PCB trace width calculator
An internal layer is buried in FR4, which is a poor conductor of heat, so it cannot shed the same power as a trace on the surface exposed to air. IPC-2221 handles that with a different constant: 0.048 for external traces against 0.024 for internal. In practice an internal trace needs roughly twice the width of an external one for the same current and temperature rise.
IPC-2221 is a conservative general guideline derived from measurements on isolated traces. Dense boards, high ambient temperatures and long traces all warrant more margin, and IPC-2152 supersedes it for serious thermal work.
IPC-2221 sizes a trace from cross-sectional area: A = (I ÷ (k × ΔT^0.44))^(1/0.725), where k is 0.048 external and 0.024 internal. Two amps on an external 1 oz layer at a 10 °C rise needs about 33 mil, or 0.84 mm.
How to size a PCB trace
The temperature rise is the design choice, and it is more forgiving than instinct suggests. A trace that runs 20 °C above ambient is entirely normal and is not damaging anything on its own — the limit is the board glass transition temperature and whatever heat-sensitive part sits nearby. What the formula does not capture is context: a trace next to a hot regulator starts from a higher ambient, a trace over a ground plane sheds heat better, and a via carrying the same current is a far more constrained path than the trace either side of it. For power designs, the vias usually fail before the traces.
Questions
For 2 A on external 1 oz copper at a 10 °C rise, about 0.84 mm. Doubling the copper weight roughly halves the width needed.