PCB trace width calculator

A track carrying current warms up, and how much depends on the cross-section of copper the current has to squeeze through. This works the sizing backwards: name the current and the temperature rise you are willing to live with, and out comes the width the IPC-2221 curve asks for at that copper thickness. It is the number every board house quotes, and it is deliberately on the generous side.

How it is calculated

A = (I ÷ (k × ΔT^0.44))^(1÷0.725) · width = A ÷ thickness · k = 0.048 outside, 0.024 inside

The curve is empirical, not derived: current, temperature rise and cross-sectional area were fitted to measurements, which is why the exponents are 0.44 and 0.725 rather than anything round. The area comes out in square mils and the width follows from dividing by the copper thickness, so doubling the copper weight halves the width you need.

Source: Texas Instruments — PCB Layout Guideline for Automotive LED Drivers, application report SNVA766 (April 2017), Section 11 Trace of Boost-Power Input: Width = (Current[Amps] / (k × (Temp_Rise[°C])b))(1/c))/ Thickness , in mils, where for IPC-2221 internal layers: k = 0.024, b = 0.44, c = 0.725 and for IPC-2221 external layers: k = 0.048, b = 0.44, c = 0.725

Questions people ask

Why does an inner layer need so much more copper?

Because it has nowhere to send the heat. The constant halves, from 0.048 to 0.024, and because the area exponent is 0.725 that does not double the copper needed — it multiplies the cross-section by 2.60. The rule of thumb that says twice the width inside is under-sizing by about a third, which is worth knowing before you copy it off a forum.

What temperature rise should I pick?

10 °C is the usual default and 20 °C is common on power tracks. It is a rise above whatever the board is already sitting at, not an absolute temperature, so a 20 °C rise inside a 70 °C enclosure puts the copper at 90 °C — check that against the glass transition temperature of the laminate, not against room temperature.

Where does this curve actually come from?

From work funded by the US Navy at the National Bureau of Standards in 1955, published as a chart labelled "Tentative", which became MIL-STD-275 and then the charts in IPC-2221. The IPC-2221 internal conductor chart represents a conductor in free air rather than one buried in laminate, which is why it runs conservative. IPC-2152 replaced it with measurements taken in real boards.

Is the answer safe to build to?

It errs on the safe side for the track itself, and says nothing at all about everything else. Nearby copper planes cool a track dramatically, adjacent tracks carrying current heat each other, vias and connectors are separate calculations, and none of that is in the curve. Treat the number as a floor to start from, not a verdict.

What does "1 oz copper" mean in millimetres?

It is a weight per unit area rather than a thickness: one ounce of copper spread over a square foot. Board houses publish the equivalent thickness alongside it — ½ oz/ft² is 17.5 µm, 1 oz/ft² is 35 µm, 2 oz/ft² is 70 µm and 3 oz/ft² is 105 µm — and those are the figures behind the menu above.

Sources

The documents this page reads its numbers out of, linked so you can check them yourself.

1. Texas Instruments — PCB Layout Guideline for Automotive LED Drivers, application report SNVA766 (April 2017), Section 11 Trace of Boost-Power Input: Width = (Current[Amps] / (k × (Temp_Rise[°C])b))(1/c))/ Thickness , in mils, where for IPC-2221 internal layers: k = 0.024, b = 0.44, c = 0.725 and for IPC-2221 external layers: k = 0.048, b = 0.44, c = 0.725
2. Michael R. Jouppi — The Value of IPC-2152, published by IPC: Use points on this line, for example, 36-sq.mils and 1-amp in addition to 400-sq.mils and 5-amps, and compare this with the 10C line in Figure 3, the IPC-2221 internal conductor sizing chart
3. Würth Elektronik — Basic Design Guide V1.0, Track Width and Conductor Spacing: 70 µm / 2 oz/ft²
4. NIST SP 811 (2008), Appendix B.8 — inch (in) to meter (m) = 2.54 E−02, so a mil is exactly 0.0254 mm

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