Copper is a good conductor, but it is not a perfect one — a trace carrying current always has some resistance, and that resistance turns part of the current's energy into heat (I²R). A trace that is too narrow for its current will heat up, and if it heats up too much it can damage the board's fibreglass substrate, discolour or lift the copper, or in extreme cases open-circuit entirely. The industry-standard way to size a trace safely is the IPC-2221 formula (the same standard used by every professional PCB design tool):
A trace on the outer layer of the board is directly exposed to the surrounding air, so heat can escape by convection straight off its surface — it runs cooler for a given current. A trace on an inner layer is sandwiched between layers of fibreglass (FR4), which is a poor thermal conductor, so the heat has to travel much further to escape. That is why internal traces get the smaller k=0.024 (half of the external k=0.048) and, for the same current and temperature rise, need roughly 2.6× more copper area than an external trace.
| Quantity | Formula / Value |
|---|---|
| Required area (given I, ΔT) | A = (I/(k×ΔT0.44))1/0.725 |
| Current capacity (given A, ΔT) | I = k×ΔT0.44×A0.725 |
| k, external layer | 0.048 |
| k, internal layer | 0.024 |
| 1 oz copper thickness | 1.378 mils (35 µm) |
If you need to control the exact temperature rise more precisely (e.g. keep a trace under 5 °C rise near a sensitive component), or want to work through the maths interactively, the companion PCB Trace Temperature Rise calculator uses the identical formula in more depth. This page focuses purely on the everyday question: "how wide does my trace need to be?"
It depends on the current, your allowed temperature rise, the copper weight, and whether the trace is on an external or internal layer. As a rough starting point, a 1 oz external trace needs about 12 mils (0.3 mm) per amp at a modest 10 °C rise, but always calculate the exact value with the IPC-2221 formula for your specific conditions.
IPC-2221 is the industry-standard PCB design guideline (successor to the older MIL-STD-275) that defines the empirical relationship between trace current, cross-sectional area, and temperature rise. It is the formula used by virtually every professional PCB CAD tool's trace-width calculator.
External traces sit exposed to open air on the top or bottom of the board, so they can shed heat easily by convection. Internal traces are buried inside layers of fibreglass, a poor heat conductor, so for the same current they run hotter — needing roughly 2.6× more copper area to hit the same temperature rise.
10 °C is a common conservative choice for general traces; 20 °C is acceptable for less critical or short runs. Traces near heat-sensitive components (crystals, precision sensors) are often designed for 5 °C or less.
Copper weight on a PCB is specified in ounces per square foot; 1 oz copper is about 1.378 mils (35 µm) thick. 2 oz is twice as thick (70 µm), and so on — thicker copper carries more current for the same trace width.
The IPC-2221 formula is independent of length — it predicts the steady-state temperature rise from cross-sectional area and current alone. Very long, unbroken high-current traces may benefit from extra margin in practice due to voltage drop (see the Trace Resistance calculator) even if the thermal calculation alone doesn't require it.
Yes. For currents where the formula demands a very wide trace, a solid copper pour (plane) behaves like an extremely wide trace and can carry much more current at a lower temperature rise — the standard approach for power and ground distribution.
On a standard 1 oz external layer at a 10 °C rise, about 1 A. On 2 oz copper it can carry noticeably more; on an internal layer it can carry meaningfully less for the same temperature budget — use the "Width → Current" tab above for your exact numbers.
Always round up, and preferably to a clean design-rule increment (e.g. the nearest 5 or 10 mils), to keep a safety margin over the bare-minimum calculated value and to match common manufacturing capabilities.
IPC-2221 was developed primarily for rigid FR4 boards. Flex circuits (polyimide substrates) have different thermal behaviour, so while the formula gives a reasonable starting estimate, flex-specific design guidelines should be consulted for critical designs.
It will run hotter than intended, potentially discolouring or delaminating the board locally, and in extreme overcurrent situations the trace can act as a fuse and burn open — sometimes desirable (intentional fuse traces) but usually a reliability failure.
A commonly quoted rough guide is about 12 mils (0.3 mm) of 1 oz external trace per amp for a modest 10 °C temperature rise, but this is only a starting estimate — always verify with the actual formula for anything current-critical, especially on internal layers or with tighter thermal budgets.
It doesn't quite — area scales as I1/0.725 ≈ I1.38, so doubling current roughly requires about 2.6× the area (and since width is proportional to area, roughly 2.6× the width) for the same temperature rise, slightly more than a straight doubling.
PCB Trace Temperature Rise • Trace Resistance & Voltage Drop • Via Current Capacity • Copper Weight Converter • All Calculators