PCB Trace Width Calculator
Based on the IPC-2221 standard — enter your current to get the required trace width, resistance, voltage drop and power loss.
Input Parameters
Recommended Trace Width
| Current (A) | External Width (mm) | Internal Width (mm) | External (mil) |
|---|
Unit conversions: 1 oz/ft² = 1.4 mil = 35.6 µm · °F = °C × 9/5 + 32 · 1 in = 25.4 mm.
Notes: ① Internal layers dissipate heat poorly — under the same conditions they need roughly 2× the width of external layers. ② Results assume an FR-4 substrate in still air; for vacuum, dense routing or continuous full load, add 20%+ margin. ③ For design reference only — validate with testing before production.
Frequently Asked Questions
1. Which standard does this calculator use?
IPC-2221. The core formula is I = k · ΔT^0.44 · A^0.725, where k = 0.048 for external layers and 0.024 for internal layers, A is the trace cross-section in mil² and ΔT is the allowed temperature rise. Given your current, it solves for the minimum trace width and also reports the trace resistance, voltage drop and power loss. IPC-2221 is deliberately conservative, which makes it a safe starting point for most boards.
2. What temperature rise should I allow?
10 °C is the standard default — the trace may run 10 °C above ambient. Smaller rises (5 °C) give wider, cooler traces for dense or high-reliability boards; 20 °C buys narrower traces at the cost of a hotter board and shorter life for nearby components. In a sealed enclosure or a high ambient (60 °C and up), keep the rise small.
3. Why do internal layers need about twice the width?
Because of the k coefficient: 0.048 external versus 0.024 internal. An internal trace is embedded in FR-4, a thermal insulator, so the heat it generates cannot escape to the air — it can only spread through the laminate into planes and vias. An external trace can also conduct into the plane and convect and radiate to the surroundings. For the same current and temperature rise, plan on roughly 2× the width on inner layers — or move the high-current path to an outer layer.
4. How much margin should I add?
The calculator flags the theoretical IPC-2221 minimum. Add at least +10% width for production designs, and +20% or more for vacuum operation, dense routing with little adjacent copper, continuous full load, or high ambient temperature. The status line tells you the margin of your current selection.
5. Does trace length matter?
Not for current capacity — heating depends on the cross-section, not the length. But length drives voltage drop and power loss: R = ρ·L/A, so a long thin run can drop real voltage even when it is wide enough thermally. The calculator reports resistance, voltage drop and power loss for your entered length; if the drop is too high, widen the trace or shorten the path. As a rule of thumb, keep power-rail drops under 1–2% of the rail voltage.
6. What copper weight should I use?
1 oz/ft² (35 µm) is the standard starting point. 2 oz doubles the cross-section for the same width — for the same current it roughly halves the width you need; 0.5 oz or 0.33 oz is used for fine-line and HDI designs. The calculator accepts copper in oz/ft² or mil. Heavier copper costs more and needs wider etch spacing — above 2 oz, confirm your fabricator’s process window.
7. Why might a real board carry a different current than the calculator says?
IPC-2221 is an empirical, conservative model based on older test data — real capacity depends on board thickness, adjacent copper planes, solder m
ask, airflow and mounting. Newer analysis (IPC-2152, based on modern measurements) often allows narrower traces than IPC-2221 for the same temperature rise. Treat this calculator as the safe lower bound: for thermally critical designs, validate with thermal imaging or a current-loading test on a coupon. LEADHUI can build test coupons with your production stack-up.
