American Wire Gauge (AWG) is the system used in North America for solid and stranded electrical conductors. The numbers run backwards — 10 AWG is thicker than 14 AWG — and sizes larger than 1 AWG are written 1/0, 2/0, 3/0 and 4/0 before switching to thousands of circular mils (kcmil). This calculator turns any gauge into the numbers you actually need: diameter, cross-sectional area, resistance at your conductor temperature, weight and, for common building sizes, the National Electrical Code ampacity.
How to use the wire gauge calculator
- Under Start from, choose AWG size, Area in mm² or Measured diameter.
- Pick the gauge, or enter the metric area or the bare-metal diameter you measured with calipers.
- Select Copper or Aluminum and the conductor temperature. 20 °C is the standard reference; use 75 °C to match NEC tables for loaded building wire.
- Optionally enter a length to get the resistance of that run.
- Read the results and the AWG table below, which lists every size from 4/0 to 40 for your metal and temperature.
AWG formulas
For 0000, 000, 00 and 0, use n = −3, −2, −1 and 0. A circular mil is the area of a circle one mil (0.001 in) across, which makes the area of a round wire simply its diameter in mils squared. The calculator uses resistivities of 1.724 × 10⁻⁸ Ω·m for annealed copper (the 100% IACS standard) and 2.82 × 10⁻⁸ Ω·m for aluminum at 20 °C, with temperature coefficients of 0.00393 and 0.00403 per °C.
Worked example
12 AWG copper at 20 °C (the default)
d = 0.005 × 9224/39 = 0.08081 in (2.053 mm), so the area is 80.81² ≈ 6,530 cmil = 3.309 mm².
R = 1.724 × 10⁻⁸ ÷ 3.309 × 10⁻⁶ m² = 5.21 Ω/km = 1.588 Ω per 1,000 ft. A 100 ft length has 0.1588 Ω, so each amp through it drops about 0.16 V.
The bare copper weighs 19.77 lb per 1,000 ft, and NEC Table 310.16 lists 20 A (60 °C) and 25 A (75 °C), with a 20 A breaker limit.
Converting a European size. Enter 2.5 mm² and the calculator reports an exact gauge of 13.2 AWG — between 14 AWG (2.08 mm²) and 12 AWG (3.31 mm²) — with 13 AWG as the next larger AWG size. In practice, 12 AWG is the next common building-wire size.
Quick reference: common sizes
| AWG | Diameter | Area | Cu Ω/1,000 ft at 20 °C | Typical use |
|---|---|---|---|---|
| 22 | 0.644 mm | 0.326 mm² | 16.14 | Hookup wire, signals |
| 18 | 1.024 mm | 0.823 mm² | 6.385 | Lamp cord, thermostat |
| 14 | 1.628 mm | 2.081 mm² | 2.525 | 15 A branch circuits |
| 12 | 2.053 mm | 3.309 mm² | 1.588 | 20 A branch circuits |
| 10 | 2.588 mm | 5.261 mm² | 0.9988 | 30 A circuits, dryers |
| 6 | 4.115 mm | 13.30 mm² | 0.3950 | Ranges, subpanels |
| 2 | 6.544 mm | 33.63 mm² | 0.1562 | Feeders |
| 4/0 | 11.68 mm | 107.2 mm² | 0.04901 | Service entrance |
Three handy rules fall out of the geometric series: three gauge steps double or halve the area (and resistance), six steps double or halve the diameter, and ten steps change the area by a factor of about ten.
Stranded wire, aluminum and ampacity
Stranded conductors of a given gauge have the same metal area as solid wire but a larger overall diameter, so measure a single strand and multiply by the strand count if you are identifying an unknown cable. Aluminum has about 61% of copper’s conductivity, so an aluminum conductor must be roughly two gauge sizes larger to carry the same current.
Ampacity — how much current a wire can carry without overheating — depends on insulation rating, ambient temperature, bundling and terminations, not just size. The NEC values shown apply to copper with no more than three current-carrying conductors at 30 °C. For long runs, check the voltage drop calculator, which often calls for a larger size than ampacity alone. To find the current a load draws in the first place, use the watts to amps calculator.
Calculations are for estimation. Wire sizing for any installation must follow your local electrical code, and installations should be done or checked by a licensed electrician.
Frequently asked questions
How is AWG wire size calculated?
ASTM B258 defines the diameter of gauge n as 0.005 in × 92^((36 − n)/39). Gauge 36 is 0.005 in and 0000 (4/0) is 0.46 in, with 39 equal ratio steps between them. Each step down in gauge number makes the wire about 12.3% wider.
What is the resistance of 12 AWG copper wire?
About 1.588 Ω per 1,000 ft (5.21 Ω/km) at 20 °C. At 75 °C, the temperature building wire is usually rated at, it rises to about 1.93 Ω per 1,000 ft because copper's resistance climbs roughly 0.39% per degree Celsius.
How do I convert mm² to AWG?
Choose the mm² option and enter the area. The calculator finds the exact equivalent gauge and the next larger standard size. For example, 2.5 mm² is about 13.2 AWG, so 12 AWG is the next size that is at least as large.
What size wire do I need for 20 amps?
For copper building wire in typical conditions, NEC Table 310.16 gives 12 AWG an ampacity of 20 A in the 60 °C column and 25 A in the 75 °C column, and NEC 240.4(D) limits 12 AWG copper to a 20 A breaker. Long runs may need a larger size for voltage drop, and a licensed electrician should confirm the choice.
Why does a smaller gauge number mean a thicker wire?
The gauge originally counted how many times wire was drawn through successively smaller dies. More drawing steps produced thinner wire, so higher numbers mean smaller diameters.