How Do You Calculate PCB Trace Width for Current Capacity?

Detailed Explanation

A copper trace heats up as current flows through it, due to its resistance, and the question a trace-width calculation answers is: how wide does this trace need to be to carry a given current without exceeding an acceptable temperature rise? The relevant variables are the trace's cross-sectional area (width multiplied by copper thickness/weight), the ambient and acceptable maximum temperature, and whether the trace sits on an external layer (open to air, dissipating heat more easily) or an internal layer (surrounded by dielectric, dissipating heat far less efficiently).

IPC-2152 is the current standard reference for this calculation, superseding the older IPC-2221 charts that many designers still default to out of habit. IPC-2152's data was derived from more representative thermal testing of real board constructions and generally produces different — often more conservative for internal layers — results than IPC-2221, which is why current guidance recommends it specifically for anything where getting this right actually matters.

Practical Examples

A 1A signal trace on an external 1oz-copper layer can typically be quite narrow — well under a millimetre — with negligible temperature rise, which is why most low-current signal routing doesn't need a trace-width calculation at all.

A 5A power trace feeding a motor driver or charging circuit is a different story: at 1oz copper on an external layer, IPC-2152 data suggests a trace width in the range of several millimetres to keep temperature rise within a reasonable margin, and the same current on an internal layer would need to be wider still — exactly the kind of calculation worth running explicitly rather than guessing, since undersizing it leads to a trace that runs hot under sustained load and degrades over time.

Design Considerations

• Always specify layer type (internal/external) and copper weight before calculating — both materially change the required width for the same target current.
• Use IPC-2152 rather than IPC-2221 for any current-capacity decision that matters — the older standard's free-air test basis tends to understate the width needed on real, internal-layer constructions.
• Don't rely on trace width alone for high-current paths — consider a dedicated plane or copper pour instead of a routed trace once current gets high enough that trace width alone becomes impractically wide.
• Account for via current capacity too, not just trace width, wherever a high-current path changes layers — a via sized only for routing convenience can become the actual bottleneck. See types of PCB vias.
• High-current design review: For power-dense boards where trace and via sizing directly affects long-term reliability, professional PCB design includes a current-capacity and thermal review as part of the layout process.

Common Mistakes

• Defaulting to an old IPC-2221 chart or a rule-of-thumb width without checking it against the actual layer type and copper weight in the real stack-up.
• Sizing a power trace for routing convenience rather than calculated current capacity, then discovering excessive temperature rise during a thermal test.
• Forgetting that internal-layer traces need to be wider than external traces for the same current, and applying an external-layer width assumption throughout the board.
• Ignoring the current capacity of a via in a high-current path, leaving a narrow via as the actual thermal bottleneck on an otherwise correctly-sized trace.