Trace Width and Current Capacity

The IPC-2221 approximation for external layers:

$I = 0.048\, \Delta T^{0.44}\, A^{0.725}$

where $I$ in amps, $\Delta T$ the allowed temperature rise in °C, and $A$ the cross-section in square mils (1 oz copper ≈ 1.37 mil, 35 µm thick). Internal layers carry about half as much.

Rules of thumb for 1 oz copper, 10 °C rise (external): about 0.25 mm for ~0.8 A, 1 mm for ~2 A, 2.5 mm for ~4 A. Use an online IPC-2152 calculator for real designs, and add vias in parallel when moving high current between layers.