PCB Trace Width Calculator
- External width
- 1.37 mm (53.82 mil)
- Internal width
- 3.56 mm (140.01 mil)
How to Use the PCB Trace Width Calculator
Printed circuit board traces carry current like miniature wires, but flat copper foils heat differently than round conductors. Too narrow a trace for the load current causes excessive temperature rise, delamination, and voltage drop that disrupts sensitive analog circuits. PCB layout engineers use IPC-2221-style curves (or the simplified formulas derived from them) to pick minimum trace width before routing power nets.
The underlying relationship estimates required cross-sectional area based on current and allowed temperature rise, then converts area to width given copper thickness:
Area (mil²) = (I / (k × ΔTb))1/c
Width (mil) = Area / Thickness (mil)
Where I is current in amperes, ΔT is temperature rise in °C above ambient, k is 0.048 for external layers or 0.024 for internal (less heat dissipation), b = 0.44, and c = 0.725. Copper thickness in oz/ft² converts to mils: 1 oz ≈ 1.378 mil.
External layers cool better than buried inner layers — the same current needs wider internal traces. Standard fab offerings are 1 oz (35 µm) and 2 oz (70 µm) copper; heavy copper (3–6 oz) suits power modules. Always confirm with your fabricator's capability and require wider traces for long runs with voltage-drop budgets.
Worked example: A 3 A power rail on 1 oz external copper, allowing 10°C rise: area ≈ (3/(0.048×100.44))1/0.725 ≈ 42 mil². Width = 42/1.378 ≈ 30.5 mil (0.77 mm). For 10 A with 20°C rise: area ≈ 320 mil² → width ≈ 232 mil (5.9 mm) — why high-current PCBs use planes or bus bars. Doubling to 2 oz copper halves required width for the same rise.
Cross-check long power traces with the wire size calculator mindset — milliohms per square sheet resistance adds up over centimeters.
Trace width guidelines (1 oz external, 10°C rise)
| Current | Width (mil) | Width (mm) | Note |
|---|---|---|---|
| 0.5 A | 10 | 0.25 | Signal/power default |
| 1 A | 15 | 0.38 | USB VBUS |
| 2 A | 24 | 0.61 | Small DC-DC output |
| 3 A | 31 | 0.77 | Motor driver |
| 5 A | 48 | 1.22 | Use plane preferred |
| 10 A | 95 | 2.41 | Heavy copper advised |
Frequently asked questions
1 oz/ft² is the default for most prototypes. Enter 2 oz for power boards or when the fab offers heavy copper upgrades.
Inner layers dissipate heat through laminate on both sides with less convection, so they run hotter unless widened.
No — it targets temperature rise. Long traces also need voltage-drop checks; use wider copper or pours than the minimum thermal width.