PCB Trace Width Calculator

This calculator uses formulas from IPC-2221 (formerly IPC-D-275), the generic standard on printed board design. The curve-fitting constants (k, b, c) are derived from IPC-2221 charts for both internal and external copper layers. Results are estimates; real-world performance may vary based on your specific application conditions.

External (outer) layer traces benefit from convective and radiative cooling directly to the surrounding air, so they dissipate heat more efficiently. Internal layer traces are embedded in the PCB laminate with much lower thermal conductivity, so they heat up more for the same current. As a result, internal traces typically need to be roughly twice as wide as external traces to carry the same current within the same temperature rise limit.

Temperature rise (ΔT) is the allowable increase in trace temperature above the ambient temperature. A common conservative design choice is 10°C, while less critical designs may allow up to 20–30°C. The IPC-2221 standard covers up to 100°C rise. Lower temperature rise values result in wider required traces, while higher values allow narrower traces.

Copper weight in oz/ft² refers to the weight of copper spread over one square foot of PCB area. 1 oz/ft² corresponds to approximately 1.378 mils (0.035 mm) of copper thickness. Standard PCBs use 1 oz copper, but 2 oz and heavier weights are used for high-current applications where wider traces would be impractical.

These values are calculated from the computed trace width and the trace length you provide. Resistance is based on copper resistivity (~1.724×10⁻⁸ Ω·m) adjusted for operating temperature, then scaled by trace length and divided by cross-sectional area. Voltage drop equals resistance × current, and power loss equals resistance × current². These calculations are only available when you enter a trace length.

The IPC-2221 trace width formula is considered valid up to approximately 35 amperes. Beyond that, the empirical curve-fit data the formula is based on becomes unreliable. For very high-current applications, you may need to use multiple parallel traces, wider copper pours, or thicker copper plating.

Ambient temperature affects the maximum allowable trace temperature (ambient + temperature rise), which is important for derating in high-temperature environments. However, the IPC-2221 trace width formula itself uses only the temperature rise value. A higher ambient temperature means your trace is already closer to its thermal limit, so designers should select conservative temperature rise values in hot environments.

To convert from mils (thousandths of an inch) to millimeters, multiply by 0.0254. For example, a 10 mil trace = 10 × 0.0254 = 0.254 mm. This calculator displays results in both mils and mm for your convenience. Most PCB design software (KiCad, Altium, Eagle) accepts trace widths in either unit.