Rebar Spacing and Count
Work out how many rebar bars fit evenly across a slab, footing or wall and the exact equal centre-to-centre spacing. Enter the placement width, edge cover and target spacing.
Fit Rebar Evenly Across a Slab or Footing
Rebar Count Formula
Where:
- \(W\) — placement width, the overall slab, footing or wall dimension across which the bars are distributed
- \(c\) — edge cover, the concrete kept clear at each edge (applied on both sides)
- \(s\) — target maximum centre-to-centre spacing between bars
- \(n\) — required number of bars
- \(s_a\) — resulting equal centre-to-centre spacing
One bar is placed at each edge of the clear span \(W - 2c\), and the remaining bars are spread evenly in between, so the actual spacing \(s_a\) is the same for every gap.
Rebar Count – Calculation Example
Example: a slab section 300 cm wide, with 5 cm of concrete cover at each edge and a target spacing of 20 cm.
\[ W - 2c = 300 - 2 \times 5 = 290 \text{ cm} \]
\[ n = \left\lfloor \frac{290}{20} \right\rfloor + 1 = 14 + 1 = 15 \text{ bars} \]
\[ s_a = \frac{290}{15 - 1} = \frac{290}{14} \approx 20.71 \text{ cm} \]
Result: 15 bars, at an equal centre-to-centre spacing of approximately 20.71 cm (≈ 8.15 in). Note that the equalized spacing is slightly wider than the 20 cm target — see the practical tips below.
How to use the calculator
- Enter the Placement width (W) — the full dimension of the slab, footing or wall across which the bars will be distributed.
- Enter the Edge cover (c) — the clear concrete distance kept at each side. It is applied on both edges, so the clear span between the two outermost bars is \(W - 2c\).
- Enter the Target spacing (s) — the maximum centre-to-centre distance you want between bars.
- Select the unit (cm, m, mm, in or ft) next to each value; the three inputs do not have to share the same unit.
- Read the results: the required number of bars, the resulting equal spacing (c/c), and the formula with notes on typical cover and spacing limits.
What rebar spacing and count mean
Reinforcing bars in a slab, footing or wall are rarely placed at random: they are laid out on a regular grid, measured centre to centre, with a clear concrete cover at the edges to protect the steel from corrosion and fire. The number of bars and their spacing directly control how much steel crosses a given section, and therefore how well the concrete resists bending, cracking and shrinkage.
This calculator answers a very practical question: given a width, a cover and a maximum allowed spacing, how many bars do I actually need, and where do they go? Because the clear span \(W - 2c\) almost never divides evenly by the target spacing, the bars cannot sit at exactly the target distance and still land symmetrically. The standard solution is to place one bar at each edge of the clear span and distribute the rest evenly, which is what the formula does — the result is a layout that is simple to mark out and build on site.
When you interpret the output, remember that the spacing is centre-to-centre, not the clear gap between bar surfaces. If you are using, say, 16 mm bars at 20 cm centres, the clear gap between bars is only about 18.4 cm. Also note that the equalized spacing \(s_a\) can end up marginally wider than the target when the division is not exact — the section below explains what to do if your specification is strict.
Assumptions, limitations and practical tips
- Cover is applied on both sides. Make sure you enter the cover for one edge only; the calculator subtracts it twice. Typical concrete cover is 40–50 mm (1.5–2 in) for slabs on ground and footings, but your design code governs — check the minimum cover for your exposure conditions.
- Spacing limits. Most codes cap the maximum bar spacing, often at about three times the slab thickness or roughly 450–460 mm, whichever is smaller. Your target spacing should already respect these limits.
- Strict compliance. Because the calculator floors the intermediate count, the equalized spacing can slightly exceed the target (as in the example above, 20.71 cm vs 20 cm). If your specification requires the spacing to stay at or below the maximum, add one more bar: 16 bars in the example would give \(290 / 15 \approx 19.33\) cm, comfortably under the limit.
- Minimum steel. This tool only counts bars for a chosen spacing; it does not check whether that amount of reinforcement satisfies minimum steel ratio, crack control or bending capacity requirements. A structural check is still needed for final design — see the section modulus calculator and the uniformly loaded simply supported beam calculator for related strength checks.
- Two-way slabs. Run the calculation separately for each direction; corner and edge details may require additional bars that this simple one-dimensional layout does not include.
For layout work on site, the resulting equal spacing is usually rounded to a convenient marking dimension (e.g. 20.7 cm marked every 20.7 cm, or slightly adjusted to hit whole numbers on your tape). For other construction planning tasks, such as stair rise and run, the same even-distribution logic applies.
Frequently Asked Questions
How do I calculate rebar count and spacing by hand?
First subtract twice the edge cover from the placement width to get the clear span: \(W - 2c\). Divide that by your target spacing and round down, then add 1 for the bar at the far edge: \(n = \lfloor (W-2c)/s \rfloor + 1\). For the exact equal spacing, divide the clear span by the number of gaps: \(s_a = (W - 2c)/(n-1)\). For example, a 5 m wall with 50 mm cover and 30 cm target gives \(n = \lfloor 4.9/0.3 \rfloor + 1 = 17\) bars at \(4.9/16 \approx 30.6\) cm centres.
Why is the actual spacing slightly wider than my target spacing?
Because the bar count is rounded down to a whole number, the remaining gaps must stretch a little to fill the clear span symmetrically — the equalized spacing \(s_a\) comes out marginally larger than the target \(s\). If your specification is strict and the spacing must not exceed the maximum, simply add one more bar: the spacing then drops below the target. Always compare \(s_a\) against your code's maximum allowed spacing before ordering or tying the grid.
What is the difference between centre-to-centre and clear spacing?
Rebar spacing is conventionally measured from the centre of one bar to the centre of the next. The clear gap between bar surfaces is smaller by one bar diameter. For example, 16 mm bars at 20 cm centres leave only about 18.4 cm of concrete between them: \(20 - 1.6 = 18.4\) cm. This matters when checking minimum clear spacing rules, which typically require enough room for aggregate to pass and for the vibrator to fit between bars.
How do I convert rebar spacing between metric and imperial units?
The calculator handles mixed units automatically — you can enter the width in feet, the cover in inches and the spacing in cm. By hand, remember that 1 inch equals 2.54 cm exactly, so 12 in centres equal 30.48 cm, and 20 cm equals about 7.87 in. A common residential layout of bars at 18 in on centre corresponds to roughly 45.7 cm. Double-check conversions on drawings that mix unit systems, since spacing limits in codes are usually quoted in one system only.
What is a normal rebar spacing and cover for a slab or footing?
Typical centre-to-centre spacing for slabs and footings is 15–30 cm (6–12 in), with 20 cm (8 in) being very common for residential work. Concrete cover at edges is usually 40–50 mm (1.5–2 in) for slabs on ground and footings, but exposure conditions can require more. Most codes also cap spacing at about three times the slab thickness or roughly 450–460 mm, whichever is smaller. Your local design code and engineer's drawings always take precedence over these rule-of-thumb values.
Can I use this calculator for walls and two-way slabs?
Yes, but run the calculation separately for each direction. In a two-way slab you compute the bar count across the short direction and again across the long direction, forming a grid; the same applies to walls with vertical and horizontal reinforcement. Note that a one-dimensional layout does not include extra corner, edge or trimmer bars that some details require, and it does not check strength — it only tells you how many bars fit evenly and where to place them.
What happens if only one bar fits in the span?
If the clear span \(W - 2c\) is smaller than the target spacing, only a single bar fits, and there is no gap to measure — the calculator notes that the bar should be placed centred between the edge covers. If twice your cover equals or exceeds the placement width, the layout is impossible and you need a wider section or less cover. Also make sure you enter the cover for one edge only; the tool subtracts it on both sides automatically.