How this is calculated. IPC-2221 gives current capacity as
I = k · ΔT0.44 · A0.725, where
A is the conductor
cross-section in mil² and
k is 0.048 for external layers or 0.024 for internal layers,
which run cooler because the board carries heat away less readily. Solving for area and dividing by
copper thickness gives the width. Resistance uses copper resistivity 1.72×10⁻⁸ Ω·m corrected for
temperature at 0.393 %/°C.
This width is below about 6 mil — many low-cost fabricators will not hold it, or will charge more. Check your fabricator's minimum before committing.