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Rebar Spacing Calculator

Enter the span, target spacing and edge cover to count the rebars needed.

Result

16
193.33 mm
2,900.00 mm

How it is calculated

All lengths are in millimetres. The clear width is the span minus the edge cover on both sides: clear = span − 2 × cover. The tool treats the spacing you enter as a maximum, so the number of gaps is rounded up: gaps = ceil(clear / spacing) and bars = gaps + 1 (one bar on each end of the clear width). The bars are then spread evenly: actual spacing = clear / gaps, which is equal to or smaller than the target. There are always at least two bars, one at each end of the clear width, even when the clear width is smaller than the spacing you entered. The first and last bars sit exactly one cover distance from the edges, so the tool measures to bar centres and does not subtract bar diameter. Span and spacing must be above zero, cover cannot be negative, and the cover must leave a clear width above zero. Check the result against the spacing limits of your design code.

Worked example

Span 3000 mm, target spacing 200 mm, cover 50 mm. Clear width: 3000 − 2 × 50 = 2,900 mm. Gaps: 2900 / 200 = 14.5, rounded up to 15. Bars: 15 + 1 = 16. Actual spacing: 2900 / 15 = 193.33 mm, which is under the 200 mm limit. Had the span been 2900 mm, the clear width of 2,800 mm would divide exactly into 14 gaps of 200 mm, so there would be 15 bars.

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Common mistakes

  • Rounding down the gap count and calling the result “about 15 bars”. Because spacing is a maximum, 14.5 gaps must become 15, giving 16 bars; fewer bars would break the spacing limit.
  • Entering the span in metres or centimetres. All three fields are in millimetres, so 3 instead of 3000 makes the cover larger than the span and returns no result.
  • Confusing the number of gaps with the number of bars. A row of 16 bars has only 15 gaps, which is the divisor in the actual spacing.
  • Forgetting that cover is applied on both sides. Clear width is the span minus twice the cover, not minus one cover.
  • Using the actual spacing as a rounded drawing dimension. 193.33 mm is the even distribution; detailers usually round it down to a practical figure such as 190 mm.

About this calculator

Reinforced concrete detailing needs an exact bar count and spacing for a given span: given the span length, the concrete cover required at each edge, and a target center-to-center spacing, this calculator treats the spacing as a maximum, finds how many bars are needed so no gap exceeds it — n = ⌈(span − 2×cover) / spacing⌉ + 1 — and then recalculates the actual even spacing, which is therefore never wider than the target, so the bars land symmetrically rather than bunched at one side, which is how spacing is normally rounded on real drawings.

Structural engineers use this when preparing reinforcement schedules to satisfy code-required minimum and maximum bar spacing (per ACI 318 or equivalent), rebar detailers use it to produce accurate shop drawings, and concrete contractors use it on site to lay out and tie bars correctly before a pour.

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Comments (1)

  • Editorial team

    The actual spacing is measured centre to centre of the bars, so it is not the gap you can see. Subtract the bar diameter to get the clear space between neighbouring bars: with the default result of 193.33 mm and 12 mm bars, the clear gap is 181.33 mm. That is the figure to hold against minimum-clearance rules for placing concrete around the bars. Which code or maximum spacing are you detailing to?

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