How much rebar do I need for a concrete slab?

For a standard 4-inch residential slab, use #4 rebar on 18-inch centers both ways. For a 20×20 ft slab, that's approximately 27 bars (14 lengthwise + 13 crosswise), totaling about 540 linear feet and weighing roughly 360 lbs. Add 10% for overlap splices and waste. Rebar should be positioned in the lower third of the slab with 3 inches of cover from the bottom.

What size rebar for a 4 inch slab?

#3 (3/8 inch) or #4 (1/2 inch) rebar is standard for 4-inch residential slabs. Use #3 at 18-inch spacing for light-duty slabs like patios and walkways. Use #4 at 18-inch spacing for garage floors and driveways. For slabs carrying heavy loads (workshops, equipment pads), step up to #4 at 12-inch spacing or #5 rebar.

How far apart should rebar be spaced?

Common rebar spacing is 12, 16, 18, or 24 inches on center. For residential slabs: 18–24 inches for light duty (patios), 12–18 inches for driveways and garage floors. For structural footings: 6–12 inches. A general rule is that rebar spacing should not exceed 3 times the slab thickness or 18 inches, whichever is less (per ACI 318). Tighter spacing = stronger slab but more material cost.

Do I need rebar chairs?

Yes — rebar chairs (supports) are essential. They hold rebar at the correct height within the slab during the pour. Without chairs, rebar sinks to the bottom and provides almost no structural benefit. Use plastic or wire chairs spaced every 3–4 feet in each direction. For a 20×20 slab, you need approximately 25–36 chairs. They cost $0.25–$0.75 each — a tiny investment that prevents catastrophic slab failure.

Rebar Calculator — Pieces, Weight & Cost

How to Calculate Rebar for Concrete Slabs

Rebar transforms a brittle concrete slab into a reinforced structure that can handle loads, temperature swings, and ground movement. Getting the right amount — size, spacing, and accessories — is critical. Here's everything you need to know.

Rebar Size Reference Table

Rebar Size	Diameter	Metric	Weight/ft	Weight/20ft bar	Common Uses
#3	⅜"	10mm	0.376 lbs	7.5 lbs	Patios, walkways, thin slabs, stirrups
#4	½"	13mm	0.668 lbs	13.4 lbs	Driveways, garage floors, standard slabs
#5	⅝"	16mm	1.043 lbs	20.9 lbs	Footings, retaining walls, heavy slabs
#6	¾"	19mm	1.502 lbs	30.0 lbs	Structural footings, columns, beams
#7	⅞"	22mm	2.044 lbs	40.9 lbs	Bridge decks, commercial foundations
#8	1"	25mm	2.670 lbs	53.4 lbs	Heavy structural, bridge piers, dams

Rebar Spacing Guide by Application

Application	Rebar Size	Spacing	Slab Thickness
Sidewalk / patio	#3	18–24" o.c.	4"
Residential driveway	#4	16–18" o.c.	4–5"
Garage floor	#4	16–18" o.c.	4–6"
Equipment pad / shop floor	#4 or #5	12–16" o.c.	5–6"
Continuous footing	#4 or #5	12" o.c. (longitudinal)	8–12"
Retaining wall	#5 or #6	8–12" o.c.	8–12"
Commercial slab	#5	12" o.c.	6–8"

Rebar Placement Rules

Minimum concrete cover: 3 inches from the bottom of the slab to the rebar (for slabs on grade). This protects the steel from moisture and corrosion.
Overlap (lap splice): When rebar isn't long enough to span the full slab, overlap pieces by at least 40 bar diameters. For #4 rebar: 40 × 0.5" = 20 inches minimum overlap.
Edge setback: Rebar should stop 3 inches from the slab edge. Don't run it all the way to the form.
Position in slab: For a 4" slab, rebar should sit at roughly 1.5–2 inches from the bottom — supported by chairs. For thicker slabs (6"+), you may need two layers (top and bottom mat).
Tie every intersection: Use tie wire at every point where bars cross. Alternating is acceptable on large slabs but every intersection is better.

Rebar vs. Wire Mesh

Factor	Rebar	Welded Wire Mesh (WWF)
Strength	Much stronger	Light reinforcement only
Best for	Driveways, garages, structural	Patios, sidewalks, light slabs
Cost per sq ft	$0.50–$1.50	$0.15–$0.40
Placement	Tied to chairs, stays in position	Often sinks during pour
Crack control	Excellent	Moderate
Code requirement	Required for structural	Acceptable for non-structural

For any slab that carries vehicle traffic or structural loads, use rebar — not wire mesh. Mesh is fine for decorative patios and sidewalks.

Money-Saving Tips

Buy from a rebar supplier, not Home Depot. Big-box stores charge $8–$15 per 20-ft bar. Rebar yards charge $0.40–$0.70/lb — typically 30–50% less.
Order pre-cut and pre-bent rebar for footings and walls. It saves hours of cutting and bending on site. Most suppliers offer this at minimal extra cost.
Use #4 at 18" spacing for most residential work. It's the most cost-effective combination that meets code for standard slabs.
Don't skip the chairs. Rebar sitting on the ground is worse than no rebar — it creates a corrosion path. Chairs cost pennies and prevent thousands in future repairs.
Tie wire goes fast. Budget 2–3 lbs of tie wire per 100 intersections. Buy an extra roll — you'll use it.

Common Rebar Mistakes

No chairs: Rebar on the ground = no reinforcement. It must be elevated to the correct position within the slab.
Insufficient overlap: Bars that barely touch don't transfer load. Overlap must be 40× the bar diameter minimum.
Too close to edges: Rebar within 1–2 inches of the surface will corrode and crack the concrete. Maintain 3" cover.
Walking on rebar mat: Workers walking on the rebar grid push chairs out of position and bend bars. Place plywood walking paths during the pour.
Wrong size for the job: Using #3 rebar where #4 or #5 is needed will result in a structurally deficient slab. Follow engineering specs.