Rebar Sizes and Weights

How US bar numbers work

A US bar number is the nominal diameter in eighths of an inch. A #4 bar is 4/8, or 1/2 in, and a #5 bar is 5/8 in. The deformations (ribs) make the bar slightly bigger than its nominal size, but weight and area are based on the nominal diameter. Canada uses metric sizes such as 10M and 15M, and Australia uses N-series bars such as N12, where the number is roughly the diameter in millimeters.

US rebar size chart

#3 and #4 are the usual sizes for residential slabs, patios and light footings. #5 and up appear in heavier footings, walls and structural work.

Metric rebar size chart

Canadian bars use slightly different nominal diameters: 10M is 11.3 mm and weighs about 0.785 kg/m, and 15M is 16.0 mm at about 1.570 kg/m. Check the mill certificate or supplier sheet for exact figures.

The weight formula

Steel weighs about 490 lb per cubic foot (7,850 kg/m³). The weight of a bar is its cross-section area times its length times that density, which simplifies to:

lb per ft ≈ 2.67 × (diameter in inches)²

kg per m ≈ 0.00617 × (diameter in mm)²

Check it on a #4 bar: 2.67 × 0.5² = 0.668 lb/ft. On a 12 mm bar: 0.00617 × 144 = 0.888 kg/m. The published weights in the charts are standard nominal values and agree with the formula to within about half a percent.

Worked example: weight of an order

A 16 × 20 ft shed slab whose plans call for #3 bars at 24 in on center, with 3 in of edge clearance, from 20 ft stock. Nine bars run the 20 ft length and eleven run the 16 ft width. Every run fits inside one 20 ft bar, so there are no laps: 20 bars and about 346 linear ft. At 0.376 lb/ft that is about 130 lb of steel, light enough for most pickups, though the 20 ft bars still need a rack or trailer. The rebar calculator works out the bar count, laps and weight for your own slab.

In metric, 100 m of 12 mm bar weighs 100 × 0.888 = 88.8 kg.

Working back from weight

Steel suppliers sometimes quote or sell by weight. To turn weight into length, divide by the weight per foot. 500 lb of #4 bar is 500 ÷ 0.668 = 749 ft, or 37 full bars of 20 ft. In metric, 200 kg of 12 mm bar is 200 ÷ 0.888 = 225 m, or 37 full bars of 6 m.

Reading the marks on a bar

US bars carry rolled-in marks along their length. Typically they show the producing mill's symbol, the bar size number, a letter for the type of steel (S for the common carbon-steel bar standard, W for weldable low-alloy bar), and a grade mark, either a number such as 60 or a continuous line. If the size on the bar does not match your plans, do not use it.

Stock lengths

In the US, rebar is commonly stocked in 20 ft lengths, with 10 ft bars at many home centers and longer bars from steel suppliers. In metric markets, 6 m and 12 m lengths are common. Long bars are awkward to transport, so ask about delivery or cutting to length.

Grade and coating

• Grade: in the US, Grade 60 (60,000 psi yield) is the most common; Grade 40 is also sold. The grade is rolled onto the bar as a mark. Use the grade your plans specify.
• Epoxy-coated (green) or galvanized bars resist corrosion in places exposed to deicing salt or seawater.
• Welded wire mesh is an alternative for some slabs on grade. It is specified differently, by wire spacing and gauge.

Safety and code notes

Bar size, spacing, grade, lap length and concrete cover for structural work are set by building codes and engineering. Use these charts to estimate and check orders, not to design a foundation. Cap exposed vertical bars, wear gloves and eye protection when cutting, and lift long bars with help.

Frequently asked questions