PCB Trace Width Converter

PCB trace width determines how much current a copper trace can safely carry. Too narrow a trace for the current causes overheating and potential board failure.

The IPC-2221 standard formula:

• Cross-sectional area (mils²) = (Current / (k × TempRise^b))^(1/c)
• Width (mils) = Area (mils²) ÷ Thickness (mils)
• Thickness (mils) = copper weight in oz/ft² × 1.378

Where the constants for external layers are:

• k = 0.048, b = 0.44, c = 0.725

And for internal layers:

• k = 0.024, b = 0.44, c = 0.725

Note the 1.378 appears once, in the thickness conversion, not again in the width division. Charts that write the width step as “Area ÷ (Thickness × 1.378)” have already converted the thickness to mils and are dividing by it twice, which yields traces about 27% too narrow.

Standard copper thicknesses:

• 1 oz/ft² = 1.378 mils = 0.035 mm (most common for standard PCBs)
• 2 oz/ft² = 2.756 mils = 0.070 mm (used for power applications)
• 0.5 oz/ft² = 0.689 mils = 0.018 mm (thin boards, fine-pitch components)

Quick reference table (external layer, 1 oz copper, 10°C temp rise):

The internal column being 2.6 times the external one is not a typo. Halving k halves the current an equal area can carry, and because the exponent 1/c is 1.379 rather than 1, that doubling of required area comes out as roughly 2.6× the width.

A practical floor the formula does not know about. Below about 6 mils the thermal answer stops being the binding constraint and the fabricator’s minimum feature size takes over. Most low-cost prototype houses quote 5 or 6 mils as their limit on standard 1 oz copper. So the 4.5 mils this table gives for half an amp is thermally honest and practically unbuildable at many shops. Draw 8 or 10 mils and stop thinking about it. Signal traces are routed at 6 to 10 mils for handling reasons long before current enters the discussion.

Important design considerations:

• The temperature rise value is relative to the ambient temperature. A 10°C rise is a conservative, commonly used design target.
• Internal traces have less cooling capacity (surrounded by FR4 insulation instead of air), so they need to be wider for the same current.
• These calculations assume DC or low-frequency current. At high frequencies, skin effect reduces the effective cross-section of the trace.
• For traces carrying high current near other components, consider the overall thermal management of the board.
• Always add a safety margin of at least 20-50% beyond the calculated minimum width.
• Vias used in the current path also have current limitations and may need multiple vias in parallel for high-current connections.

Unit conversions:

• 1 mil = 0.001 inch = 0.0254 mm
• 1 mm = 39.37 mils