Rebar Spacing Calculator
Work out how many bars fit across a span at a given maximum spacing, and the total bar length for a one-way or two-way mat.
Formula
Spacing in a specification is a maximum. With n bars there are (n − 1) gaps, so the number of gaps is the clear span divided by the maximum spacing, rounded up. Bars = gaps + 1.
- Clear span (m) = Span − 2 × Cover
- Gaps = ceiling( Clear span across ÷ Maximum spacing )
- Number of bars = Gaps + 1
- Achieved spacing (m) = Clear span across ÷ Gaps (always ≤ maximum)
- Cut length (m) = Clear span along the bar
- Total length (m) = Number of bars × Cut length, summed over both directions
Worked example
A 6 m × 4 m slab with 200 mm maximum spacing both ways and 40 mm cover, reinforced two ways.
- Clear A = 6 − 0.08 = 5.92 m; Clear B = 4 − 0.08 = 3.92 m
- Direction A bars: gaps = ceiling(3.92 ÷ 0.2) = ceiling(19.6) = 20, so 21 bars
- Achieved spacing = 3.92 ÷ 20 = 0.196 m = 196 mm, within the 200 mm maximum
- Direction B bars: gaps = ceiling(5.92 ÷ 0.2) = ceiling(29.6) = 30, so 31 bars
- Achieved spacing = 5.92 ÷ 30 = 0.1973 m = 197 mm, within the maximum
- Total length = 21 × 5.92 + 31 × 3.92 = 124.32 + 121.52 = 245.84 m
52 bars, 245.84 m of bar, with both directions inside the 200 mm maximum spacing.
Assumptions and limitations
- The spacing you enter is treated as a maximum, which is how specifications normally express it. If your drawing gives an exact spacing to be set out, the achieved spacing shown here may be slightly tighter.
- Spacing is uniform across the whole slab. Real detailing often has different spacing in different zones.
- No laps, hooks or bends are included. Cut length here is the straight clear length only.
- This tool counts bars. It does not check whether the spacing or bar size satisfies any design requirement.
What this calculator does not do
- Does not size reinforcement or check any structural requirement.
- Does not calculate lap lengths, hooks, bends or chair supports.
- Does not handle varying spacing zones, additional bars at openings, or edge trim bars.
Practical tips
- Rounding a bar count down looks harmless and is not. One bar fewer across a 4 m slab pushes 200 mm spacing out to 206 mm, which fails a specification written as a maximum.
- Add laps separately. On slabs longer than the available bar stock, lap lengths can add several percent to the steel weight.
- The intersection count is a useful proxy for tying labour and tie wire quantity.
Frequently asked questions
How many rebars do I need at 200 mm spacing?
Divide the clear span across the bars by 0.2, round the result up, then add one. For a 4 m width with 40 mm cover each side that is ceiling(3.92 ÷ 0.2) + 1 = 21 bars, giving 196 mm actual spacing.
Why round up rather than down?
Because specifications give spacing as a maximum. Rounding down gives one bar fewer and pushes the actual spacing wider than allowed — on a 4 m slab at 200 mm nominal, that produces 206 mm, which does not comply.
Why do you add one bar?
Because spacing counts the gaps between bars, not the bars themselves. Twenty gaps means twenty-one bars.
Should cover be deducted from the bar length?
Yes. Bars must finish inside the concrete with the specified cover at each end, so the cut length is the slab dimension minus cover at both ends.
Does this include lap lengths?
No. Laps depend on bar size, concrete grade, bar position and the design code, so they should be taken from your bar bending schedule and added separately.
Related calculators
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- Concrete Slab Calculator Size a floor or ground slab: volume, ready-mix load, weight and area, including an edge thickening if there is one.
- Concrete Volume Calculator Work out the volume of concrete needed for a rectangular pour, with an allowance for wastage.
- Area Calculator Calculate the area of a rectangle, triangle, circle or trapezoid, with results in metric and imperial units.
Estimating tool, not a design. This calculator works out quantities from the dimensions you enter. It does not size any structural member, verify a design, or confirm compliance with any building code. Check every result against your project specification and applicable local standards, and involve a qualified engineer where the work requires one.