Wrench size


Width across flats is the distance between two parallel surfaces on the head of a screw or bolt, or a nut, mostly for torque transmission by positive locking.
The term width across flats is used for the following forms:
The width across flats indicates the "size" of the spanner. It is imprinted on the spanners in millimeter values. Older English and current American spanners have inch sizes that are imprinted in intermediate sizes in fractions.
The two systems are in general not compatible, which can result in rounding of nuts and bolts. There are some exceptions with a few sizes being close enough in sizes to interchange. This includes 19 mm and in, which are interchangeable for most purposes. Sizes that may interchange, depending on the precision needed, includes 2 mm, 4 mm and 8 mm.

Width across flats

The width across flats of the fastener is nominally the same as that on the tool. The table below shows dimensions of standard metric spanners:
Nominal thread diameter M1.6M2M2.5M3M3.5M4M5M6M8M10M12M14M16M18M20M22M24M27M30M33M36M39M42
Width across flats 3.2455.567891011121314151719212427303436414650556065

Widths for bicycles

In addition to industry standards, there are special thread standards, such as the bicycle threads according to DIN 79012, a fastening thread on metric-inch size basis of the designation.
Modern bicycle spokes carry the bike thread FG 2.3.