How to use the PCB trace width calculator
- Enter the current the trace must carry continuously (use the worst case, for example a motor's stall current if it can stay stalled).
- Choose the allowed temperature rise — 10 °C is a safe default, 20 °C is common for compact boards.
- Select the copper weight (most hobby PCBs are 1 oz) and whether the trace is on an external (top/bottom) or internal layer.
- Enter the trace length to see its resistance, voltage drop and the power it wastes as heat.
The chart shows how the required width grows with current for both layer types, and the table compares 0.5, 1 and 2 oz copper side by side.
Formula
This calculator uses the IPC-2221 (formerly IPC-D-275) approximation:
- I – current in amps
- ΔT – temperature rise above ambient in °C
- 1.378 – the thickness in mils of 1 oz/ft² copper (about 35 µm)
- T – trace temperature = ambient + ΔT, used for the resistance
It is an empirical curve fit, valid roughly up to 35 A, up to 400 mil (10 mm) wide and for a 10–100 °C rise. Outside that range the calculator warns you.
Typical widths (1 oz copper, 10 °C rise)
| Current | External layer | Internal layer |
|---|---|---|
| 0.5 A | 0.12 mm (4.5 mil) | 0.30 mm (11.8 mil) |
| 1 A | 0.30 mm (11.8 mil) | 0.78 mm (30.8 mil) |
| 2 A | 0.78 mm (30.8 mil) | 2.03 mm (80.0 mil) |
| 3 A | 1.37 mm (53.8 mil) | 3.56 mm (140 mil) |
| 5 A | 2.77 mm (109 mil) | 7.19 mm (283 mil) |
Doubling the copper to 2 oz halves these widths. Allowing a 20 °C rise instead of 10 °C makes an external 2 A trace about 0.51 mm instead of 0.78 mm.
Why inner layers need wider traces
Outer traces lose heat to the air. Inner traces are buried in FR-4, which conducts heat poorly, so IPC-2221 uses half the k value. That means about 2.6 times the copper area for the same current and temperature rise. The newer IPC-2152 standard is based on more detailed measurements and often allows narrower inner traces, but IPC-2221 remains a safe and widely used quick estimate.
Common mistakes
- Using average instead of peak current. Size motor and heater traces for the highest current they can carry for more than a moment.
- Forgetting the return path. The ground trace carries the same current as the supply trace — it needs the same width.
- Assuming 2 oz copper by default. Most low-cost boards are 1 oz unless you pay for heavier copper.
- Putting a narrow "neck" in a wide trace. One thin section near a pad becomes the hot spot.
Related tools and guides
Sizing the wires to your board? Use the wire gauge voltage drop calculator. Designing a regulator section? Try the LM317 calculator, and estimate run time with the battery life calculator. For motor currents in robot projects, see our L298N motor driver guide and the robot battery guide. Ready to build? Browse our robotics kits.
