How the width is calculated
IPC-2221 relates the current a trace can carry to its cross-section and the temperature rise you accept. The copper thickness then turns the cross-section into a width.
I = k × ΔT^0.44 × A^0.725 k = 0.048 outer layers, 0.024 inner layers A in mil², ΔT in °C, I in A width = A ÷ copper thickness (mil)
Example: 1 A on an outer layer with 1 oz copper and a 10 °C rise needs 16.3 mil². With 1.378 mil of copper that is 11.8 mil, or 0.30 mm.
| Current | Outer layer | Inner layer |
|---|---|---|
| 0.5 A | 0.12 mm · 4.5 mil | 0.3 mm · 11.8 mil |
| 1 A | 0.3 mm · 11.8 mil | 0.78 mm · 30.8 mil |
| 2 A | 0.78 mm · 30.8 mil | 2 mm · 80 mil |
| 3 A | 1.4 mm · 53.8 mil | 3.6 mm · 140 mil |
| 5 A | 2.8 mm · 109 mil | 7.2 mm · 283 mil |
| 10 A | 7.2 mm · 283 mil | 18.7 mm · 737 mil |
Minimum widths per IPC-2221 with 1 oz copper and a 10 °C rise.
Copper weight
PCB copper is specified in ounces per square foot: 1 oz of copper spread over a square foot is about 35 µm thick, 2 oz about 70 µm. Doubling the copper roughly halves the width needed for the same current.
IPC-2221 and IPC-2152
The IPC-2221 curves go back to measurements from the 1950s. IPC-2152, published in 2009, is based on new tests. Inner traces turned out to run about as cool as outer ones, so the IPC-2221 inner-layer widths are very conservative. For outer layers IPC-2221 is in the right range but not always on the safe side: a board in still air with no copper planes can run hotter than it predicts, while planes nearby or a thicker board keep traces cooler. For a hobby board, the outer-layer figure with a 10 °C rise is a sound starting point.
Frequently asked questions
How wide should a 1 A trace be?
About 0.3 mm (12 mil) on an outer layer with 1 oz copper and a 10 °C rise. Accepting a 20 °C rise, about 0.2 mm.
Can a short trace be thinner?
The temperature rise depends on the width, not the length. A short neck between large copper areas runs a little cooler, because the copper around it draws the heat away. Length adds resistance and voltage drop.
What does 1 oz copper mean?
One ounce of copper spread evenly over one square foot, about 35 µm thick. It is the standard for the outer layers of most boards; 2 oz is a common option for power boards.
Why is the inner-layer trace so much wider?
IPC-2221 halves the current for inner layers, which makes them about 2.6 times wider. The newer IPC-2152 found inner traces cool about as well as outer ones, so the inner figure is very conservative.