Wire Gauge Converter (AWG to mm²)
Turn an AWG number into the numbers that decide a job: diameter in mm and inches, area in mm² and circular mils, resistance per 1,000 ft for copper and aluminum, and the ampacities of NEC 310.16. The other tab goes the other way, from a metric cable size to the closest gauge.
Diameters follow the AWG definition (ASTM B258): d = 0.005 in × 92^((36 − n) ÷ 39). Circular mils for 14 AWG to 4/0 are the nominal areas NEC Chapter 9 Table 8 prints, and the resistance columns use the Table 8 DC constants at 75 °C (12.9 Ω·cmil/ft for copper, 21.2 for aluminum), so copper values match Table 8 to the precision it prints — about 1% of published tables elsewhere. Ampacities are the 14–4/0 subset of NEC 310.16 for not more than three current-carrying conductors at 30 °C. Amperage depends on insulation rating, ambient temperature, conduit fill and terminal ratings: your local code and the manufacturer data always govern the final size.
Formula and calculation
Worked example
12 AWG — the standard 20 A household circuit conductor — is 2.0525 mm across, 3.3088 mm² of copper and 6,530 circular mils. Its copper resistance is 1.9755 Ω per 1,000 ft, which is why long runs of it lose voltage. Switch to the mm² tab and a 2.5 mm² metric cable comes back as 13 AWG.
Reading an AWG number
AWG runs backwards to intuition: the bigger the number, the thinner the wire. 4/0 (also written 0000) is the largest size on this page and 40 AWG the smallest, a range of about 92 to 1 in diameter. The steps are geometric — every 3-gauge change doubles or halves the cross-section and therefore halves or doubles the resistance. That is why two 12 AWG conductors in parallel carry roughly the copper of one 9 AWG, and why going from 14 AWG to 12 AWG — a change of just two numbers on paper — buys you 59% more copper area.
Copper, aluminum and the temperature columns
The two resistance and two ampacity columns here are not decoration. Aluminum has about 61% of copper's conductivity, so an aluminum conductor needs roughly two AWG sizes more than copper to carry the same current, and its terminations must be listed for aluminum (AL/CU lugs and antioxidant compound). The 60 °C and 75 °C columns come from the insulation and the termination ratings — most breakers and lugs are 75 °C, so the 75 °C column governs most branch circuits — and both assume a 30 °C ambient with no more than three current-carrying conductors in the raceway. Hot attics and bundled conductors derate from there.
What the results mean
- Diameter
- The conductor diameter in millimetres, from the AWG definition (ASTM B258). In mm² mode it is the diameter of the nearest AWG size, named in the label.
- Diameter
- The same diameter in inches — the unit the AWG standard is defined in (36 AWG is exactly 0.005 in).
- Area
- Cross-sectional area in square millimetres. A metric cable size like 2.5 mm² sits between two AWG sizes, which is why the answer is 'closest'.
- Circular mils
- Area in circular mils — the square of the diameter in mils (thousandths of an inch), the unit NEC tables use. For 14 AWG to 4/0 this is the nominal area printed in Chapter 9, Table 8.
- Copper resistance
- Copper DC resistance per 1,000 ft at 75 °C, from the Table 8 constants (12.9 Ω·cmil/ft), so it matches published Table 8 values to the precision they print.
- Aluminum resistance
- Aluminum resistance per 1,000 ft at 75 °C (21.2 Ω·cmil/ft). Aluminum conducts about 61% as well as copper, so its number is about 1.64× higher for the same size.
- Ampacity
- Current the size may carry under NEC 310.16 assumptions, or a note saying why no table value applies — below 14 AWG and above 4/0 the code uses other tables.
- Metric equivalents
- The sizes either side: metric equivalents of an AWG size, or the two neighbours of the closest AWG for a mm² figure, plus the kcmil alternative above 4/0.
- Substituting your numbers
- The diameter, area and resistance arithmetic with your numbers substituted, so the result can be checked by hand.
Frequently asked questions
How do I convert mm² to AWG?
Match by cross-section: 2.5 mm² is closest to 13 AWG (2.624 mm²), 4 mm² to 11 AWG, 6 mm² to 9 AWG and 16 mm² to 5 AWG. The mm² tab shows the nearest size and the two either side. Above about 107 mm² — 4/0 AWG — conductors are traded by kcmil instead, like 250 or 300 kcmil.
What does AWG mean?
American Wire Gauge, the US conductor standard defined by ASTM B258: diameter = 0.005 in × 92^((36 − n) ÷ 39), where n is the gauge number. A larger number is thinner wire, so 12 AWG is 2.05 mm across while 24 AWG is 0.51 mm. The range here runs from 4/0 (0000), the largest, down to 40 AWG.
How many amps can 12 AWG carry?
NEC 310.16 lists 12 AWG copper at 20 A with 60 °C insulation and 25 A with 75 °C, and 240.4(D) caps 12 AWG branch circuits at 20 A regardless. Those numbers assume no more than three current-carrying conductors in a 30 °C ambient — bundled wires in a hot attic carry less, so derate with the code tables before deciding.
What is a circular mil?
A unit of conductor area built for round wires: square the diameter in mils (thousandths of an inch) and you have the circular mils. A 12 AWG conductor is 80.8 mils across, so 80.8² = 6,530 cmil, which is the 3.309 mm² printed next to it. NEC resistance tables are all in circular mils, which is why the wire tables on this site use them.
Should I use copper or aluminum?
Aluminum costs less and weighs about a third of copper for the same current, so it dominates big service feeders and utility lines. It conducts roughly 61% as well, which is why the ampacity table shows an aluminum size or two behind the copper one, and it needs terminations listed for aluminum. Small branch circuits are almost always copper.