Rebar Calculator
Rebar grid for a slab: bars, stock lengths, weight and ties.
About this tool
Enter the slab size, bar spacing, edge cover, bar size and the stock length you'll buy. The tool calculates the bars in each direction, the total length including laps, stock bars to order, weight and tie points.
Laying out a rebar grid
Slab reinforcement is usually a grid of bars in both directions at a set spacing. Bars stop short of the edges by the concrete cover, typically 50โ75 mm (2โ3 in), so the steel stays protected from corrosion. The number of bars running one way is the other dimension minus the cover at both ends, divided by the spacing, plus one. A 6 ร 4 m slab with 75 mm cover and 300 mm spacing has 13 bars 5.85 m long and 20 bars 3.85 m long, about 153 m in total. That's 27 six-metre stock bars with 5% spare, and about 240 kg in 16 mm bar. Bars longer than the stock length are spliced with a lap (here 40 bar diameters, 640 mm for 16 mm bar), and each crossing point is tied with wire, 260 ties in this example. Weights use standard kg per metre for US bar numbers (#4 is about 1 kg/m) and metric sizes. Honest limits: bar size, spacing, cover, lap length and whether you need top and bottom mats are structural decisions for an engineer or local code. This estimates quantities for a design you already have. Not engineering advice. The concrete calculator and the paver calculator are related.
Frequently asked questions
How do I calculate rebar for a slab?
For each direction: (slab dimension โ 2 ร cover) รท spacing + 1 bars, each as long as the other dimension minus the cover.
What is concrete cover?
The thickness of concrete between the rebar and the surface, typically 50 to 75 mm for slabs, which protects the steel.
How long should a rebar lap be?
Often about 40 bar diameters, but follow your engineer's or code's requirement.
How much does #4 rebar weigh?
About 0.668 lb per foot (0.994 kg per metre).
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.