Maker Tools
PCB Trace Width Calculator
Size a copper trace to IPC-2221: enter a target current to get the minimum width, or enter a width to find the current it can safely carry. Every result includes trace resistance, voltage drop and power loss for your board's copper weight and layer.
How the PCB trace width calculator works
This trace width calculator uses the IPC-2221 conductor sizing formula, the same standard fabricators and EDA tools follow. It relates the current a copper trace carries to its cross-sectional area and the temperature rise you allow above ambient. Size a trace by giving a target current to get the minimum width, or enter a width to find the current it can carry. Both directions use the same equation, just solved for a different unknown.
Trace ampacity and temperature rise
Ampacity is the current a trace can carry continuously for a chosen temperature rise. IPC-2221 models it as I = k · ΔT0.44 · A0.725, where A is the copper cross-section in square mils and ΔT is the rise in °C. A wider or thicker trace has more area, so it carries more current for the same heating. The default 10 °C rise is a conservative, safe starting point; many designs allow 20 °C or more where the copper can shed heat into a plane or open air.
Copper weight and internal vs external traces
Copper weight sets the trace thickness: 1 oz/ft² is about 35 µm (1.37 mil), 2 oz doubles it, and so on. Doubling the copper weight roughly halves the width you need for the same current. IPC-2221 uses k = 0.048 for external traces on the outer layers, which radiate heat to the air, and k = 0.024 for internal traces buried between layers, which run hotter and carry only about half as much current at a given width. Always pick the internal setting for inner-layer routing.
Copper trace resistance and voltage drop
Meeting the ampacity limit keeps a trace from overheating, but a long thin trace can still drop meaningful voltage. Resistance is R = ρ · L / (W · t) with copper resistivity ρ = 1.72 × 10⁻⁸ Ω·m, so a 0.5 mm wide 1 oz trace is roughly 1 mΩ per mm. Voltage drop is V = I · R and power lost as heat is P = I² · R. On power rails, keep the drop small so parts at the far end still see a solid supply, and widen the trace or add copper pour where the current is high.
Frequently asked questions
How wide should a PCB trace be for a given current?
Enter your target current, copper weight and allowed temperature rise, and the calculator solves the IPC-2221 formula for the minimum width. As a rough guide, a 1 oz external trace needs about 0.3 mm (12 mil) for 1 A at a 10 °C rise, but always size for your own copper weight and layer.
What is the IPC-2221 trace width formula?
IPC-2221 models the current as I = k · ΔT^0.44 · A^0.725, where A is the copper cross-section in square mils, ΔT is the temperature rise in °C, and k is 0.048 for external traces or 0.024 for internal traces. The calculator inverts this equation to solve for width, or evaluates it directly to find ampacity.
Why do internal traces carry less current than external ones?
Internal traces are buried between layers, so they cannot shed heat into the surrounding air as easily as outer-layer traces and run hotter for the same current. IPC-2221 reflects this with a lower constant (k = 0.024 versus 0.048), so an internal trace of a given width carries only about half the current of an external one and must be made correspondingly wider.
How does copper weight affect the required trace width?
Copper weight sets the trace thickness: 1 oz/ft² is about 35 µm (1.37 mil), and 2 oz is double that. Since the ampacity depends on cross-sectional area, doubling the copper weight roughly halves the width you need for the same current, which is why heavy-copper boards are common for power circuits.
What temperature rise should I use for trace sizing?
A 10 °C rise is a conservative default that assumes still air and no nearby copper helping to spread heat, so real boards usually run cooler. You can allow 20 to 30 °C where the trace can dump heat into a plane or open air, which lets you use a narrower trace, but only if you have measured headroom.
Do I need to check voltage drop as well as ampacity?
Yes. Meeting the ampacity limit stops a trace from overheating, but a long thin trace can still drop meaningful voltage on a power rail. The calculator also reports resistance, voltage drop (V = I · R) and power loss (P = I² · R) for your trace length, so you can widen the trace or add copper pour where the drop is too high.
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