PCB Trace Width and Current Capacity Calculator (IPC-2221)
How wide must a trace be to carry a given current without overheating? This calculator applies the IPC-2221 relation between current, temperature rise and copper cross-section to give a minimum trace width in mils and millimeters.
Your numbers
Continuous current the trace must carry
Rise above ambient you accept for the trace
Your results
IPC-2221 curves are empirical and derived from older data, valid for modest currents and temperature rises. Results ignore nearby heat sources, vias, airflow, pulsed current and board thickness. Check against IPC-2152 or thermal simulation for high-current or high-reliability designs.
Required trace width (mm) vs current
Sweeps Trace current from 0.5 to 10, holding your other inputs fixed.
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How to use the calculator
Enter the continuous current, the temperature rise you accept above ambient, the copper weight and whether the trace is on an outer or an inner layer. The result shows the cross-section area, the minimum width and a width with a 20 percent margin. The chart sweeps the current from 0.5 to 10 amps so you can see how width grows. One ounce copper is 1.378 mils thick, which is why a 3 amp trace at a 10 C rise comes out near 1.4 mm on the outer layer.
The formula behind it
IPC-2221 gives I = k * dT^0.44 * A^0.725, where I is current in amps, dT is temperature rise in degrees C, A is cross-section in square mils, and k is 0.048 for external traces and 0.024 for internal ones. Rearranged, A = (I / (k * dT^0.44))^(1/0.725). Width equals A divided by copper thickness. Internal layers need a wider trace because they shed heat less easily than outer layers exposed to air. The model is empirical and best for currents up to about 35 amps.
- External: k = 0.048
- Internal: k = 0.024
- 1 oz copper = 1.378 mil
- Lower allowed rise means a wider trace
Limits and good practice
The formula does not model voltage drop, so calculate resistance separately for long runs. IPC-2152 is the later standard with more complete data, including the effect of board thickness and nearby copper planes, and it is often more accurate and sometimes less conservative. For defense products operating hot, use a lower temperature rise, add margin, derate for the maximum ambient, and treat pulsed currents with a thermal analysis. Always confirm with your design rules and the fabricator's minimum trace and space limits.
Frequently Asked Questions
What trace width do I need for 3 amps?
With 1 oz copper, a 10 C rise and an external layer, the IPC-2221 relation gives roughly 54 mils, about 1.4 mm. Internal layers need about twice the cross-section. Add margin for hot ambient conditions and nearby heat sources.
Why are internal traces wider?
Internal traces are surrounded by dielectric and cannot lose heat to air as easily as outer traces. IPC-2221 uses a smaller k constant for internal layers, which increases the area needed for the same current and temperature rise.
Should I use IPC-2221 or IPC-2152?
IPC-2152 is newer and based on more extensive measurements, including board thickness and plane effects. IPC-2221 is quick and widely used for first estimates. For high-reliability or high-current work, check against IPC-2152 or run a thermal simulation.
Does this include voltage drop?
No. It only addresses heating. Voltage drop depends on trace length, thickness and temperature. Compute resistance as resistivity times length over cross-section, and check that the drop is acceptable for your load.
Netray works on the ERP and manufacturing side of electronics, such as BOMs, revisions and quality data. Contact us if you want design data and ERP records linked privately.
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