Rebar in a concrete slab does one job: hold the slab together after it cracks. Concrete cracks — that's a chemical certainty, not a quality issue — and the rebar grid stops the cracks from opening into trip hazards or structural failures. The math for ordering is simple. Spacing and bar size are the decisions that drive cost. Here's the practical version for driveways, patios, garage floors, and shed pads.

The grid math

Rebar in slabs runs in two directions, perpendicular to each other, forming a grid:

bars_long = ceil(width ÷ spacing) + 1 bars_short = ceil(length ÷ spacing) + 1 total_linft = (bars_long × length) + (bars_short × width)

Example: a 20 × 12 ft patio with 18 in OC grid. Spacing in feet = 1.5. Bars long direction = ceil(12/1.5) + 1 = 9 bars × 20 ft = 180 ft. Bars short direction = ceil(20/1.5) + 1 = 15 bars × 12 ft = 180 ft. Total = 360 linear feet, or 18 sticks of 20 ft rebar.

The rebar calculator does this and adds total weight by bar size so you can compare lumber-yard pricing.

Pick the bar size — #3, #4, or #5

Three sizes cover residential slab work. Each is named by the diameter in eighths of an inch:

  • #3 (3/8 in, 0.38 lb/ft): Sidewalks, light-duty slabs, ties for column cages. Cheapest. Bends by hand. Not strong enough for vehicle slabs.
  • #4 (1/2 in, 0.67 lb/ft): The residential default. Patios, garage floors, sheds, basement slabs. Bends with a manual bender or bar of pipe.
  • #5 (5/8 in, 1.04 lb/ft): Driveways, heavier garage slabs, slabs supporting masonry walls. Needs a hydraulic bender.

For most homeowner slabs, #4 at 18 in OC is the right answer. Stepping up to #5 doubles the steel weight but barely improves crack control unless the slab is taking vehicle loads.

Pick the spacing — 12, 18, or 24 in OC

Spacing controls how tight the grid is and therefore how small the cracks stay when concrete shrinks:

  • 12 in OC: Driveways, slabs supporting columns, slabs in expansive soil. Maximum crack control. Doubles material vs 24 OC.
  • 18 in OC: Patios, garage floors, basement slabs. Standard residential. Good balance of cost and crack control.
  • 24 in OC: Sidewalks, garden paths, decorative slabs without vehicle loads. Cheapest grid that still meets ACI 318 minimum reinforcement for crack control.

Tighter than 12 in OC is rarely useful for residential — beyond that, you're adding cost without adding crack resistance. Looser than 24 in is also a problem: cracks can open wider than 1/8 in before the bars engage.

Wire mesh vs rebar — when each works

Welded wire mesh (typically 6×6 W2.9 or 6×6 W4.0) is the cheaper alternative for thin slabs (under 4 in) and light-duty applications like patios. It comes in flat sheets or rolls, ships pre-spaced, and saves the hour of tying rebar.

Mesh fails because it's impossible to keep in the middle of the slab during the pour. Workers walk on it and push it to the bottom, where it does nothing. Rebar on chairs stays at the right height. For any slab over 4 in, take the extra time and use rebar.

Fiber-reinforced concrete is a third option — synthetic or steel fibers mixed into the concrete itself. It controls micro-cracking better than mesh or rebar but doesn't replace rebar for structural reinforcement on driveways and load-bearing slabs.

Cover and chair use — the part that gets skipped

Rebar at the bottom of a slab does nothing — the bars must sit in the middle of the slab thickness (or 1/3 down from the top for slabs that bend that direction). ACI 318 cover minimums:

  • 3 in clear cover from the bottom (slab on grade, against earth)
  • 1.5 in clear cover from the top (finished surface)

Use rebar chairs — small plastic or wire stands that hold the rebar at the right height while concrete is poured around it. Place chairs every 4 ft along each bar. Skipping chairs and assuming the workers will lift the bars during the pour is the #1 reason DIY slabs fail at 5 years.

What pros do differently

Tie, don't weld. Welding rebar in the field damages the steel and creates stress concentrations that crack first. Use 16-gauge wire ties at every intersection — a tying gun ($150) makes this 10x faster than hand ties.

Lap splices for long bars.Rebar comes in 20 ft sticks. For a slab longer than 20 ft, overlap two bars by 40 bar diameters (20 in for #4, 25 in for #5). Tie the overlap. Don't butt-weld or short-lap.

Bend on site, not at the yard. Yards charge a premium for pre-bent bars. A manual rebar bender from the home center is $30 and bends #3-#5 cleanly. Order straight 20 ft sticks and bend in the field.

Quick FAQ

How much rebar for a 20 × 20 garage slab? At 18 in OC #4: 30 sticks of 20 ft rebar (600 linear feet, ~400 lb). Drop to 12 in OC for vehicle loads and that climbs to about 44 sticks.

Do I need rebar in a 4 in patio? Technically not for structure if the soil is well-compacted, but yes for crack control. Mesh is acceptable; rebar is better.

Can I just throw rebar in after pouring?No — “throwing it in” (pulling rebar up into wet concrete with a hook) almost always leaves the bars at random depths. Lay the grid on chairs first, then pour. There's no shortcut.

Order the concrete in the same trip. Use the concrete calculator to size the cubic yards (or 80 lb bags) for the same slab so the truck and the rebar arrive together.