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Concrete Slab Rebar Calculator

Enter your slab size and bar spacing to see the bar count, total rebar length and tie wire.

Result

52
250.00
651
227.85

How it is calculated

The slab is treated as a single flat grid of bars. Slab length and width are in metres; spacing is in centimetres and is converted with s = spacing ÷ 100. Bars running across the width are A = floor(length ÷ s) + 1, and bars running along the length are B = floor(width ÷ s) + 1. Total bars are A + B, total bar length is A × width + B × length in metres, ties are A × B (one per crossing) and tie wire is ties × 0.35 m. All three fields are required. Spacing below 5 cm is rejected as too tight to place or tie, and so is a spacing longer than either side of the slab; those inputs show no result. The numbers are for one layer only: edge cover, lap splices, bar bending and waste are not added, and bar size and spacing are your design decision, not something the tool selects.

Worked example

A slab measures 6 m by 4 m with bars at 20 cm, so s = 0.20 m. Bars across the width: floor(6 ÷ 0.2) + 1 = 31. Bars along the length: floor(4 ÷ 0.2) + 1 = 21. Total bars: 31 + 21 = 52. Bar length: 31 × 4 + 21 × 6 = 124 + 126 = 250.00 m. Crossings: 31 × 21 = 651 ties, and tie wire at 0.35 m each is 227.85 m. Add your own allowance for laps and cover before ordering stock lengths.

Try this example →

Common mistakes

  • Entering the spacing in metres. The field is centimetres: 20 means 20 cm, while 0.2 (meant as 0.2 m) is read as 0.2 cm, which is below the 5 cm minimum and gives no result.
  • Entering the slab in feet or centimetres. Length and width are metres.
  • Treating the total as ready to order. The bar length has no edge cover, no laps where bars join and no waste, so it is a minimum for one layer.
  • Forgetting a second mat. A slab with top and bottom reinforcement needs the quantities doubled, since the tool counts one grid.
  • Mixing up bar count and bar length. The 52 bars in the example are individual pieces of different lengths; the 250 m figure is what you buy in stock lengths.

About this calculator

Rebar count for a concrete slab is found by dividing the slab's length and width by the chosen bar spacing and adding one, done separately in each direction for a grid layout, then multiplying by the number of bars needed for the total length of reinforcement. Tie wire quantity follows from the number of intersections in the grid — each crossing point needs a tie — multiplied by the length of wire used per tie.

Concrete contractors and DIY slab-pourers run this calculation before ordering rebar because reinforcement spacing directly affects a slab's crack resistance and load capacity — tighter spacing (denser grid) is used for driveways and slabs bearing vehicle loads, while wider spacing suffices for patios and light-duty slabs. Getting the bar count and tie wire estimate right avoids a mid-pour materials run, which is far more disruptive for rebar placement than for concrete itself.

Lay out a grid at your chosen bar spacing over the slab size and you see how many bars it takes, their total length and the tie wire for the crossings.

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