Calculation method and supported range
Exact length definitions: 1 inch = 25.4 mm; 1 mil = 0.001 inch. Copper conversion uses the common nominal convention 1 oz/ft² ≈ 35 μm, linearly scaled.
Nominal copper conversion is not a mass-density calculation or a plated finished-copper specification. Suppliers round foil classes (for example, ½ oz is often labeled 18 μm); confirm actual foil and plating tolerances. Zero is valid; negative dimensions are rejected.
Technical sources
Using the result
A mil is one thousandth of an inch. Nominal foil designations are procurement shorthand, not measurements of finished outer-layer copper after plating. Retain both the original unit and the permitted tolerance on fabrication drawings.
From a calculation to a released design
Keep the input assumptions with the result. Recalculate when the ordered material, dielectric height, copper or operating conditions change. Confirm the final construction with the fabricator and use field simulation or measurement wherever the simplified model omits an important effect.
See the controlled-impedance guide and material selection workflow for the next checks.