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Generated on: July 20, 2026 at 09:33 PM

Rebar Quantity Calculator

Inputs

Slab length5 m
Slab width4 m
Bar spacing30 cm
Overlap & waste10 %

This is a planning estimate for a simple rectangular grid. It does not size the reinforcement or set the spacing, lap length, cover, or bar diameter — those are engineering decisions governed by the slab’s loads and local code. Use your approved structural drawings for any load-bearing slab, footing, or driveway.

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Construction & DIY

Rebar Quantity Calculator

Estimate the number of reinforcing bars and the total bar length for a rectangular slab reinforced with a grid at a fixed spacing, plus an overlap and waste allowance.

Inputs

≥ 0 m
≥ 0 m
≥ 0.1 cm
%
0 – 50 %

Results

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Details

A 5 m × 4 m slab at 30 cm spacing needs ... bars and about ... of rebar, including overlap.

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Rebar Quantity

Reinforcing a concrete slab means laying a grid of steel bars at a regular spacing in both directions. The number of bars and the total length follow from the slab size and the spacing, and this calculator works them out along with an allowance for the overlaps where bars are spliced.

Counting the grid

Bars that run the long way are spaced across the width, so their count is the width divided by the spacing, plus one for the bar at the far edge:

nL=⌊Ws⌋+1\begin{aligned} n_L &= \left\lfloor \frac{W}{s} \right\rfloor + 1 \end{aligned}

Bars that run the short way are spaced along the length the same way:

nW=⌊Ls⌋+1\begin{aligned} n_W &= \left\lfloor \frac{L}{s} \right\rfloor + 1 \end{aligned}

Each lengthwise bar spans the length LL and each widthwise bar spans the width WW, so the total steel — with an overlap fraction kk for lap splices and offcuts — is:

T=(nL L+nW W)(1+k)\begin{aligned} T &= \left( n_L \, L + n_W \, W \right) (1 + k) \end{aligned}

Worked example

Consider a 5 × 4 m slab reinforced on a 0.3 m grid, with 10% added for overlap:

nL=⌊40.3⌋+1=13+1=14nW=⌊50.3⌋+1=16+1=17T=(14×5+17×4)(1.10)=138×1.10=151.8\begin{aligned} n_L &= \left\lfloor \frac{4}{0.3} \right\rfloor + 1 = 13 + 1 = 14 \\ n_W &= \left\lfloor \frac{5}{0.3} \right\rfloor + 1 = 16 + 1 = 17 \\ T &= (14 \times 5 + 17 \times 4)(1.10) = 138 \times 1.10 = 151.8 \end{aligned}

So the slab needs 31 bars and about 152 m of rebar.

Consider a 16 × 13 ft slab reinforced on a 12 in grid, with 10% added for overlap:

nL=⌊131⌋+1=13+1=14nW=⌊161⌋+1=16+1=17T=(14×16+17×13)(1.10)=445×1.10=490\begin{aligned} n_L &= \left\lfloor \frac{13}{1} \right\rfloor + 1 = 13 + 1 = 14 \\ n_W &= \left\lfloor \frac{16}{1} \right\rfloor + 1 = 16 + 1 = 17 \\ T &= (14 \times 16 + 17 \times 13)(1.10) = 445 \times 1.10 = 490 \end{aligned}

So the slab needs 31 bars and about 490 ft of rebar.

Choosing inputs

The spacing is a structural choice, not a preference: it depends on the slab thickness, the load, the bar size, and local code, so take it from your approved plan for anything that carries weight. The overlap allowance covers lap splices, where two bars overlap by roughly forty bar diameters; long runs that need several splices waste more, so raise it for big slabs.

For the concrete that surrounds the grid, see Concrete Volume Calculator.

Frequently Asked Questions (FAQ)

How do I calculate rebar quantity?

For a grid, divide the slab width by the bar spacing and add one to get the bars running lengthwise, then divide the slab length by the spacing and add one for the bars running widthwise. Multiply each count by the length those bars span and add them together, then add an allowance for lap splices. A 5 × 4 m slab at 0.3 m spacing needs 14 + 17 = 31 bars and about 152 m of rebar with 10% overlap.

What rebar spacing should I use?

Residential slabs are commonly reinforced on a 20–30 cm (8–12 in) grid, but the correct spacing depends on the slab thickness, the load, the bar size, and local code. Closer spacing or larger bars are used for driveways, footings, and slabs that carry vehicles. Use the spacing on your approved plan rather than a rule of thumb for anything structural.

How much overlap should I add for rebar?

Where two bars are spliced end to end, they overlap by a lap length — often around 40 times the bar diameter — so the load transfers between them. Adding roughly 10% to the bar length covers typical splices on a slab plus offcuts; long runs that need several splices per bar waste more. Check the specified lap length for your bar size and code.

Disclaimer

This is a planning estimate for a simple rectangular grid. It does not size the reinforcement or set the spacing, lap length, cover, or bar diameter — those are engineering decisions governed by the slab’s loads and local code. Use your approved structural drawings for any load-bearing slab, footing, or driveway.

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