AWG Wire Size Calculator

American Wire Gauge is not a measurement system so much as amanufacturing log. Wire is drawn by pulling a rod through a die that squeezes it slightly narrower, then through a smaller die, and again, and the gauge number is a count of those passes. More passes means a thinner wire, which is the whole reason the numbering appears to run backwards.

It was later formalised as a geometric series between two fixed points: 36 AWG is 0.005 inches in diameter, 4/0 is 0.46 inches, and there are 39 steps between them. So every step multiplies the diameter by the 39th root of 92, about 0.8905. Three consequences fall straight out of that ratio and are worth carrying around: six gauge steps halve the diameter, six steps quarter the area, and ten steps divide the area by roughly ten.

Area is quoted in circular mils, a unit invented to make this arithmetic painless. One circular mil is the area of a circle one mil across, which means the area in circular mils is just the diameter in mils squared — no pi anywhere. A 12 AWG conductor measures 80.8 mils, and 80.8 squared is 6,530 circular mils, precisely what the tables print.

The series stops at 4/0, since continuing would need negative gauge numbers. Beyond it, conductors are named directly by area in thousands of circular mils: 250 kcmil, 500 kcmil and so on. Resistance is then simply resistivity divided by area — about 10.371 ohm-circular-mils per foot for copper at 20 °C — which is why resistance falls as area rises, and why aluminium at roughly 17.0 comes out around 64% more resistive for the same gauge.

The tool runs in two directions, and both are worth walking through.

Forward: you have a size and want its properties. Take12 AWG copper, the most common branch-circuit conductor in North American residential work. Its diameter is 0.0808 inches, which is 2.052 mm. Squaring the diameter in mils gives 80.8² = 6,530 circular mils, or 3.31 mm². Resistance follows from area: 10.371 divided by 6,530, times 1,000, comes to about 1.59 ohms per 1,000 feet at 20 °C. The NEC works at 75 °C instead, where the same conductor is listed at 1.98 ohms per 1,000 feet — roughly 25% higher, because copper gets more resistive as it warms.

Reverse: you have a wire and want to know what it is. Suppose you are working on an existing installation with no markings legible on the jacket. You strip a short length and a caliper across the bare conductor reads about0.064 inches. Scanning the table for the closest standard diameter gives 14 AWG at 0.0641 inches — a difference of one ten-thousandth of an inch, well inside the tolerance of a hand measurement. The next size up, 12 AWG at 0.0808 inches, is 26% away, so there is no ambiguity about which it is.

Two cautions on that second case. Measure the bare conductor, because a reading across the insulation will be several gauges out on small wire. And if the conductor is stranded, a caliper reads wider than the equivalent solid diameter quoted here, since the strands do not pack perfectly — so treat a stranded reading as an upper bound and check the area figure instead where you can.

Both tables below cover the full standard range, 14 AWG through 750 kcmil. They are split by what you are looking for rather than crammed into one very wide table: dimensions first, then resistance for both materials at both reference temperatures.

Use the dimensions table when you are matching a physical wire or converting between AWG and metric. Use the resistance table for voltage-drop work, and pick the column that matches your basis — 20 °C for a bench calculation or a manufacturer's datasheet, 75 °C for anything following NEC practice. The calculator above does the same lookups and additionally works backwards from a measurement, which the tables cannot.

Unlike the other calculators on this site, nothing here depends on a code edition. These are physical constants of the wire itself and do not change when a new NEC is published.