PCB Trace Current Calculator
Estimate PCB trace width or current capacity, resistance, voltage drop, and power using the IPC-2221 approximation.
About trace current estimates
I = k × ΔT^0.44 × A^0.725
A is the copper cross-section in square mils; k is 0.048 for external layers and 0.024 for internal layers (IPC-2221). Switch the mode to size a trace for a target current, or to find the current a given width can carry for the chosen temperature rise.
These are first-pass estimates. Real board temperature depends on copper pours, airflow, layer stackup, solder mask, and nearby heat sources — validate high-current designs thermally.
How it works
A trace carries current through a copper cross-section of width × thickness. Push current through it and the copper's small resistance turns some power into heat; the trace settles at whatever temperature makes that heat escape as fast as it's generated. The IPC-2221 formula captures that balance — more copper area lets a trace run cooler at the same current, and allowing a bigger temperature rise lets a given trace carry more. The exponents matter: area helps a lot (A^0.725) while the allowed rise helps less (ΔT^0.44), so widening the trace is usually the more effective lever.
Why external and internal layers differ
The constant k is twice as large for outer-layer traces (0.048) as for inner ones (0.024) because a trace on the surface can shed heat to the surrounding air, while a buried trace is wrapped in dielectric and runs hotter for the same current. So an internal trace needs roughly double the width of an external one to carry the same load — an easy detail to miss when routing power on inner layers.
Worked example
For 3 A on an outer layer with a 10 °C rise in 1 oz copper (35 µm thick), the formula asks for roughly a 1.4 mm (55 mil) trace. Bump the allowed rise to 20 °C and the required width drops noticeably, since the trace is allowed to run hotter. For the same net's vias, see thevia current calculator, and convert copper weight to thickness with thecopper weight calculator.
What IPC-2221 does not know
These curves are empirical, fitted to measurements of isolated traces on a board in still air, and they know nothing about the rest of your layout. A trace over a ground plane sheds heat into it and runs cooler than predicted; three high-current traces side by side each heat the others and run hotter. The model also assumes a steady state, so a trace carrying a brief inrush behaves quite differently from one carrying that current continuously. And on any long run,voltage drop usually becomes the binding constraint before temperature does. The guide tosizing a PCB trace covers all three in more detail.