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Rebar Spacing and Bar Count Calculator

Bars and actual spacing from run length and design spacing, with total weight.

Quick answer

With the default inputs: Run length 6000 · Design spacing 200 · Bar diameter 12 etc.Number of bars 30 bars

Edit the inputs below to recalculate.
Design spacing mmBar diameter mm
The pitch is design; actual layout rounds length ÷ pitch and back-calculates the real pitch. End cover changes the count.

Inputs

levels

Result

Number of bars
30bars
Net run5950mm
Actual spacing205.2mm
Deviation from design5.2mm
Total with layers30bars
Total length180.0m
Unit weight0.888kg/m
Total weight159.9kg

Counting is net run ÷ spacing + 1, so actual spacing comes out slightly wider — that is normal. If it exceeds design by 5%, add a bar. Slab bars usually start half a spacing from the edge, not at the cover line.

Formula

Net runL = run − 2 × cover
Bar countn = floor(L ÷ spacing) + 1
Actual spacings = L ÷ (n − 1)
Total weightW = n × L × unit weight
A 6000 mm run with 25 mm cover at 200 mm spacing, 12 mm bars: 5950 mm net gives 30 bars at 205.2 mm actual spacing, 178.5 m total and 158.5 kg.

How to work it out

  1. Clear length L = placing length − 2 × cover, cover being measured to the outside of the outermost bar.
  2. Count n = ⌊L ÷ design spacing⌋ + 1. Round down then add one: spacing must never exceed the design value, so you may add a bar but never drop one.
  3. Actual spacing = L ÷ (n − 1). Space them evenly rather than leaving one odd gap.
  4. Total length = n × bar length × number of layers.
  5. Weight W = total length × 0.00617 × d², with d in mm.

Where it goes wrong

Material and cost

An extra bar is cheap (about 32 for a 6 m Φ12) while one too few can leave the section under-reinforced. Crowd rather than skimp — but not so much that concrete cannot enter: clear spacing below 1.25 times the aggregate size causes honeycombing.

On site

Basis

FAQ

Why 30 bars and not 29?
Both ends carry a bar, so the count is always the number of spaces plus one. 5950 ÷ 200 = 29.75 gives 29 full spaces and therefore 30 bars. Forgetting the plus-one is the classic error, and across hundreds of beams it adds up to tonnes of steel.
How far from the edge does slab reinforcement start?
Half a spacing, not one cover. At 200 mm spacing the first bar sits 100 mm from the beam or wall face. Support moments are high at the edge and bars set too far in leave an unreinforced strip, while half a spacing is also easier to work around. Rafts and wall boundary elements follow similar rules — read the drawing notes.
Is slightly wider spacing acceptable?
Tolerances exist (typically ±10 mm and not more than 5% of design), but wider spacing means less steel per metre and lower capacity. If rounding pushes you past the tolerance, add a bar rather than spreading the spacing evenly.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Rebar Spacing and Bar Count Calculator. 2026-10-25. https://jianzhujisuan.com/en/rebar-spacing

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.