Rebar Calculator: How Much Rebar for a Concrete Slab?

How the rebar calculator works

Reinforcement in a slab on the ground is usually a square grid of bars tied where they cross. The calculator first pulls every bar back from the edges by the cover you enter, so the steel stays inside the concrete and away from moisture. It then counts how many bars fit across each direction without exceeding your spacing, which means rounding up and adding one for the far edge.

Runs longer than a stock bar are joined with a lap splice: two bars overlap by 40 bar diameters and are tied together. The pieces left to cut, from both directions together, are then fitted into stock bars longest first, so a bar that yields two or three runs is used that way whichever direction they go. What cannot be reused becomes offcut, which is why the bar count depends on the stock length as much as on the slab.

Worked example

A 6 m × 4 m slab with a 30 cm grid and 7.5 cm cover leaves a reinforced zone of 5.85 m × 3.85 m. Across the 3.85 m width, ⌈3.85 ÷ 0.30⌉ + 1 = 14 bars run lengthwise, each 5.85 m. Across the 5.85 m length, ⌈19.5⌉ + 1 = 21 bars run widthwise, each 3.85 m. That is 162.75 m of 12 mm bar in the slab and 294 crossings to tie. With 6 m stock, every run needs its own bar, so you buy 35 bars (210 m, about 186 kg) and throw away twenty-one 2.15 m offcuts. With 12 m stock the same grid takes only 14 bars, because each 12 m bar gives two long runs or three short ones. On a square slab both directions share the same cut: a 10 × 10 ft slab needs 22 runs of 9.5 ft, which 11 bars of 20 ft cover exactly, where counting each direction on its own would give 12.

Common bar sizes

US bar numbers are the diameter in eighths of an inch. Merchants in Europe sell straight bars in 6 m and 12 m lengths and price them by weight, while North American yards usually stock 10 ft and 20 ft lengths priced per bar.

Placing tips and frequent mistakes

• Bars on the ground do nothing. Support the grid on chairs or concrete spacers, roughly one every metre (3 ft) each way, so it ends up in the middle or upper third of the slab.
• Stagger the laps. Do not put all the splices in one line across the slab; offset neighbouring laps by at least a lap length.
• Keep cover consistent. Concrete poured against soil generally needs about 7.5 cm (3 in) of cover; too little and the steel rusts, too much and it stops working.
• Buy tie wire. Allow about 20–25 cm (8–10 in) of wire per tie; a coil of 1.5 kg covers a few hundred crossings.
• Know the limits of a rule of thumb. The 40-diameter lap and the spacings above are common practice for simple slabs on the ground. Suspended slabs, footings, retaining walls and anything that carries a building need bar sizes, spacing and laps from an engineer or your local code.

Frequently asked questions

Last reviewed: 2026-09-27 · How we calculate