Required trace width per IPC-2221B Table 6-1. Enter current, allowable temperature rise, and copper weight to get minimum width for both external (outer) and internal (inner) PCB layers.
IPC-2221 PCB Trace Width
Layer
Width (mm)
Width (mil)
External
--
--
Internal
--
--
Formula & Theory
IPC-2221B Table 6-1 empirical formula:
Area: A (mil²) = I / (k × ΔT0.44)1/0.725
Width: W (mil) = A / (Tcu × 1.378)
k = 0.048 for external layers, 0.024 for internal layers
Tcu = copper weight (oz/ft²) — 1 oz = 1.378 mil thickness
ΔT = allowable temperature rise above ambient (°C)
Worked Example
10 A, 10°C rise, 1 oz copper:
Aext = [10 / (0.048 × 100.44)]1/0.725 ≈ 75.6 mil²
Wext = 75.6 / (1 × 1.378) ≈ 54.9 mil ≈ 1.40 mm
Aint ≈ 151 mil² → Wint ≈ 109.7 mil ≈ 2.79 mm
Assumptions & Limitations
Still air — no forced convection. Airflow reduces required width significantly.
Uniform DC current — no AC ripple content included in the model
Single isolated trace — bundle of traces requires 50% derating
Based on IPC-2221B (2003) Table 6-1; IPC-2152 provides updated coefficients for some applications
Common Mistakes
Ignoring via current capacity: A via neck carrying the same current as the trace must be sized separately. A 0.3 mm drill via typically handles 500 mA–1 A.
Not accounting for ambient temperature: At 70°C ambient with a 130°C Tg board, your ΔT budget is 60°C — set ΔT accordingly, not to a fixed 10°C.
Treating copper pour as a derating factor: A copper pour (polygon fill) helps dissipate heat but IPC-2221 doesn't model this. Treat it as additional safety margin only.
Engineering component digestGet new calculators and power-design notes when they publish.
1 oz/ft² (35 µm) is standard for most PCBs. Use 2 oz for power planes or traces carrying more than 5 A. Some fabs offer 0.5 oz for fine-pitch signal layers.
Why are internal layer traces wider?
Internal layers dissipate heat less effectively — they are sandwiched in FR4 with limited convection. IPC-2221 uses k=0.024 (internal) vs k=0.048 (external) in its empirical formula, resulting in approximately 40% wider required traces for internal layers.
What temperature rise is acceptable?
IPC-2221 uses 10°C as a typical reference. The actual limit depends on board material Tg and ambient temperature. For a 130°C Tg board at 70°C ambient, you have 60°C of headroom. A 10°C rise gives ample margin; 30–40°C rise is acceptable in non-critical applications.