Rebar Calculator

Rebar for a concrete slab: bars each way at your spacing and cover, laps, stock bars to buy and total weight, for US, UK or Australian bar sizes.

Rebar for a slab is laid as a grid, with bars running both ways at the same spacing and kept back from the edges by the concrete cover. Each direction takes the slab's width (less cover) divided by the spacing, plus one. A 20 × 12 ft slab at 12 in centres with 3 in cover takes 13 bars 19½ ft long and 21 bars 11½ ft long: 495 ft of bar, 34 bars of 20 ft stock, weighing about 331 lb in #4. Runs longer than a stock bar are joined with laps, which the calculator adds (40 bar diameters by default). It counts bars the way they're cut, packing shorter pieces into stock bars, and works for US, UK and Australian bar sizes.

Units
Bar sizes
ft
ft
in
in
in
Stock bars
34

#4 (½ in), 20 ft

Bars each way
13 × 21

at 12 in

Total length
495 ft

including laps

Weight
331 lb

0.17 tons

How it was worked out
  1. Bars along the length: 11.5 ft (width less cover) at 12 in = 13 bars of 19.5 ft
  2. Bars across: 19.5 ft at 12 in = 21 bars of 11.5 ft
  3. Runs longer than a 20 ft bar are joined with 20 in laps
  4. Total 495 ft, cut from 34 stock bars of 20 ft (pieces packed into bars, longest first)
  5. Weight: 495 ft × 0.668 lb/ft = 331 lb

The default lap is 40 bar diameters, a common rule of thumb; bar size, spacing, cover and lap lengths for structural slabs come from your engineer or code. Weights are nominal: 0.668 lb/ft for #4 (½ in). Mesh is often used instead of bars for slabs on the ground.

Common use cases

  • Order rebar for a patio, shed or garage slab
  • Compare #3 and #4 bar, or 12 mm and 16 mm
  • Work out the weight of steel for delivery
  • Check a supplier's cutting list

How to use

  1. Choose your units and country (for bar sizes).
  2. Enter the slab length and width, the bar spacing and the edge cover.
  3. Choose the bar size and stock length, and check the lap length.
  4. Read the stock bars, bars each way, total length and weight.

FAQ

How much rebar do I need for a slab?

Bars each way = (slab dimension − 2 × cover) ÷ spacing + 1. Multiply by the bar lengths for the total, add laps where runs are longer than a bar, then work out how many stock bars that is.

How much does rebar weigh?

#3 bar weighs 0.376 lb per foot, #4 0.668, #5 1.043 and #6 1.502. Metric bars weigh their diameter squared ÷ 162 kg per metre: 0.89 kg/m for 12 mm and 1.58 kg/m for 16 mm.

How much cover does rebar need?

For concrete cast against the ground, ACI 318 calls for 3 in of cover; slabs on the ground also need the bars held up on chairs, not lying on the base. Your engineer or code sets the cover, spacing, size and laps for structural work.

Related tools

Results are estimates for planning and buying materials, worked out from the figures you enter and typical values (weights, coverage, sizes) that vary by product and supplier. They are not engineering, building or professional advice. Check your local building code, the product maker's instructions and your supplier's figures before you buy or build, and use a qualified professional for structural, electrical and gas work. Sizes, spans, depths and reinforcement for anything structural must come from span tables, your building code or an engineer.