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PCB Trace Width Calculator

The minimum trace width to carry a current at a chosen temperature rise, by IPC-2221, for inner or outer copper.

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A PCB trace is a thin copper wire, and like any wire it heats up with the current it carries. Too narrow and it runs hot, and at the extreme it acts like a fuse. This sizes the minimum trace width to carry your current while holding the temperature rise you choose, following the IPC-2221 standard, for copper on the outside or buried inside the board.

What sets a trace's width

Three things decide it: how much current the trace carries, how much temperature rise you will accept, and how thick the copper is. More current needs more copper to carry it without heating up. Allowing a larger rise lets you get away with a narrower trace. And thicker copper, measured in ounces, carries more in the same width. The IPC-2221 formula ties these together, and this page runs it for you.

External and internal are not the same

A trace on the outer surface of the board sheds heat into the air, so it can run cooler and narrower for the same current. A trace on an inner layer is wrapped in board material and other copper, so it holds heat and has to be noticeably wider, often two to three times, to stay within the same rise. Pick the layer your trace is actually on, because guessing external for an internal trace undersizes it.

Pick a sensible temperature rise

Ten degrees Celsius is a safe, common default, and it is what many designers use without thinking. Push to twenty or thirty where a warm trace is acceptable and you want to save board space, and the trace can be narrower. For anything where heat is a worry, or for a trace near sensitive parts, stay conservative. The rise you choose is a real engineering call, not a formality, so set it with the board in mind.

Where this stops

This sizes a trace for the heat its own resistance makes while carrying current. It is not a voltage-clearance or creepage calculator, and it is not for controlled-impedance work, which are separate problems. It also ignores the heat a trace loses into pads, vias and copper pours, so real traces often run a little cooler than the number suggests. Treat the result as a safe minimum, give power traces extra, and keep high-voltage spacing to its own rules.

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