PCB Trace Width Calculator

PCB trace width must be wide enough to carry the target current without excessive temperature rise, which can damage the board or cause intermittent failures. The IPC-2221A standard provides the governing formula.

For External Traces (outer copper layers):

W = (I / (k × ΔT^b))^(1/c) / (T × 1.378)

Simplified using the IPC approximation:

Area (mils²) = (Current / (k × ΔT^0.44))^(1/0.725)

Where:

• k = 0.048 for external layers; 0.024 for internal layers
• ΔT = Allowable temperature rise in °C (typically 10°C for standard boards)
• Area is then converted to width by: Width (mils) = Area / (Thickness in mils)

Standard 1oz copper = 1.4 mils thick (35 µm). 2oz = 2.8 mils.

Worked Example — 3A trace on 1oz external copper, ΔT = 10°C:

• Area = (3 / (0.048 × 10^0.44))^(1/0.725)
• 10^0.44 ≈ 2.754 → 0.048 × 2.754 = 0.1322
• (3 / 0.1322)^1.379 = (22.69)^1.379 ≈ 74 mils²
• Width = 74 / 1.378 = 54 mils (1.37 mm)

Quick Reference Table (external, 1oz copper, ΔT = 10°C):

• 1A → ~12 mils (0.30 mm)
• 2A → ~31 mils (0.78 mm)
• 3A → ~54 mils (1.37 mm)
• 5A → ~109 mils (2.8 mm)
• 10A → ~283 mils (7.2 mm)

Those widths climb faster than the current does, which surprises people. Going from 1A to 10A is ten times the current and roughly twenty-four times the trace width, because the exponent in the IPC formula is 1.38 rather than 1. Past about 5A on a normal board you are usually better off pouring a copper plane than trying to route a trace that wide.

Internal layers are the other trap. The constant k drops from 0.048 to 0.024 because a buried trace has fibreglass on both sides instead of open air, so it cannot shed heat. Working that through the formula, an internal trace needs roughly 2.6 times the width of an external one for the same current and the same temperature rise, not the “about double” that gets quoted. Select the layer type above and the calculator handles it.

Safety note: add a 20% margin for high-reliability or automotive boards, and remember IPC-2221 is about temperature rise, not voltage drop. A long trace can be thermally fine and still drop more voltage than your circuit tolerates.