Method and bounds

Start with the table. Keep its precision.

Gaugepoint uses table 1 of NBS Handbook 100, Copper Wire Tables, issued 21 February 1966. The selected range is 4/0 through 40 AWG. These are nominal bare-round reference diameters at 20 °C from an explicitly dated archive. The introduction describes rounding nominal diameters to 0.1 mil in this part of the table. A lookup is not a measurement of a particular wire.

Exact arithmetic after a rounded starting value

Each stored integer T is the number of tenths of a mil. Divide T by 10 for mils, by 10,000 for inches, and multiply T by 254 then divide by 100,000 for millimetres. The NIST conversion reference gives the exact unit relationships: one inch is 25.4 mm and one mil is one thousandth of an inch. Gaugepoint uses integer decimal arithmetic for these conversions.

A circular mil is the area of a circle with a diameter of one mil. For a nominal diameter d in mils, the geometric area is d² circular mils. This is why 80.8 mil gives exactly 6,528.64 circular mils in this reference model. A circular mil is an area unit, despite its name; it is not a linear mil.

Why mm² is marked approximate

Geometric circle area in square millimetres is π × d² ÷ 4 with d in millimetres. Its display uses eight significant digits and a visible approximate sign. Extra output digits do not mean a physical wire has been measured to that precision. The JSON export retains the approximate numerical result and labels its display policy.

One declared model

The ideal AWG progression is useful background, but Gaugepoint does not substitute an unrounded logarithmic formula for the rounded nominal table. For example, 40 AWG starts at 3.1 mil and converts to 0.07874 mm here. The ideal progression gives a different result because it starts from a different mathematical representation.

The circle is schematic, not print-to-scale and not a fit template. Stranding, insulation, actual manufactured dimensions, temperature changes, electrical sizing, material properties and compliance are outside the model. Do not use this reference lookup to choose a conductor for a circuit or certify a product. On every selection edit, inspect again to produce fresh notes.