Overview
Rebar gets sized by a number, and the number tells you the diameter in eighths of an inch. A #4 bar is 4/8, or 1/2 inch. A #8 is 8/8, a full inch. Once you know that, the whole chart reads itself: #3 is 3/8 inch, #6 is 3/4 inch, #11 is 1-3/8 inch. The common range runs #3 to #11, with #14 and #18 reserved for heavy industrial and bridge work. That numbering is the imperial system, which dominates US jobsites. There's also a soft-metric designation (a #4 shows up as a #13, since it's roughly 13 mm), and you'll see it stamped on the bar right next to the imperial mark. The metric number is just the diameter in millimeters; it doesn't change the steel. Size alone doesn't tell you how strong the placement is. Three things set that: the bar size (how much steel cross-section you get), the grade (the yield strength of that steel), and the spacing (how close the bars sit and how they're laid out). Read the structural drawings for all three. A slab with #4 at 12 inches on center behaves nothing like one with #5 at 6 inches, even though both use small bars.
The Size Chart: #3 to #11 and Up
Here's the working set, with nominal diameter and approximate weight per foot. #3 is 3/8 inch at about 0.376 lb/ft. #4 is 1/2 inch at about 0.668. #5 is 5/8 inch at about 1.043. #6 is 3/4 inch at about 1.502. #7 is 7/8 inch at about 2.044. #8 is 1 inch at about 2.670. From there the heavy bars climb fast: #9 (1-1/8 in, ~3.400 lb/ft), #10 (1-1/4 in, ~4.303), and #11 (1-3/8 in, ~5.313). The jumbo #14 and #18 exist for columns and bridge piers and rarely show up on a house. Where do they go? #3 and #4 handle slabs-on-grade, driveways, sidewalks, and footings on residential work. #4 and #5 cover most residential footings, foundation walls, and grade beams. #5, #6, and #7 show up in retaining walls and heavier foundations. #8 and up belong to columns, large beams, and commercial work where the loads justify the steel. Memorize the #4 weight at roughly 0.668 lb/ft and you can sanity-check a takeoff in your head, because every other bar scales off the cross-section.
Rebar Grades
Grade is yield strength, and the grade number is that strength in ksi (thousands of psi). Grade 60 yields at 60,000 psi, Grade 40 at 40,000, and so on up through Grade 75, 80, and 100. Grade 60 is the workhorse; it covers the overwhelming majority of footings, walls, slabs, and beams you'll pour. Grade 40 still exists for light work and easier field bending, but it's getting rare. Grades 75, 80, and 100 are high-strength steels for columns and heavily loaded members where the engineer wants more capacity without packing in more bars. The grade pairs with an ASTM spec. ASTM A615 is plain carbon-steel rebar, the default for general construction. ASTM A706 is low-alloy steel built to be weldable and to bend without cracking, with tighter limits on its chemistry and strength range. That controlled behavior is why A706 is specified in seismic zones and anywhere bars get welded. The two are not interchangeable. If the drawings call out A706, A615 of the same grade will not satisfy the spec, even though the yield number matches. The Concrete Reinforcing Steel Institute (CRSI) publishes the detailing standards behind these calls.
Spacing, Cover, and Lap Length
Three placement dimensions decide how much bar you actually buy. Spacing is the center-to-center distance between parallel bars, written as "#4 @ 12 in O.C." Tighter spacing means more bars and more steel per square foot, so it drives your count directly. Cover is the concrete between the bar and the nearest surface, and it protects the steel from corrosion. Cover requirements change with exposure: a footing cast against earth needs about 3 inches, formed surfaces against the ground get 2 inches, and interior bars need far less. Skimp on cover and the bar rusts, swells, and spalls the concrete off. Lap splice length is how far two bars overlap where one length ends and the next begins, since rebar ships in finite lengths and has to be continuous in the structure. Laps run roughly 40 to 60 times the bar diameter depending on the splice class (Class A or Class B), the grade, the concrete strength, and the bar spacing. A #5 Grade 60 lap can easily run 30-plus inches. Multiply that across a wall and the overlap steel adds up, so account for it in the takeoff instead of guessing.
Bends, Hooks, and Fabrication
Most rebar doesn't go in straight. Footings need hooked dowels, walls need corner bars, and stirrups and ties wrap the main steel in beams and columns. The two standard end hooks are the 90-degree and the 180-degree, and each has a defined extension past the bend so the hook can actually develop its strength in the concrete. Bends have a minimum inside diameter set by the bar size, because steel cracks if you bend it too tight; bigger bars need a bigger pin. That minimum climbs with grade too, which is one more reason A706 matters when bars get bent hard. Fabrication runs off a bar list, sometimes called a cut-and-bend schedule or a rebar schedule. It spells out every bar's mark, size, length, bend type, and quantity. The fabricator cuts and bends to that list and ships it tagged and bundled by mark. That shop work is a separate line item from the raw steel, because you're paying for cutting, bending, handling, and delivery on top of the bar itself. A complicated bend schedule with lots of unique marks costs more to fabricate than a pile of straight stock.
Common Mistakes
Cover is the one that bites most often. Too little, and the bar sits too close to the surface, water reaches it, and you get rust, spalling, and a footing that fails inspection. Too much, and the steel sits too deep to do its structural job, which quietly weakens the member even though it looks fine. Hit the number on the drawings, top and bottom. Under-lapping splices is the next one: a lap that's shorter than the spec doesn't transfer load, so the two bars behave like they're not connected. Mixing grades is a real failure too. If the engineer designed for Grade 60 and someone sets Grade 40, the capacity drops below what the calculation assumed. The A615-where-A706-is-required swap is the same problem wearing a different mask; the yield matches but the weldability and ductility don't, and in a seismic detail that gap is dangerous. Last, people ignore the size callout entirely and grab whatever bar is on the truck. The spec says #5 for a reason. Substituting #4 because it's handy changes the steel area and undercuts the design. Read the size, grade, spacing, and cover off the drawings, then set exactly that.
FAQ
What does the rebar number actually mean? The number is the bar's nominal diameter in eighths of an inch. A #4 is 4/8, or 1/2 inch; a #8 is 8/8, a full inch. Divide the number by eight and you have the diameter.
What's the difference between Grade 60 and Grade 40? The grade is yield strength in ksi. Grade 60 yields at 60,000 psi and Grade 40 at 40,000 psi. Grade 60 is the standard on most modern work; Grade 40 is lighter and now uncommon.
When do I need ASTM A706 instead of A615? A706 is low-alloy steel made to be weldable and to bend and stretch without cracking. Use it where bars get welded or in seismic zones. If the drawings call out A706, A615 of the same grade does not satisfy the spec.
What size rebar do I need for a residential footing? Most house footings use #4 or #5 Grade 60, but the size, spacing, and cover come from your structural drawings or local code, not a rule of thumb. Read the sheet and set exactly what it specifies.