Rebar Size & Spacing Chart (Weight, Diameter & Grid)
Part of Concrete & Masonry
Quick answer
Rebar is numbered in eighths of an inch: #3 is 3/8 in, #4 is 1/2 in, #5 is 5/8 in, #6 is 3/4 in. Weight per foot is 0.376, 0.668, 1.043 and 1.502 lb respectively. For slabs, space the bars 12 in on center for driveways and footings, 16–18 in for patios and light slabs — the code maximum is the lesser of 3× the slab thickness or 18 in.
Rebar is sized by number, and the number is the bar's diameter in eighths of an inch — so #4 is 4/8 (1/2) inch. The bigger the number, the thicker, heavier and stronger the bar. These four cover almost all residential flatwork.
Rebar size chart (#3–#6)
| Bar size | Diameter | Weight (lb/ft) | Weight per 20 ft stick | Typical use |
|---|---|---|---|---|
| #3 | 3/8" (0.375 in) | 0.376 | ~7.5 lb | Sidewalks, light slabs |
| #4 | 1/2" (0.500 in) | 0.668 | ~13.4 lb | Driveways, patios, residential slabs |
| #5 | 5/8" (0.625 in) | 1.043 | ~20.9 lb | Heavy/commercial slabs, footings |
| #6 | 3/4" (0.750 in) | 1.502 | ~30.0 lb | Foundations, columns, beams |
Spacing by application
| Application | Bar size | Spacing (on center) |
|---|---|---|
| Sidewalk / light slab | #3–#4 | 18" |
| Patio / shed floor | #4 | 16–18" |
| Residential driveway | #4 | 12" |
| Garage / commercial floor | #4–#5 | 12" |
| Footings | #4–#5 | 12" |
How spacing changes the steel
Tighter spacing puts more steel in the slab, which controls cracking better but costs more. Going from 18 in to 12 in on center roughly doubles the number of bars. The code ceiling is the lesser of three times the slab thickness or 18 inches — so a 4-inch slab maxes out at 12-inch spacing.
Once you've picked a size and spacing, the rebar calculator turns your slab dimensions into a bar count, total linear feet, sticks to buy and the steel weight — and shows how the stick count changes at 12, 16, 18 and 24 inch spacing.
FAQs
What does the rebar number mean?
The number is the bar diameter in eighths of an inch. #4 rebar is 4/8 = 1/2 inch in diameter, #5 is 5/8 inch, and so on. Higher numbers are thicker and stronger but heavier and more expensive.
How much does a stick of rebar weigh?
A standard 20-foot stick weighs about 7.5 lb for #3, 13.4 lb for #4, 20.9 lb for #5 and 30 lb for #6. Multiply the per-foot weight (0.376, 0.668, 1.043, 1.502 lb) by your total linear footage to size a load.
What is the maximum rebar spacing in a slab?
For slabs, the maximum spacing is the lesser of three times the slab thickness or 18 inches (per ACI 318 / IRC). So a 4-inch slab is capped at 12 inches and a 6-inch slab at 18 inches. Closer spacing is always allowed and gives better crack control.