Two numbering systems, one bar
Reinforcing bar is sized two different ways depending on where you are, and confusing them is an expensive mistake.
- Imperial (US) sizes: A #3, #4, #5 and so on. The number is the diameter in eighths of an inch. A #4 is four eighths — half an inch, about 12.7 mm.
- Metric sizes: Named directly by diameter in millimetres. A 10M bar is nominally 10 mm; a 16 mm bar is 16 mm.
They are not interchangeable, and the near-misses are the dangerous part. A #4 at 12.7 mm sits between a 12 mm and a 13 mm metric bar. Substituting one for the other without checking changes the steel area, and steel area is what the design is based on.
Weight is what you actually buy
Rebar is priced and delivered by weight, but specified by diameter and counted in lengths. The bridge between the two is mass per metre, which follows from the cross-sectional area and the density of steel:
Metric bars, named by diameter:
- 10 mm: 0.617 kg per metre
- 12 mm: 0.888 kg per metre
- 16 mm: 1.579 kg per metre
- 20 mm: 2.466 kg per metre
- 25 mm: 3.854 kg per metre
- 32 mm: 6.313 kg per metre
US bars, numbered in eighths of an inch:
- #3 (9.5 mm): 0.560 kg per metre
- #4 (12.7 mm): 0.994 kg per metre
- #5 (15.9 mm): 1.552 kg per metre
- #8 (25.4 mm): 3.973 kg per metre
- #10 (32.3 mm): 6.404 kg per metre
- #14 (43.0 mm): 11.38 kg per metre
Note the gap in the US series: it runs #3 to #11, then jumps straight to #14 and #18. There is no #12, #13 or #15, and a schedule calling for one has a typo in it.
Note how fast that climbs. Weight goes with the square of the diameter, so a #5 is not 25 per cent heavier than a #4 — it is more than half as heavy again. Stepping a bar size up on a large job is a much bigger change to the delivery, and to the crane, than the size numbers suggest.
Why bigger is not simply stronger
It is tempting to treat a larger bar as a free upgrade. Two reasons it is not:
- Bond and crack control: A given area of steel spread over many small bars bonds to the concrete over far more surface than the same area in a few large bars, and it holds cracks tighter. That is why slabs use many small bars rather than a few thick ones.
- Bending and handling: Large bars need serious equipment to cut and bend, and minimum bend radii grow with diameter. A detail that works in 10 mm may be physically impossible in 25 mm.
The specification names a size for reasons that are not only about total strength.
Which sizes turn up where
As a rough orientation for ordinary building work:
- 6 to 8 mm: Links, stirrups, light mesh wire.
- 10 to 12 mm (#3 to #4): Domestic slabs, ground beams, small footings. The most common sizes on a house.
- 16 to 20 mm (#5 to #6): Structural beams, columns, retaining walls.
- 25 mm and above: Heavy civil work, transfer structures, piles.
Reading a bar on site
Bars are rolled with marks identifying the mill, the diameter and the grade. If you have a bundle with no paperwork, the marks are the truth — not the label on the rack, and not what the last delivery was.
Where marks are unreadable, measure across the core of the bar, between the ribs rather than over them, and compare against the diameter table. A caliper and thirty seconds is cheaper than pouring the wrong steel into a foundation.