Rebar Calculator

How much rebar does a concrete slab actually need?

Rebar in a slab-on-grade is not there to carry the load — the compacted subgrade does that. Steel is there to hold the two halves of an inevitable crack tightly together so the slab keeps behaving like one rigid plate instead of two tilting ones. That single idea explains every decision on this page: the grid, the spacing, the edge clearance and why the bars have to sit in the middle of the concrete rather than on the ground.

The quantity itself is simple geometry. You lay bars in two directions, count them, add up their lengths, then divide by the length of the sticks the yard sells. Everything else is trade practice layered on top: a couple of inches of cover at the edges so the steel does not rust out through the side of the slab, a 5% allowance for lap splices, and enough chairs to stop the whole mat being trodden into the base.

Worked example: a 20 ft x 10 ft driveway strip

Take a typical single-car driveway apron, 20 ft long by 10 ft wide, reinforced at 18 in centres with 3 in of clearance at every edge. Clearance comes off both ends of each dimension, so the steel actually covers 19.5 ft by 9.5 ft.

Bars running lengthwise are spaced across the 9.5 ft width: 9.5 ft is 114 in, and 114 / 18 = 6.33, so you fit 6 full gaps plus the starting bar — 7 bars, each 19.5 ft long. Bars running widthwise are spaced along the 19.5 ft length: 234 in / 18 = 13 gaps, plus the starting bar — 14 bars, each 9.5 ft long. That is 7 x 19.5 + 14 x 9.5 = 136.5 + 133 = 269.5 linear feet of steel.

Add 5% for laps and offcuts and you need 283 ft, which at 20 ft per stick is 14.1 sticks — order 15. The two directions cross 7 x 14 = 98 times, so budget about 100 tie wires or clips, and at roughly one chair per 4 sq ft of mat you want around 47 supports. Buy a few spare of everything; nobody has ever finished a pour wishing they had fewer ties.

What tightening the spacing costs

Run the same slab at 12 in centres and the counts jump to 10 bars lengthwise and 20 widthwise: 385 linear feet, 21 sticks, 200 intersections. You have added 43% more steel and roughly doubled the tying labour. That is the right call under a heavy vehicle, a shop floor or expansive clay, and the wrong call for a garden patio. For most residential driveways 18 in centres with #4 bar is the sweet spot, and 24 in centres is fine for patios and shed pads.

Chairs matter more than people expect

Steel laid straight on the gravel does almost nothing. To work in bending it needs to sit near the middle of the slab depth — for a 4 in slab, about 2 in up. Rebar chairs, dobies or precast blocks hold that height while the concrete goes in. The old habit of hooking the mat up with a rake mid-pour is unreliable: parts of it drop straight back down and you cannot see where. Chairs at roughly 3 to 4 ft in each direction, which is what the one-per-4-sq-ft estimate reflects, keep the mat where the calculation assumed it would be.

Cutting, bending and lapping on site

#3 and #4 bar cut cleanly with a portable band saw, an angle grinder and cut-off wheel, or a manual rebar cutter for the smaller sizes. Bend corner bars rather than butting them: an L-shaped bar around a corner is stronger than two straight bars tied together. Where sticks have to meet, overlap them by 40 bar diameters — about 15 in for #3, 20 in for #4 — tie both ends of the lap, and stagger the joints so they do not all land on the same grid line.

Limits of this estimate

These numbers describe a plain rectangular slab-on-grade with a single mat of bar. They do not cover thickened edges and footings, which need their own longitudinal bars and stirrups; two-layer mats in structural slabs; post-tensioned slabs; or anything an engineer has stamped, where the drawing overrules any rule of thumb. Local codes also vary on cover, lap length and minimum bar size, and inspectors in freeze-thaw regions are often stricter. Check the permit conditions before you order steel — a phone call is cheaper than a rejected inspection.