Rebar Spacing Calculator

Calculate rebar spacing and bar quantity for slabs, walls, and footings

This rebar spacing calculator helps you plan evenly spaced reinforcement along a straight run, like a slab strip, wall length, footing run, or any one dimensional section where bars are placed in a line. You can use it in two practical ways. If you know the spacing you want (for example 150 mm or 200 mm), it will estimate how many bars you should place and what the resulting actual spacing will be after accounting for the real usable length. If you already know the number of bars you have or the number you want to use, it will calculate the exact center-to-center spacing that fits them evenly.

The key input most people forget is cover. Real reinforcement rarely starts exactly at the edge. Concrete cover is the gap from the outside face to the reinforcement, and it reduces the usable length for bar centers. This calculator treats cover as an end allowance on both sides. That means the spacing is calculated across the “effective length” between the first and last bar centers, not the full outside-to-outside length. If you do not know the cover, you can leave it blank and the calculator will assume a typical value (you can override it any time).

For quick estimates, you can input a length in meters and a target spacing in millimeters, then get a clear count of bars and the practical spacing you will actually build. For more accurate planning, switch to the “exact spacing from number of bars” mode. That is useful when your drawing, supplier bundle, or site constraints force a fixed bar count and you need to check the resulting spacing against a requirement or typical practice.

Assumptions and how to use this calculator

• The run is straight and bars are placed in a single line with equal center-to-center spacing between adjacent bars.
• Concrete cover is applied at both ends of the run. The calculator uses an “effective length” equal to total length minus two times the cover.
• If you leave cover blank, the calculator assumes 40 mm cover as a practical default. You should replace this with your project requirement where possible.
• When you enter a target spacing, the calculator aims to keep actual spacing at or below the target by increasing the number of spaces as needed.
• This tool does not replace structural design. It does not check bar diameter, lap lengths, anchorage, bend shapes, or code compliance for your specific application.

Common questions

What does “actual spacing” mean and why is it different from the target spacing?

Actual spacing is the exact center-to-center distance that fits evenly across the effective length after cover is removed. If the effective length is not a perfect multiple of your target spacing, you have to choose between uneven gaps or slightly adjusting the spacing. This calculator keeps spacing uniform and adjusts it to the nearest practical value that fits the full run.

Why does adding cover change the spacing and bar count?

Cover reduces the usable distance between the first and last bar centers. With less effective length, the same number of spaces produces smaller spacing, or the same target spacing results in fewer spaces. If you ignore cover, you can end up with a bar layout that does not fit on site or that shifts bars too close to the edges.

What if I do not know the cover for my slab or wall?

Leave it blank and use the default, then treat the result as a planning estimate. When you get drawings or a specification, update the cover and re-calculate. If your project has exposure conditions, fire requirements, or aggressive environments, the required cover may be higher than typical.

How do I use this for a slab grid where bars run both directions?

Run the calculator twice: once for the slab length direction and once for the slab width direction. Use the relevant run length in each direction, and apply the same cover rule. The outputs give you the bar count and spacing per direction. You can then estimate total bars by multiplying counts by the number of strips or by using a separate quantity calculator.

When does this calculator not apply?

If your reinforcement layout is not evenly spaced, uses variable spacing, includes openings, requires bar curtailment, or follows detailed structural drawings with different zones, this tool is only a rough guide. Also, if bars must start from a fixed reference point (like a starter bar position) and the end is allowed to vary, you may want to set the first bar position on site and accept a non-uniform last gap instead of forcing equal spacing.