Calculator KB-07
PCB trace width calculator
Find out how wide a PCB trace needs to be for a given current, and how much power it dissipates.
Result
In brief: Calculate PCB trace width based on current and allowable temperature rise per IPC-2221 for external and internal layers, plus resistance and voltage drop. Example: Minimum width: 0.898 mm.
Calculation example
| Current | 3 A |
|---|---|
| Allowable trace temperature rise | 20 °C |
| Copper thickness | 35 µm (1 oz) |
| Layer | Outer (air-cooled) |
| Trace length | 50 mm |
| Minimum width | 0.898 mm |
|---|---|
| Same in mil | 35.3 mil |
| Trace cross-sectional area | 0.0314 mm² |
| Trace resistance | 30.06 mΩ |
| Voltage drop | 90.2 mV |
| Trace power loss | 270.6 mW |
How it is calculated
The calculation uses the empirical formula from the IPC-2221 standard: trace cross-sectional area is determined by current and allowable temperature rise.
W = A / (T · 1,378) [mil]
Here, k = 0.048 for outer layers and 0.024 for inner layers; T is the copper thickness in ounces. Resistance is calculated for copper heated to 25 °C plus the specified temperature rise.
What to look for
- The formula is intended for currents up to 35 A, temperature rises of 10–100 °C, and widths up to 10 mm. Outside these limits, the result is approximate.
- Temperature rise isn't the only constraint. Voltage drop can be an issue in a long trace: check it separately in feedback circuits and controller power supply lines.
- Adjacent power traces heat each other. Where space allows, make the trace wider than calculated or use a copper pour.
Frequently asked questions
How wide should a trace be for 3 A?
With 35 µm copper on the outer layer and a 20 °C temperature rise, the minimum is about 0.9 mm according to IPC-2221. On an inner layer, it needs to be about 2.6 times wider.
How does copper thickness affect trace width?
It’s inversely proportional: 70 µm instead of 35 µm lets you make the trace half as wide at the same current and temperature rise.
How much current can a 1 mm-wide trace handle?
About 3.2 A on an outer layer with 35 µm copper and a 20 °C temperature rise. For long traces, also check the voltage drop.
