PCB Trace Width Calculator
A printed-circuit trace heats up as current flows through it, and IPC-2221 gives a standard estimate of how wide the trace must be to hold the temperature rise within a chosen limit. This calculator applies the IPC-2221 relationship using your current, allowed temperature rise, copper thickness, and the layer constant k (external or internal). It returns the required cross-sectional area and the trace width in mils and millimetres. The constant k and the temperature rise are user inputs because they depend on your layer choice and thermal budget. Always add manufacturing margin.
IPC-2221 trace width formula
area (sq mils) = (I / (k * dT^0.44))^(1 / 0.725)
thickness (mils) = copper (oz) * 1.378
width (mils) = area / thickness (mils)
width (mm) = width (mils) * 0.0254
I is current in amps, dT the allowed temperature rise in degrees Celsius, and k is 0.048 for external traces or 0.024 for internal traces per IPC-2221. One ounce of copper is about 1.378 mils thick.
Design notes
- Use k = 0.048 for outer-layer traces and k = 0.024 for inner-layer traces.
- Internal traces run hotter, so they need to be wider for the same current.
- One mil is one-thousandth of an inch, equal to 0.0254 mm.
- The result is a guideline; add margin for tolerance, vias, and reliability.
- The temperature rise and k constant are user inputs that depend on your board.
PCB trace width: frequently asked questions
What is the IPC-2221 trace width formula?
IPC-2221 gives the cross-sectional area in square mils as A = (I / (k * dT^0.44))^(1/0.725), where I is the current in amps, dT is the allowed temperature rise in degrees Celsius, and k is a constant. The width in mils is then A divided by the copper thickness in mils.
What are the k constants for internal and external traces?
IPC-2221 uses k = 0.048 for external traces (on the outer surface, better cooled) and k = 0.024 for internal traces (buried, poorer cooling). The constant is a user input here so you can match your layer; internal traces need to be wider for the same current.
How is copper thickness converted to mils?
Copper is specified in ounces per square foot. One ounce of copper is about 1.378 mils thick (0.0347 mm). The area is divided by the thickness in mils to get the trace width, so thicker copper allows a narrower trace for the same current.
What temperature rise should I allow?
A common design choice is a 10 to 20 degree Celsius rise above ambient, but the right value depends on your board's thermal environment and the components nearby. It is a user input because there is no single correct figure; consult your thermal budget.
Is the IPC-2221 result a minimum or a recommendation?
It estimates the width needed to keep the temperature rise at your chosen limit for a given current. It is a guideline, not a hard safety limit. Add margin for manufacturing tolerance, vias, connectors, and high-reliability applications, and follow your board house's minimums.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. Formula per IPC-2221 generic standard. See our methodology.