beats width — stiffness goes with the cube.
- Driveway Gravel Calculator — The sub-base carries the load, not the gravel.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
How do I calculate rebar for a slab?
Divide the slab width by the spacing, add one, and repeat for the other direction. Then add length for laps where bars join — typically 40 times the bar diameter.
How much cover does rebar need?
Typically 40 to 50 mm from the concrete surface for external work, less internally. Reinforcement too close to the surface rusts, expands, and spalls the concrete off — which is how concrete structures fail.
Where in the slab should rebar sit?
It depends on where the tension is. In a ground-bearing slab that is usually the lower third; in a suspended slab it varies across the span. Chairs or spacers hold it at the right height during the pour.
What is a lap length?
The overlap where two bars join so the force transfers between them. Around 40 bar diameters is a common figure and the exact requirement is set by code and by the concrete grade.
Can I use mesh instead of loose bars?
For many slabs, yes — welded mesh is faster to place and its spacing is fixed by manufacture. Loose bars allow the spacing and size to vary where the structure needs it.
Does a garden path need reinforcement?
Usually not, though fibre reinforcement in the mix helps with shrinkage cracking. Anything carrying vehicle loads or spanning ground that may move should be designed by an engineer.
What size rebar should I use?
That comes from a structural design rather than a rule of thumb — bar size, spacing and position are all determined together for the load and the span. For anything structural it needs an engineer.
How do I tie rebar?
With wire ties at intersections, enough to hold the layout during the pour. The ties carry no structural load; their only job is stopping the mat shifting when concrete is placed on it.
What are chairs and spacers?
Supports that hold the reinforcement at the specified height in the slab. Reinforcement lying on the ground does nothing useful, and it is one of the most common site failures.
Can I use mesh instead of loose bars?
For many slabs, yes — welded mesh places faster and its spacing is fixed. Loose bars let size and spacing vary where the structure needs them to.
What is fibre reinforcement?
Fibres mixed into the concrete that control shrinkage cracking. They do not replace structural reinforcement — they address a different problem, and the marketing sometimes blurs the two.
Why does rebar need to be clean?
Light surface rust actually improves bond and loose flaking rust and oil do not. Anything that stops the concrete gripping the bar defeats the purpose of putting it there.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/rebar-spacing-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Rebar Spacing Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/rebar-spacing-calculator" style="color:#64748b">Rebar Spacing Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.
Related calculators
Concrete Slab Calculator
Dry materials occupy 54% more than the wet mix.
OpenJoist Spacing Calculator
Depth beats width — stiffness goes with the cube.
OpenDriveway Gravel Calculator
The sub-base carries the load, not the gravel.
OpenRing Size Calculator
Finger measurement to US, UK, EU and Japanese sizes.
Open