What Is a PCB Trace Width Calculator and How Does It Work?
A PCB trace width calculator determines the minimum copper trace width needed to carry a given current without the trace overheating. It applies the IPC-2221 standard, the industry-recognized specification for printed circuit board design, which relates current, allowable temperature rise, and copper thickness to the trace's cross-sectional area.
The IPC-2221 formula is: I = k × ΔT^0.44 × A^0.725, rearranged to solve for area as A = (I / (k × ΔT^0.44))^(1/0.725). Here I is current in amps, ΔT is the allowable temperature rise in °C, A is the required cross-sectional area in square mils, and k is a constant of 0.048 for external traces or 0.024 for internal traces. Dividing the resulting area by the copper thickness (1.378 mils per ounce of copper weight) gives the minimum trace width.
According to IPC-2221, a 1 oz/ft² external copper trace carrying 2A at a 10°C temperature rise needs a minimum width of roughly 1.1 mm (about 43 mils) — undersizing it risks resistive heating that can delaminate the board or degrade nearby components.
How to Use This PCB Trace Width Calculator
Enter your circuit values and the recommended trace width updates instantly. Here's what each field means:
- Current (Amps): The maximum continuous current the trace will carry — use the worst-case load, not the average.
- Max Temp Rise: How much hotter the trace is allowed to get above ambient. 10°C is a common conservative default; 20-30°C is acceptable for traces with more thermal margin.
- Trace Location: External traces sit on the top or bottom copper layer and cool by convection into open air. Internal traces are buried between layers of FR-4 laminate, which insulates heat, so they need roughly double the width for the same current.
- Copper Weight: The thickness of the copper foil, measured in ounces per square foot. 1 oz/ft² (35 microns) is the standard for consumer electronics; 2 oz/ft² and heavier is common on power boards.
- Trace Length (optional): If provided, the calculator also estimates the trace's resistance, voltage drop, and resistive power loss over that length.
This calculator works in the mils and ounces-per-square-foot units the PCB industry uses worldwide, and also shows the result in millimeters, so it's useful whether you design boards in Eagle, KiCad, Altium, or any other layout tool.
PCB Trace Width Reference Table (1 oz Copper, External Layer)
The table below shows minimum external trace widths for common currents at a 10°C temperature rise, using 1 oz/ft² copper — a quick reference for early-stage design before running the exact numbers above.
| Current | Min. Width (external) | Typical Use |
|---|---|---|
| 0.3 A | ≈ 0.15 mm (6 mils) | Digital signal / logic line |
| 1 A | ≈ 0.6 mm (24 mils) | Low-power sensor / peripheral rail |
| 2 A | ≈ 1.1 mm (43 mils) | USB power / small motor driver |
| 5 A | ≈ 2.6 mm (102 mils) | Battery charging / power supply output |
Figures use k = 0.048 (external layer) and a 10°C temperature rise per IPC-2221; internal traces need roughly double these widths.
Frequently Asked Questions
What trace width do I need for 2A?
For 2A on a 1 oz/ft² external copper trace with a 10°C allowable temperature rise, IPC-2221 calls for a minimum width of about 1.1 mm (43 mils). Internal traces carrying the same 2A need roughly double that width, around 2.2 mm (85 mils), to reach the same temperature rise.
How accurate is this PCB trace width calculator?
This calculator applies the standard IPC-2221 curve-fit equation used by most PCB manufacturers and design tools, giving a reliable estimate for general-purpose boards. It does not account for altitude derating, multiple parallel traces sharing heat, or forced airflow, so high-power or safety-critical designs should add margin and verify against your fabricator's capabilities and IPC-2152 (which supersedes IPC-2221 for thermal modeling with additional variables).
What is the difference between internal and external trace width?
External traces are on the outer copper layers and dissipate heat directly into the surrounding air, so IPC-2221 uses a constant of k = 0.048 for them. Internal traces are sandwiched between FR-4 layers with poor thermal conductivity, so they use k = 0.024 — roughly half as efficient at shedding heat — meaning an internal trace needs about twice the cross-sectional area of an external one to carry the same current at the same temperature rise.
How do I reduce voltage drop on a PCB trace?
Widen the trace, use heavier copper weight (2 oz/ft² instead of 1 oz/ft²), or shorten the trace length — all three directly reduce resistance. Doubling the trace width roughly halves its resistance and voltage drop for the same current and length.