Rebar Calculator

HomeMath takes the edge cover off each side, then fits bars across the rest at the spacing, both edges included: the number of bars is the span ÷ the spacing, rounded up, plus one, because the spacing is a maximum and the bars spread evenly between the edges. Bars run both ways, each as long as the slab inside its cover. A run longer than a stick is made of sticks lapped end to end, the lap being the bar's diameter × the lap you set (40 to start). The linear feet with laps, plus waste, ÷ the stick length gives the sticks; × the bar's weight per foot, the steel. Chairs sit at the chair spacing both ways. Wire mesh is the area plus waste ÷ what one sheet or roll covers once overlapped. In metric, sizes are in meters and centimeters and the results in meters and kilograms.

The published rules it uses: rebar sizes and mesh; US customary units; metric conversion. Their sources, and how HomeMath tests every result, are on the about page.

1. Take off the cover: 20 ft − 2 × 3 in = 19.5 ft each way.
2. Bars: 19.5 ft ÷ 1 ft, rounded up, + 1 = 21 each way, 42 in all.
3. Length: 42 × 19.5 ft = 819 ft; no laps, as each bar fits one 20 ft stick.
4. Sticks: 819 × 1.05 ÷ 20, rounded up = 43. Steel: 819 × 0.668 = 547 lb.
5. Chairs, one at each crossing of a grid at 4 ft: 36 chairs for 20 × 20 ft.

A run longer than a stick gets laps, which join two sticks by overlapping them, tied with wire. In a 16 × 40 ft driveway with #4 at 18 inches, each long bar is 39.5 ft: 3 sticks lapped 20 inches, 42.8 ft of bar per run, 24 laps in all for that slab.

The spacing sets how many bars a slab takes: (the span inside the cover ÷ the spacing), rounded up, + 1. The spacing is a maximum, so the bars spread evenly with one at both edges, and wider spacing means fewer bars and less steel. For a 20 × 20 ft slab with #4 bars:

A US bar's number is its diameter in eighths of an inch: #3 is 3/8 inch, #4 is 1/2 inch, #5 is 5/8 inch. The lap is the calculator's starting 40 diameters.

• Chairs hold the grid up so the concrete surrounds it. The calculator puts one at every crossing of a grid at the chair spacing: 36 chairs for 20 × 20 ft at 4 ft.
• Edge cover keeps the bars inside the slab, away from the soil and the air; the bars end that far from each edge.
• Laps join two sticks by overlapping them, tied with wire. The length a lap needs depends on the bar and the concrete, so use your plans' figure.

The calculator starts at 3 in of cover, laps of 40 bar diameters, 20 ft sticks and chairs every 4 ft. These are starting values, not rules: your plans or local code set the bar, the spacing, the cover and the lap.

Welded wire mesh (6 × 6 in squares of W1.4 wire) comes in 5 × 10 ft sheets and 5 × 150 ft rolls. Overlapped one 6 in square on a side and an end, a sheet covers 42.8 sq ft; with 5% waste:

Your plans say whether a slab gets rebar, mesh or both. Pair this with the concrete calculator for the pour, and the concrete weight calculator for the slab's weight with the steel in it.