Rebar Spacing and Quantity Calculator

How do you count reinforcing bars?

When bars are laid at a given spacing in a slab or footing, deduct the edge distance at both ends, divide the remaining length by the spacing and add one to the number of gaps.

n = (L − 2c) / s + 1

If the division is not exact, round the number of gaps up. Rounding down places the bars further apart than the drawing asks for.

Spacing from a bar count

When fitting a set number of bars into a narrow section such as a beam, work the other way: s = (L − 2c) / (n − 1). What matters here is the clear gap between bars, which is the centre spacing minus the bar diameter. Concrete and the vibrator must pass through it.

Spacing from a required area

Structural calculations usually give a steel area per metre (for example 5 cm²/m). Divide the area of the chosen bar by that value to get the largest spacing: s = (π ø² / 4) / a.

Example

12 mm bars at 150 mm spacing across a 4 m width, starting 50 mm from each edge: (4 − 0.10) / 0.15 = 26 gaps, so 27 bars. The area per metre is 113.1 / 0.15 = 754 mm², which is 7.54 cm²/m.

Limits

The tool does the geometry. It does not check code limits on maximum spacing, minimum clear gap or minimum reinforcement. Choosing the bar size and spacing belongs to the structural design.