Standard rebar sizes
| Size | Diameter | Weight | Area | Lap (40d) | Typical use |
|---|---|---|---|---|---|
| #3 | 3/8" (0.375) | 0.376 lb/ft | 0.11 in² | 15" | Slab mesh replacement, small footings |
| #4common | 1/2" (0.500) | 0.668 lb/ft | 0.20 in² | 20" | The residential standard — slabs, footings, walls |
| #5 | 5/8" (0.625) | 1.043 lb/ft | 0.31 in² | 25" | Driveways, garage slabs, heavier footings |
| #6 | 3/4" (0.750) | 1.502 lb/ft | 0.44 in² | 30" | Retaining walls, structural footings |
| #7 | 7/8" (0.875) | 2.044 lb/ft | 0.60 in² | 35" | Commercial columns and beams |
| #8 | 1" (1.000) | 2.670 lb/ft | 0.79 in² | 40" | Commercial structural |
| #9 | 1-1/8" (1.128) | 3.400 lb/ft | 1.00 in² | 45" | Heavy commercial |
| #10 | 1-1/4" (1.270) | 4.303 lb/ft | 1.27 in² | 51" | Heavy commercial |
| #11 | 1-3/8" (1.410) | 5.313 lb/ft | 1.56 in² | 56" | Heavy commercial |
Imperial ASTM A615 designations. The size number is the diameter in eighths of an inch, so #4 = 4/8" = ½". Lap lengths are the common 40-diameter rule for typical conditions — high-stress and seismic details require more.
Concrete cover requirements
| Condition | Minimum cover |
|---|---|
| Cast against and permanently exposed to earthcommon | 3" |
| Exposed to earth or weather, #6 and larger | 2" |
| Exposed to earth or weather, #5 and smaller | 1-1/2" |
| Slabs and walls not exposed to weather | 3/4" |
| Beams and columns not exposed to weather | 1-1/2" |
Per ACI 318. Cover is what protects the steel from moisture — too little and the bar corrodes, expands and spalls the concrete from the inside out.
Working out weight from length
Suppliers price rebar by weight, so the conversion matters. Total weight = total linear feet × pounds per foot.
Worked example: a 20 × 12 ft slab with #4 bar at 18 inches on centre needs roughly 317 linear feet of steel. At 0.668 lb/ft that is 212 lb, or about a tenth of a ton.
Standard stock length is 20 feet, though 10 ft and 40 ft are available. Ordering 20 ft stock where you can handle it means fewer splices, less waste and a stronger mat.
What people get wrong
- Rebar lying on the ground does nothing. It has to sit near mid-depth of the slab to resist tension, which means chairs or bolsters — not pulling it up with a hook during the pour and hoping.
- Splices must overlap and be tied, not butted. A butted joint transfers no load at all and becomes a planned crack.
- Grade matters as much as size. Grade 60 is the standard for structural work; Grade 40 is softer and is not an equivalent substitute where the drawings call for 60.
- Do not weld rebar unless it is specified as weldable (ASTM A706). Standard A615 bar becomes brittle at the weld and can fail without warning.
- Count the "+1". Bars across a dimension are (dimension ÷ spacing) + 1, because you need one at each edge. Forgetting it is the most common reason an order comes up short.
Frequently asked questions
What does the rebar number mean?
It is the nominal diameter in eighths of an inch. A #4 bar is 4/8 of an inch, or ½ inch; a #8 bar is 8/8, or 1 inch.
How much does #4 rebar weigh per foot?
0.668 pounds per linear foot. A standard 20 ft stick weighs about 13.4 lb.
What size rebar for a residential slab?
#4 at 18 inches on centre each way is the usual standard for a 4 inch slab on grade. Driveways and garage floors typically tighten to 12 inches, or step up to #5.
How long should a rebar lap splice be?
Forty bar diameters for typical conditions — 15 inches for #3, 20 inches for #4, 25 inches for #5. Always tie the overlap rather than butting the ends together.
How much concrete cover does rebar need?
Three inches where concrete is cast directly against earth, 1½ to 2 inches where it is formed but exposed to weather, and ¾ inch for interior slabs and walls.