PCB Trace Width Calculator — IPC-2221

Minimum trace width for a target current and temperature rise, for internal and external layers, per IPC-2221.

Example: Carrying 2 A on an external 1 oz layer with a 10 °C rise needs a trace at least 0.78 mm (30.8 mil) wide.

Inputs

Current(A)

Temperature rise(°C)

10 °C is conservative, 20 °C common, 40 °C aggressive.

Copper weight(oz)

1 oz is standard. 2 oz costs more but halves the width needed.

Layer

Trace length (optional)(m)

Gives resistance, voltage drop and power lost in the trace.

Result

Minimum trace width

0.781 mm

In mils: 30.8 mil
Cross-section: 42.4 mil²
Copper thickness: 0.035 mm
Resistance: 614.3 mΩ/m

IPC-2221 is an empirical fit from the 1950s and is generally conservative. It assumes still air, a single isolated trace, and no nearby heat sources — a trace bundled with others or next to a hot regulator runs warmer than this predicts.

Width by current (external, 1 oz, 10 °C rise)

Current	Width (mm)	Width (mil)	Cross-section
500 mA	0.115	4.5	6.3 mil²
1 A	0.3	11.8	16.3 mil²
2 A	0.781	30.8	42.4 mil²
3 A	1.367	53.8	74.2 mil²
5 A	2.765	108.9	150 mil²
10 A	7.194	283.2	390.3 mil²
15 A	12.585	495.5	682.8 mil²

Formula

A = (I / (k × ΔT^0.44))^(1/0.725)    width = A / thickness

Calculations follow IPC-2221B.

Worked example

Carrying 2 A on an external 1 oz layer with a 10 °C rise needs a trace at least 0.78 mm (30.8 mil) wide.

A = (I / (k × ΔT^0.44))^(1/0.725)
A = (2 / (0.048 × 10^0.44))^(1/0.725)
42.39 mil²
k is 0.048 for external traces — internal layers cool much worse, so they need twice the copper.
width = A / thickness
42.39 mil² / 1.378 mil
30.76 mil (0.781 mm)

Frequently asked questions

How wide does a PCB trace need to be for 1 amp?

On a standard 1 oz external layer with a 10 °C rise, about 0.5 mm (20 mil). The same current on an internal layer needs roughly twice that, because there is no air on either side to carry the heat away.

Why do internal traces need to be wider?

They are buried in FR4, which is a poor thermal conductor, so heat has nowhere to go. IPC-2221 halves its constant for internal layers, which works out at about twice the width for the same current and temperature rise.

What temperature rise should I allow?

10 °C is conservative and a good default. 20 °C is common in commercial designs. Going to 40 °C lets you use much narrower traces but leaves little margin if the board runs hot for other reasons — and copper resistance rises with temperature, which compounds it.

Is IPC-2221 conservative or optimistic?

Generally conservative — it assumes still air and an isolated trace. But it also assumes nothing else is heating the board. A trace running beside a hot regulator, or one of several carrying current in parallel, will run warmer than the standard predicts.

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