How wide should a PCB trace be? IPC-2221 current capacity, explained

A PCB trace is a thin ribbon of copper, and like any conductor it has resistance. Push too much current through a trace that is too narrow and it heats up; push enough and it acts like a fuse. The industry rule of thumb for how wide a trace needs to be is IPC-2221, and it is worth understanding rather than just plugging into a calculator.

The three inputs that matter

• Current — how many amps the trace carries continuously.
• Allowable temperature rise — how much warmer than ambient you'll let the trace get, typically 10 °C.
• Copper weight — how thick the copper is, quoted in ounces per square foot. 1 oz copper is about 1.37 mils (35 µm) thick; 2 oz is about 2.75 mils.

The IPC-2221 formula

IPC-2221 gives the current a trace can carry for a chosen temperature rise as a function of its cross-sectional area:

I = k × ΔT^0.44 × A^0.725

  I  = current (amps)
  ΔT = temperature rise (°C)
  A  = cross-sectional area (mils²) = width × thickness
  k  = 0.048 for external (outer-layer) traces
       0.024 for internal (inner-layer) traces

Two things fall straight out of this. First, outer-layer traces carry roughly twice the current of inner-layer ones for the same width, because they shed heat to the air (the k constant is double). Second, to find the width you need, you solve for area at your target current and temperature rise, then divide by the copper thickness.

A worked feel for the numbers

On standard 1 oz outer-layer copper with a 10 °C rise, a trace around 0.25 mm (about 10 mil) wide comfortably handles roughly an amp, and you scale up from there. Signal traces carrying milliamps can be far thinner — routing density, not current, sets their width. It is the power and ground traces where this math matters.

Don't forget voltage drop

Ampacity keeps the trace from overheating, but a long thin trace can also drop meaningful voltage. Copper's resistance is about 1.7 × 10⁻⁸ Ω·m; a long 5 V or 3.3 V rail through a skinny trace can sag enough to matter for sensitive analog or RF. For power distribution, check both the temperature rise and the end-to-end voltage drop.

The trace width calculator does all of this — enter current, copper weight, and temperature rise and it returns the minimum width plus resistance and voltage drop. When Banana Board routes a board, its power traces are sized against these same rules automatically.