American wire gauge. American Wire Gauge (AWG) is a logarithmic stepped standardized wire gauge system used since 1857, predominantly in North America, for the diameters of round, solid, nonferrous, electrically conducting wire. Dimensions of the wires are given in ASTM standard B 258. The cross-sectional area of each gauge is an important factor for determining its current-carrying capacity.

Origin 
The AWG originated in the number of drawing operations used to produce a given gauge of wire. Very fine wire (for example, 30 gauge) required more passes through the drawing dies than 0 gauge wire did. Manufacturers of wire formerly had proprietary wire gauge systems; the development of standardized wire gauges rationalized selection of wire for a particular purpose.

While the AWG is essentially identical to the Brown & Sharpe (B&S) sheet metal gauge, the B&S gauge was designed for use with sheet metals as its name suggests. These are functionally interchangeable but the use of B&S in relation to wire gauges, rather than sheet metal gauges, is technically improper.

Specifications 
Increasing gauge numbers denote logarithmically decreasing wire diameters, which is similar to many other non-metric gauging systems such as British Standard Wire Gauge (SWG). However, AWG is dissimilar to IEC 60228, the metric wire-size standard used in most parts of the world, based directly on the wire cross-section area (in square millimetres, mm2). 

The AWG tables are for a single, solid and round conductor. The AWG of a stranded wire is determined by the cross-sectional area of the equivalent solid conductor. Because there are also small gaps between the strands, a stranded wire will always have a slightly larger overall diameter than a solid wire with the same AWG.

Formulae 
By definition, 36 AWG is 0.005 inches in diameter, and 0000 AWG is 0.46 inches in diameter. The ratio of these diameters is 1:92, and there are 40 gauge sizes from 36 to 0000, or 39 steps. Because each successive gauge number increases cross sectional area by a constant multiple, diameters vary geometrically. Any two successive gauges (e.g.,  and ) have diameters whose ratio  is  (approximately 1.12293), while for gauges two steps apart (e.g., , , and ), the ratio of the  to  is about 1.122932 ≈ 1.26098. Similarly for gauges n steps apart the ratio of the first to last gauges is about 1.12293n.

The diameter of an AWG wire is determined according to the following formula:

(where  is the AWG size for gauges from 36 to 0,  for 00,  for 000, and  for 0000. See below for rule.)

or equivalently:

The gauge can be calculated from the diameter using 

and the cross-section area is
.

The standard ASTM B258-02 (2008), Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes of Solid Round Wires Used as Electrical Conductors, defines the ratio between successive sizes to be the 39th root of 92, or approximately 1.1229322. ASTM B258-02 also dictates that wire diameters should be tabulated with no more than 4 significant figures, with a resolution of no more than 0.0001 inches (0.1 mils) for wires thicker than 44 AWG, and 0.00001 inches (0.01 mils) for wires 45 AWG and thinner.

Sizes with multiple zeros are successively thicker than 0 AWG and can be denoted using "number of zeros/0", for example 4/0 AWG for 0000 AWG.  For an /0AWG wire, use  in the above formulas. For instance, for 0000 AWG or 4/0 AWG, use .

Rules of thumb 
The sixth power of  is very close to 2, which leads to the following rules of thumb:
 When the cross-sectional area of a wire is doubled, the AWG will decrease by 3. (E.g. two 14 AWG wires have about the same cross-sectional area as a single 11 AWG wire.)  This doubles the conductance.
 When the diameter of a solid round wire is doubled, the AWG will decrease by 6. (E.g. 1 mm diameter wire is ≈18 AWG, 2 mm diameter wire is ≈12 AWG, and 4 mm diameter wire is ≈6 AWG.) This quadruples the cross-sectional area and conductance.
 A decrease of ten gauge numbers (E.g. from 24 AWG to 14 AWG) multiplies the area, weight, and conductance by approximately 10.
Convenient coincidences result in the following rules of thumb for resistances:
 The resistance of copper wire is approximately  for 10 AWG,  for 20 AWG,  for 30 AWG, and so on.  For an arbitrary gauge n, it's approximately 10n/10 Ω per .
 Because aluminum wire has a conductivity of approximately 61% of copper, an aluminum wire has nearly the same resistance as a copper wire that is two sizes smaller, which has 62.9% of the area.

Tables of AWG wire sizes
The table below shows various data including both the resistance of the various wire gauges and the allowable current (ampacity) based on a copper conductor with plastic insulation.  The diameter information in the table applies to solid wires.  Stranded wires are calculated by calculating the equivalent cross sectional copper area.  Fusing current (melting wire) is estimated based on  ambient temperature.  The table below assumes DC, or AC frequencies equal to or less than 60 Hz, and does not take skin effect into account.  "Turns of wire per unit length" is the reciprocal of the conductor diameter; it is therefore an upper limit for wire wound in the form of a helix (see solenoid), based on uninsulated wire.

In the North American electrical industry, conductors thicker than 4/0AWG are generally identified by the area in thousands of circular mils (kcmil), where 1 kcmil = 0.5067 mm2. The next wire size thicker than 4/0 has a cross section of 250 kcmil. A circular mil is the area of a wire one mil in diameter. One million circular mils is the area of a circle with 1,000 mil (1 inch) diameter. An older abbreviation for one thousand circular mils is MCM.

Stranded wire AWG sizes
AWG can also be used to describe stranded wire. The AWG of a stranded wire represents the sum of the cross-sectional diameter of the individual strands; the gaps between strands are not counted. When made with circular strands, these gaps occupy about 25% of the wire area, thus requiring the overall bundle diameter to be about 13% larger than a solid wire of equal gauge.

Stranded wires are specified with three numbers, the overall AWG size, the number of strands, and the AWG size of a strand. The number of strands and the AWG of a strand are separated by a slash. For example, a 22AWG 7/30 stranded wire is a 22AWG wire made from seven strands of 30AWG wire.

As indicated in the Formulas and Rules of Thumb sections above, differences in AWG translate directly into ratios of diameter or area. This property can be employed to easily find the AWG of a stranded bundle by measuring the diameter and count of its strands. (This only applies to bundles with circular strands of identical size.) To find the AWG of 7-strand wire with equal strands, subtract 8.4 from the AWG of a strand. Similarly, for 19-strand subtract 12.7, and for 37 subtract 15.6.

Measuring strand diameter is often easier and more accurate than attempting to measure bundle diameter and packing ratio. Such measurement can be done with a wire gauge go-no-go tool or with a caliper or micrometer.

Nomenclature and abbreviations in electrical distribution

Alternative ways are commonly used in the electrical industry to specify wire sizes as AWG.
 4 AWG (proper)
 #4 (the number sign is used as an abbreviation of "number")
 № 4 (the numero sign is used as an abbreviation for "number")
 No. 4 (an approximation of the numero is used as an abbreviation for "number")
 No. 4 AWG
 4 ga. (abbreviation for "gauge")
 000 AWG (proper for thick sizes)
 3/0 (common for thick sizes)  Pronounced "three-aught" or "triple-aught"
 3/0 AWG
 #000

Pronunciation
AWG is colloquially referred to as gauge and the zeros in thick wire sizes are referred to as aught . Wire sized 1 AWG is referred to as "one gauge" or "No. 1" wire; similarly, thinner sizes are pronounced " gauge" or "No. " wire, where  is the positive-integer AWG number. Consecutive AWG wire sizes thicker than No. 1 wire are designated by the number of zeros:
 No. 0, often written 1/0 and referred to as "one-aught" or "single-aught" wire
 No. 00, often written 2/0 and referred to as "two-aught" or "double-aught" wire
 No. 000, often written 3/0 and referred to as "three-aught" or "triple-aught" wire
and so on.

See also
IEC 60228, international standards for wire sizes
French gauge
Brown & Sharpe
Circular mil, North American Electrical industry standard for wires thicker than 4/0.
Birmingham Wire Gauge
Stubs Iron Wire Gauge
Jewelry wire gauge
Body jewelry sizes, which commonly uses AWG (especially for thinner sizes), even when the material is not metallic.
Electrical wiring
Number 8 wire, a term used in the New Zealand vernacular

References

Wire gauges
Customary units of measurement in the United States
Logarithmic scales of measurement