How many rebar bars do I need at a given spacing?

Take the end cover off both ends of the run, divide by the spacing, drop the part-space and add one for the first bar. A 24 ft run with 3 in cover at 12 in centres is (288 − 6) ÷ 12 = 23.5, so 23 + 1 = 24 bars. In metric, 7.2 m with 75 mm cover at 300 mm is (7,200 − 150) ÷ 300 = 23.5, also 24 bars.

What happens to the leftover space at the end?

With the spacing held exactly, the part-space ends up at the far end, so the last bar sits a little short of the cover line. If your spacing is a maximum rather than an exact figure, add one more bar and spread them evenly — the tool shows that count and the even spacing it gives.

How do I count laps when bars are longer than the stock length?

Each lap loses one lap length, so a bar made of k sticks covers k × stock − (k − 1) × lap. With 20 ft stock and a 24 in lap from the drawings, a 30 ft bar needs 2 pieces and 1 lap, 32 ft of steel. The lap length comes from your drawings or your engineer — this page never supplies one.

Does this tell me what spacing or bar size to use?

No. Size, spacing, cover and lap length come from the drawings or your local requirements. This page counts and totals the bars you were told to place. For a two-way slab grid, use the rebar grid calculator instead.

Rebar spacing calculator

Run length

The length the bars are spread along. Feet, or feet-inches like 24' 6".

Spacing (on centre)

End cover, each end

From your drawings.

Bar size

Length of each bar

Stock length

What your yard sells — commonly 20 ft sticks.

Lap length

Only needed when a bar is longer than the stock. From your drawings.

— bars

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Show the working — with your numbers

Counting and totalling only: bars along one run at the spacing you type, and laps at the lap length you type. It does not choose bar size, spacing, cover or lap length and says nothing about strength — those come from the drawings. Sticks assume offcuts are not reused; bends and hooks add length it does not count.

Type the run, the spacing and the end cover from the drawings, and how long each bar is. The tool counts the bars, totals the length, works out how many stock sticks to buy and — where a bar is longer than a stick — how many laps at the lap length you enter. For a slab laid both ways, use the rebar grid calculator.

Worked example

Imperial: #4 bars across a 24 ft run at 12 in centres with 3 in end cover, each bar 10 ft long. Usable run 288 − 6 = 282 in; 282 ÷ 12 = 23.5, so 23 + 1 = 24 bars, 240 ft of bar (73.2 m). Two 10 ft bars cut from each 20 ft stick: 12 sticks. The half space (6 in) lands at the far end — or use 25 bars evenly at 11.75 in.

Metric: 15M bars across 7.2 m at 300 mm with 75 mm cover, each bar 3 m. Usable 7,200 − 150 = 7,050 mm; 7,050 ÷ 300 = 23.5, so 24 bars, 72 m of bar. Two 3 m pieces from each 6 m stock length: 12 lengths. Evenly, 25 bars at 293.75 mm.

On the job

Laps add up. Sixteen 30 ft bars from 20 ft stock with a 24 in lap (the drawing’s figure): each bar is two pieces, one lap, 32 ft of steel — 512 ft, not 480. The short pieces are 12 ft, one per stick, so that is 32 sticks, half of them leaving an 8 ft offcut. Stagger the laps if the drawings ask for it.

Exact or maximum. “12 in o.c.” on a drawing is usually a maximum, and the half space at the end is where an inspector’s tape lands. Spreading one extra bar evenly keeps every space under the number.

Cut from the stick you buy. A 20 ft stick gives two 10 ft bars, three 6 ft bars (2 ft offcut) or one 11 ft bar with 9 ft left over. Set the bar length to what the shop cuts and the stick count follows.

What went wrong?

Quick referencefor bars along a run

Bar count: floor((run − 2 × cover) ÷ spacing) + 1
Per 10 ft at 12 in (no cover): 10 spaces, 11 bars · 3 m at 300 mm: 11 bars
Spacings: 12 in = 305 mm · 16 in = 406 mm · 300 mm = 11.8 in · 200 mm = 7.9 in
Stock: 20 ft = 6.10 m · 6 m = 19.7 ft · 12 m = 39.4 ft
Laps: pieces per bar = (bar − lap) ÷ (stock − lap), rounded up; laps = pieces − 1

The formula

bars = floor((run − 2 × cover) ÷ spacing) + 1

Total length is bars × bar length, plus one lap for every joint. A bar longer than the stock takes k pieces where k × stock − (k − 1) × lap reaches the bar length. Sticks are the whole pieces plus the cut pieces grouped as many to a stick as fit.

Questions

How many rebar bars do I need at a given spacing?: Take the end cover off both ends of the run, divide by the spacing, drop the part-space and add one for the first bar. A 24 ft run with 3 in cover at 12 in centres is (288 − 6) ÷ 12 = 23.5, so 23 + 1 = 24 bars. In metric, 7.2 m with 75 mm cover at 300 mm is (7,200 − 150) ÷ 300 = 23.5, also 24 bars.
What happens to the leftover space at the end?: With the spacing held exactly, the part-space ends up at the far end, so the last bar sits a little short of the cover line. If your spacing is a maximum rather than an exact figure, add one more bar and spread them evenly — the tool shows that count and the even spacing it gives.
How do I count laps when bars are longer than the stock length?: Each lap loses one lap length, so a bar made of k sticks covers k × stock − (k − 1) × lap. With 20 ft stock and a 24 in lap from the drawings, a 30 ft bar needs 2 pieces and 1 lap, 32 ft of steel. The lap length comes from your drawings or your engineer — this page never supplies one.
Does this tell me what spacing or bar size to use?: No. Size, spacing, cover and lap length come from the drawings or your local requirements. This page counts and totals the bars you were told to place. For a two-way slab grid, use the rebar grid calculator instead.

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