A PCB trace is just a thin copper conductor, so like any conductor it heats up as current flows through its resistance. The trace width must be large enough that this self-heating stays within an acceptable temperature rise. The long-standing IPC-2221 formula relates the current, the allowed temperature rise and the copper cross-sectional area; dividing the area by the copper thickness gives the width. Internal layers need wider traces because they cannot shed heat to the air as easily.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: IPC-2221B and IPC-2152 trace current-capacity standards.
The IPC-2221 formula
Where I is current in amps, ΔT is temperature rise in °C, A is cross-sectional area in mils², and k = 0.048 for external (outer) layers or 0.024 for internal layers. Rearranged to find the area you need:
Copper thickness comes from the weight: 1 oz/ft² ≈ 1.37 mils (35 µm). So a 2 oz board has twice the thickness and needs about half the width for the same current.
Worked example — a 3 A power trace
Scenario: 3 A on a 1 oz external layer with a 10 °C rise.
On a 2 oz board the same 3 A trace would be about 27 mils (0.69 mm) — half the width, because the copper is twice as thick. If the trace were on an internal layer, double the width again.
Frequently Asked Questions
Use IPC-2221: I = k × ΔT0.44 × A0.725 (k = 0.048 external, 0.024 internal). Solve for area A in mils², then divide by copper thickness for the width. 2 A, 1 oz, 10 °C external ≈ 0.78 mm (31 mils).
1 oz/ft² ≈ 35 µm (1.37 mils); 2 oz ≈ 70 µm; 0.5 oz ≈ 17.5 µm. Thicker copper carries more current for the same width.
Buried traces can't shed heat to air as well, so IPC-2221 uses k = 0.024 (vs 0.048) — about twice the cross-section, and therefore roughly twice the width, for the same current and rise.
10 °C is a common conservative default; 20 °C is often used for power traces. A higher allowed rise lets the trace be narrower, but keep the total board temperature within the laminate and component ratings.
Yes. IPC-2221 is older, simpler and more conservative (wider traces). IPC-2152 is newer and accounts for board material and copper planes, often allowing narrower traces. IPC-2221 is a safe first estimate.