PCB Trace Width Calculator

How to Calculate PCB Trace Width

A PCB trace carrying current heats up due to its own resistance, and a trace that's too narrow for the current it carries can overheat, damage the board, or fail outright. The IPC-2221 standard provides a widely used empirical formula — based on lab-measured heating data rather than pure theory — for finding the minimum copper cross-section needed to keep a trace's temperature rise within a chosen limit.

$$\text{Area} = \left(\frac{I}{k \cdot \Delta T^{0.44}}\right)^{1/0.725}$$

Once the required area is known, dividing it by the copper's actual thickness (set by its weight in ounces per square foot) gives the trace width:

$$\text{Width (mils)} = \frac{\text{Area}}{\text{Copper Thickness (mils)}}$$

Worked Example

Using the calculator's own defaults — 2 amps, a 10°C allowable temperature rise, 1 oz external copper — the required area works out to roughly 42 square mils. Dividing by 1 oz copper's 1.378-mil thickness gives a trace width of about 31 mils, or roughly 0.78 mm. Switching Layer Type to Internal (same current and temperature rise) roughly doubles the required area, since internal traces are sandwiched between insulating layers and can't shed heat into open air the way an external trace can.

External vs. Internal Traces

An external trace runs along the outer surface of the board where it's directly exposed to open air, letting heat escape efficiently by convection. An internal trace is buried between layers of fiberglass substrate, which insulates it thermally far more than air does — the same current produces a bigger temperature rise in an internal trace than in an external one of identical width, which is exactly why the IPC-2221 formula uses a smaller k constant (0.024 vs. 0.048) for internal traces, requiring roughly double the copper area for the same current and temperature rise.

Common PCB Trace Width Mistakes

Using the external-trace constant for a trace that's actually buried between layers is a frequent and consequential error — it can undersize an internal trace by roughly half the copper it actually needs. Forgetting that copper weight sets the trace's thickness, not its width, sometimes leads to skipping that unit conversion step entirely. Treating the IPC-2221 result as an exact physical guarantee, rather than an empirical approximation that doesn't account for nearby copper pours, board stack-up, or ambient airflow, is the broader mistake — always leave margin and check against your manufacturer's actual capabilities.

PCB Trace Terms You Should Know

Copper Weight — the thickness of copper on a PCB layer, measured in ounces per square foot (1 oz = 1.378 mils thick); a common shorthand rather than a literal weight measurement.

Mil — one-thousandth of an inch (0.0254 mm), the standard small unit for PCB trace dimensions.

Temperature Rise (ΔT) — how much hotter the trace is allowed to get above the surrounding ambient temperature under full load current.

External / Internal Layer — whether a copper layer sits on the board's outer surface (exposed to air) or buried between inner layers (insulated by substrate on both sides).

This calculator implements the IPC-2221 empirical formula as an industry-standard approximation, not an exact physics calculation. Always verify the result against your PCB manufacturer's minimum trace width and spacing capabilities — this tool is not a substitute for a full design review.