PCB Trace Current and Width Calculator
How much current a copper trace carries for a given temperature rise. This is the IPC-2221 chart fit - the one every tool uses. Solve for width to size a trace against a target current, and read the resistance and drop that come with it.
Choosing something to solve for makes Current capacity an editable target and computes the chosen input from it.
Formula
| w, t | trace width and copper thickness (m) |
| A | cross-sectional area, in square mils for the IPC fit |
| dT | allowed temperature rise above ambient (K) |
| k | IPC-2221 layer constant |
What this model assumes, and where it stops
Assumptions
- A single isolated trace on a board with no other heat sources.
- Steady-state DC or low-frequency current - no skin effect.
- Still air, no forced convection, no heat sink.
- Uniform copper thickness at the nominal plating weight.
- The trace is long enough that the ends do not sink heat.
Limitations
- This is IPC-2221, not IPC-2152. IPC-2152 measured what IPC-2221 had guessed and generally permits more current, sometimes much more, because board thermal conductivity, adjacent copper and nearby planes all help. Treat this as the conservative answer.
- The internal-layer halving is the least defensible part of IPC-2221; buried traces are surrounded by laminate that conducts heat away, and IPC-2152 shows they often do better than external ones.
- No adjacent copper. A trace beside a plane or a copper pour runs cooler than this predicts.
- Nothing about vias, which are usually the real thermal bottleneck in a power path.
- Fusing current is a completely different question and is not addressed here.
- AC above a few tens of kHz will crowd current toward the trace edges and raise the effective resistance.
When you need a 3D field solution instead
Closed-form models like the one above hold on idealised geometry. These are the cases where they stop being good enough and a full 3D electromagnetic and thermal solution is the only way to get a trustworthy answer:
- Any board where the trace runs near other heat sources - which is most real boards.
- Power planes, pours and irregular copper shapes, where a single width does not describe the geometry.
- Thermal vias and layer transitions carrying current between planes.
- Cases where you need the actual peak temperature rather than a rise allowance, especially near components with their own dissipation.
- High-frequency current where the distribution across the trace is not uniform.
- Coupled electrical and thermal behaviour, where resistance rises with temperature and temperature rises with resistance.
Common questions
How much current can a 1 mm trace carry?
On 1 oz external copper with a 10 K rise, about 1.9 A by IPC-2221. Doubling the allowed rise to 20 K raises it by 2^0.44, around 36%, to roughly 2.6 A.
Should I use IPC-2221 or IPC-2152?
IPC-2152 supersedes IPC-2221 and is based on measurement rather than the older extrapolated charts. It generally permits more current, sometimes considerably more, because it accounts for board thermal conductivity, nearby copper and planes. This page implements IPC-2221, so treat it as the conservative answer.
Why does IPC halve the current for internal layers?
IPC-2221 assumed buried traces cool worse than surface ones. IPC-2152 measurements largely contradicted that, since internal traces are surrounded by laminate that conducts heat away in all directions. The halving here is conservative and often unnecessarily so.
References
- IPC-2221B - Generic Standard on Printed Board Design, conductor sizing charts
- IPC-2152 - Standard for Determining Current Carrying Capacity, the measured successor