Rebar size refers to the nominal diameter of the steel bar used inside concrete. A higher bar number means a larger nominal diameter, a heavier bar, and greater load capacity, while a lower number suits lighter structures. Choosing the right size matters as much as choosing the right material: just as shop vacuum hose diameters determine airflow and which tools attach, rebar diameter controls how much tension the concrete section can carry. The size also sets the practical side of the job: bar spacing, cover, splice lengths, and even the equipment used to cut and bend the steel all start from the diameter. Contractors and designers read sizes off structural drawings, and the numbers correspond to standard diameters and weights published in national codes.
How Rebar Sizes Are Numbered
Rebar sizes follow a numbering system that started in the United States and spread to most English-speaking markets. Imperial sizes run from #3 to #18, and through #8 the number approximates the bar diameter in eighths of an inch: a #3 bar is about 3/8 inch, a #4 bar is about 1/2 inch, and a #8 bar is 1 inch. Larger bars such as #9, #10, #11, #14, and #18 use nominal diameters that do not follow the same rule, so the numbers act as labels rather than exact fractions. In the United States the designations come from ASTM A615, while most metric countries follow ISO 6935, and both systems describe the same physical product.
Nominal Diameter Versus Actual Diameter
The nominal diameter is the bar size used in design calculations. It is derived from the cross-sectional area of the bar converted to an equivalent circle, so it excludes the height of the ribs and deformations. A 16 mm deformed bar therefore measures slightly less than 16 mm across the flats, while the ribs bring the overall height above 16 mm. Metric bars are sold by nominal diameter in millimeters: 8, 10, 12, 16, 20, 25, 32, and 40 mm are common stock sizes. Designers who work across markets convert between the two systems, just as builders compare log profiles and diameters when they size timber for a mountain home, where the chosen profile changes both appearance and structural capacity.
Why Weight per Meter Matters
Weight drives purchasing, transport, and placement labor. Bar weight per meter is calculated from the nominal diameter and the density of steel, about 7,850 kilograms per cubic meter. The same relationship explains why thicker members cost more in any material: more mass, more handling, more cost.
Rebar Size Chart: Diameters and Weights
The table below lists the standard imperial bar numbers with their nominal diameters in millimeters and unit weights in kilograms per meter, based on ASTM A615 and equivalent international standards.
| Bar number | Nominal diameter (mm) | Weight (kg/m) |
|---|---|---|
| #3 | 9.5 | 0.560 |
| #4 | 12.7 | 0.996 |
| #5 | 15.9 | 1.556 |
| #6 | 19.1 | 2.235 |
| #7 | 22.2 | 3.042 |
| #8 | 25.4 | 3.973 |
| #9 | 28.7 | 5.060 |
| #10 | 32.3 | 6.404 |
| #11 | 35.8 | 7.907 |
| #14 | 43.0 | 11.384 |
| #18 | 57.3 | 20.239 |
Reading the Size Chart
Each column follows a clear relationship. Weight grows roughly with the square of the diameter, so a #8 bar weighs about seven times as much as a #3 bar even though its diameter is only about 2.7 times larger. Engineers use the chart to convert design forces into bar sizes and to estimate tonnage for ordering. The chart also feeds cost estimates, because suppliers price rebar by weight, and a footing schedule that drops one bar size can shift the total by several percent. Homeowners planning a patio or footing can apply the same logic at a smaller scale, and studies of whether log size affects house size show the identical principle in wood construction: bigger members carry bigger loads up to the point where the structure, not the material, sets the limit.
Metric Equivalents
Most metric projects specify bars by diameter in millimeters rather than by number. A #5 bar and a 16 mm bar are nearly interchangeable in practice, and a #6 bar matches a 19 mm bar. When drawings mix units, convert before ordering, because a one-size error changes tonnage by 20 percent or more.
Types of Rebar and Available Sizes
Rebar is manufactured in several material families, and each family is offered in a particular range of sizes.
- Carbon steel rebar, the default choice, from #3 to #18
- Stainless steel rebar, usually #4 to #11, for marine and corrosive work
- Galvanized rebar with a zinc coating, generally #3 to #11
- Glass fiber reinforced polymer (GFRP) rebar, typically #3 to #8
- Epoxy-coated rebar with a fusion-bonded layer, in standard sizes #3 to #11
- Welded wire fabric (WWF), sheets of smooth or deformed wire in light diameters
- Expanded metal rebar, flattened mesh used for crack control in slabs
Size Availability by Material
Carbon steel dominates because it is available in every standard size at the lowest price. Stainless and GFRP bars cover the most common sizes but not the full range, and specialty products are often stocked in fewer diameters, which can force a redesign when a project calls for an unusual bar. Concrete follows the same logic: a grain size analysis of aggregates measures the particle size distribution of sand and stone, and that distribution dictates how much paste is needed to fill voids, just as bar size dictates how much steel is needed to carry a given force.
Welded Wire Fabric and Expanded Metal
WWF and expanded metal are reinforcement products rather than discrete bars. They arrive in sheets or rolls, with wire diameters typically from 3 mm to 8 mm, and they speed up slab and wall construction because one sheet replaces many individual bars. Their smaller diameters limit them to crack control and light load paths.
Matching Rebar Size to the Application
Application dictates size. Light members need small bars, heavy members need large ones, and the pattern repeats in every structure type. Slabs on grade, driveways, and pool decks sit at the light end, while transfer girders and bridge piers sit at the heavy end, and the difference between those extremes is roughly a factor of thirty in weight.
Light Loads Versus Heavy Loads
Small bars such as #3 and #4 handle slabs, driveways, and light footings where loads stay modest. Medium bars from #5 to #8 appear in beams, columns, and heavier footings. Large bars from #9 to #18 carry the forces in high-rise columns, bridge piers, and transfer girders, where a single bar can hold more than a hundred tons in tension. Organizing stock by size pays off on site, like single-size socket sets that keep one fastener size on one rail, so the crew grabs the right bar without sorting through mixed bundles.
Rebar in Concrete Driveways and Pools
A driveway slab typically uses #3 or #4 bars on a grid at 450 mm centers, supported on chairs so the steel sits in the middle of the slab thickness. Pool shells and decks need closer spacing and sometimes coated bars, because chlorinated water attacks bare steel. Piers and foundations use the largest bars: a concrete pier carrying a house point load can require #5 or #6 bars tied into a footing mat.
Practical Size Selection Steps
Selecting the right bar size is a repeatable process. The sequence below works for most small projects and mirrors the logic behind structural drawings.
- Identify the member type and the loads it carries
- Read the drawing note that specifies bar size, grade, and spacing
- Convert units if the drawing mixes metric and imperial
- Confirm the required cover, which protects the bar from fire and corrosion
- Estimate total tonnage using the weight per meter from the size chart
- Verify delivery against the schedule and inspect bar markings
Exposed and Rusted Rebar
Bars left exposed after formwork is stripped or during a pause in concreting develop surface rust. Light surface rust is acceptable and can even improve bond, but heavy scaling or pitting reduces the effective cross-section. Wire-brush loose scale and remove the cause of exposure before placing concrete. In existing structures, exposed rebar needs treatment that restores cover and prevents further corrosion, and the same judgment applies when choosing the right log diameter for your floor plan: the member has to match the load, the exposure, and the budget, not just the drawing.
Sizing, Spacing, and Code Checks
Codes set minimums and maximums that protect the structure from too little and too much steel. Minimum bar sizes prevent fragile detailing, maximum spacing keeps cracks narrow, and cover rules shield the steel from fire and moisture.
Cover, Spacing, and Stirrups
Spacing and cover checks follow from the size chart. For slabs, maximum spacing is typically three times the slab thickness and never more than 450 mm. Column bars are sized from the axial load, with ties and stirrups in #3 or #4 bars spaced closer near supports where shear peaks. Cutting a concrete slab that contains rebar calls for a scanner and a diamond blade, because a blade that grabs a steel bar can kick violently. The same logic that sets minimum height and size standards for rooms applies to reinforcement: codes translate engineering judgment into numbers that any crew can check, so the finished structure matches the design.
Sizing rebar is a matching exercise. The bar number, diameter, and weight per meter give the designer a vocabulary, the chart supplies the numbers, and the application provides the context. Choose the size that fits the load, protect it with the correct cover, and confirm it on site before the concrete goes in.