IPC-2221: A = (I / (k · ΔT^0.44))^(1/0.725) · k = 0.048 external, 0.024 internal · width = A / thickness
Trace width reference table
1 oz copper, 10 °C temperature rise — the most common starting point in PCB layout.
| Current (A) | External (mil / mm) | Internal (mil / mm) |
|---|---|---|
| 0.1 | 0.5 / 0.013 | 1.3 / 0.033 |
| 0.25 | 1.7 / 0.044 | 4.5 / 0.115 |
| 0.5 | 4.5 / 0.115 | 11.8 / 0.300 |
| 0.75 | 8.0 / 0.202 | 20.7 / 0.525 |
| 1 | 11.8 / 0.300 | 30.8 / 0.781 |
| 1.5 | 20.7 / 0.525 | 53.8 / 1.367 |
| 2 | 30.8 / 0.781 | 80.0 / 2.033 |
| 3 | 53.8 / 1.367 | 140.0 / 3.556 |
| 4 | 80.0 / 2.033 | 208.2 / 5.288 |
| 5 | 108.9 / 2.765 | 283.2 / 7.194 |
| 7.5 | 190.5 / 4.838 | 495.5 / 12.585 |
| 10 | 283.2 / 7.194 | 736.8 / 18.715 |
How the IPC-2221 formula works
IPC-2221 sizes a trace by the copper cross-section needed to carry current without exceeding an allowed temperature rise. First solve for the area, then divide by the copper thickness to get a width:
width[mil] = A / (copper[oz] × 1.378)
k = 0.048 for external (outer) layers and k = 0.024 for internal layers, which is why an inner-layer trace needs roughly twice the width for the same current. Copper weight converts to thickness at about 1.378 mil per oz/ft².
Choosing temperature rise and copper weight
A 10 °C rise is the usual default for signal and light power traces. 20–30 °C is acceptable on short power runs where the board has airflow or a copper pour to spread heat. Heavier copper (2 oz or 3 oz) buys width back on dense power designs, but raises fabrication cost and limits minimum spacing — check your fab's capabilities first.
Remember IPC-2221 covers heating only. Long traces also drop voltage, so for power rails check the resistive loss separately, and add margin for vias, connectors, and thermal neighbours. Working in mils and millimetres at once? Use the mil to mm converter.
Frequently asked questions
- How wide should a 1 A trace be?
- About 12 mil (0.30 mm) on an external 1 oz layer with a 10 °C rise, or roughly 30 mil on an internal layer.
- Why do internal traces need to be wider?
- Inner layers are buried in laminate and cannot cool by convection, so IPC-2221 halves k — roughly doubling the required width.
- Is IPC-2221 conservative?
- It is intentionally cautious for general designs. IPC-2152 uses measured data and often permits narrower traces, but needs more inputs such as board thickness and copper pour.
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