Understanding Rebar Spacing: How Much Steel Reinforcement Do You Need?

Rebar sizes in plain English

Rebar is sized in eighths of an inch. A #3 bar is 3/8 inch, a #4 is 1/2 inch, a #5 is 5/8 inch. Residential flatwork uses #3 and #4 almost exclusively; #5 shows up in footings and structural walls.

Bar size	Diameter	Weight per ft	Typical residential use
#3	0.375 in	0.376 lb	Patios, sidewalks, thin slabs
#4	0.500 in	0.668 lb	Driveways, garage slabs, footings
#5	0.625 in	1.043 lb	Foundation walls, heavy footings
#6	0.750 in	1.502 lb	Structural columns and grade beams

Standard spacing by application

Spacing is specified as bars each way on a grid. Common residential specs:

Application	Bar size	Spacing	Slab thickness
Sidewalk / patio	#3	18 in OC each way	4 in
Driveway (cars)	#4	18 in OC each way	4–5 in
Driveway (RV / truck)	#4	12 in OC each way	6 in
Garage slab	#4	16 in OC each way	4–6 in
Strip footing	#4	2 bars continuous	8–12 in deep

Cover and position matter more than quantity

Steel only works if it sits in the right part of the slab. For a slab on grade, rebar belongs in the middle third, or one-third up from the bottom for slabs in bending. Minimum cover is 3 inches when concrete is cast against earth and 1.5 inches for formed surfaces exposed to weather.

Rebar laid on the ground and 'pulled up' during the pour never ends up where it should. Use chairs or dobies every 3–4 feet in both directions.

Step-by-step

1. 1

   Calculate bars in each direction

   Bars = (slab dimension in inches ÷ spacing) + 1. For a 20 x 24 ft driveway at 18 inches on-center: across the 20 ft (240 in) direction, 240 ÷ 18 + 1 = 14 bars, each 24 ft long. Across the 24 ft (288 in) direction, 288 ÷ 18 + 1 = 17 bars, each 20 ft long.

2. 2

   Subtract the edge cover

   Bars stop 3 inches short of each edge, so a 24 ft bar is really 23.5 ft. This does not change the count, but it does change how you cut stock lengths.

3. 3

   Convert to purchased stock lengths

   Rebar is sold in 10 and 20 ft lengths. 14 bars x 23.5 ft = 329 linear ft; 17 bars x 19.5 ft = 332 linear ft. Total 661 linear ft, or 34 pieces of 20 ft #4 rebar before splices.

4. 4

   Add lap splices where bars join

   Where two bars overlap, the lap must be at least 40 bar diameters — 20 inches for #4 bar, 15 inches for #3, and never less than 12 inches. Tie every lap with wire in at least two places. On our driveway, the 24 ft runs need one splice each: 14 splices x 20 in = 23 extra linear feet.

5. 5

   Add the grid ties

   Tie every intersection on the perimeter and every second or third intersection in the field. A 14 x 17 grid has 238 intersections; tying two-thirds of them uses about 160 ties, or roughly one 3.5 lb roll of 16-gauge tie wire.

6. 6

   Set chairs and re-check before the pour

   Place chairs on a 3–4 ft grid, then walk the slab and confirm nothing has sagged. Take a photo of the finished grid — many inspectors accept photo evidence, and it documents the work if you ever sell the house.

7. 7

   Plan the control joints around the steel

   Saw-cut or tool control joints within 6–18 hours of the pour, at intervals of 24–30 times the slab thickness (8–12 ft for a 4-inch slab), cutting one-quarter of the depth. Rebar continues through control joints; only expansion joints break the steel.

Safety note: this guide is educational. Always follow local building codes, pull required permits, and hire a licensed pro for structural, electrical, gas or plumbing work when in doubt.