PCB Trace Width and Current Formulas
PCB trace formulas: the current an outer copper trace carries for a temperature rise, the trace resistance and the voltage drop and heat at a current.
The formulas
| To find | Formula |
|---|---|
| Cross-section | A = width × thickness (in mil²: width in mil, thickness in mil) |
| Current for a temperature rise (IPC-2221 fit) | I = k × ΔT^0.44 × A^0.725 |
| Trace resistance | R = ρ × L / (w × t) |
| Voltage drop and heat | ΔV = I × R and P = I² × R |
k is 0.048 for an outer layer and 0.024 for an inner layer, ΔT the allowed temperature rise in °C and A the area in square mils (1 mil = 0.0254 mm, and 1 oz of copper is about 1.378 mil or 35 µm thick). The current formula is the empirical fit from the older IPC-2221 charts, and later work (IPC-2152) shows that the real value depends on the board, the planes and the air flow, so treat it as a first estimate. The resistance uses ρ = 1.724e-8 Ω·m for copper at 20 °C and rises when the trace is hot.
Worked example: a 50 mil outer trace, 1 oz copper, 10 °C rise
- A = 50 × 1.378 = 68.9 mil²
- I = 0.048 × 10^0.44 × 68.9^0.725 = 2.844 A
Worked example: a 100 mm long, 1 mm wide, 35 µm thick trace at 2 A
- R = 1.724e-8 × 0.1 / (0.001 × 35e-6) = 0.04926 Ω
- ΔV = 2 × 0.04926 = 0.09851 V and P = 2² × 0.04926 = 0.197 W
Current for the 50 mil trace by temperature rise (outer layer)
| Temperature rise | Current |
|---|---|
| 5 °C | 2.097 A |
| 10 °C | 2.844 A |
| 20 °C | 3.858 A |
| 30 °C | 4.612 A |
Frequently asked questions
Why is an inner trace rated lower?
It cannot lose heat to the air on its own, so for the same rise it carries less current than an outer trace.
Is the formula accurate?
It is a rough fit to old charts. Real boards differ with the copper planes, the vias and the air flow, so keep a margin and check with the board maker.
Does a wider trace always help?
It lowers the resistance and the heat, and it carries more current for the same rise. It must also fit the layout and the clearance rules for the voltage.
What about short, high current pulses?
A short pulse heats the trace less than a steady current. Use the fusing current article to see when a trace can melt.
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