Uses the industry-standard IPC-2221 formula. Calculate trace width from current, or maximum current from trace width. Internal and external layers, common-value quick picks.
The IPC-2221 standard is the industry's classic reference for sizing PCB traces. Its formula is empirical — a curve-fit of experimental data from the 1950s — but it's conservative and safe, which is why almost every quick PCB calculator uses it.
The formula: Area = (I / (k · ΔT0.44))1/0.725. The trace width is then just width = area / copper_thickness.
The k constant depends on where the trace lives: 0.048 for external (top/bottom) layers, 0.024 for internal (buried) layers. Internal traces need roughly 2.6× more copper cross-section for the same current, because they can't shed heat to open air — the surrounding fiberglass traps it.
Copper weight in ounces? One "ounce of copper" means one ounce spread over a square foot — about 35 µm thick, or 1.378 mils. 2 oz doubles the thickness. Standard PCBs ship 1 oz; power designs go 2 oz or higher.
One thing to know: IPC-2221 is conservative. The newer IPC-2152 standard (2009) accounts for board thickness, adjacent copper planes, and airflow — often allowing narrower traces. For everyday designs the IPC-2221 numbers here are safe. For tight power designs, look up IPC-2152 or run a thermal simulation.