How this calculator works
The legacy IPC-2221 fit relates current to copper cross section and temperature rise. Its layer coefficient is 0.048 externally and 0.024 internally. Resistance uses copper resistivity at ambient plus calculated rise. This is a preliminary empirical estimate; airflow, adjacent copper, stackup and enclosure conditions require thermal verification. IPC-2152 uses more detailed thermal information. Temperature rise is a difference, so it is entered in K or Celsius degrees without an absolute-temperature conversion.
Formula
I = k x rise^0.44 x A^0.725, with A in square mils. Width = A/thickness. R = rho20[1 + alpha(T - 20 Celsius)]L/A, rho20 = 1.724e-8 ohm m, alpha = 0.00393/K.
Worked example
For an external trace carrying 2 A, with 0.035 mm copper and a 10 K allowed rise, the legacy IPC fit gives a width of about 0.7814 mm. An internal layer needs more width under the same fit.
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Frequently asked questions
Is this a certified current rating?
No. It is a curve-fit estimate. Confirm the actual board temperature and fabrication stackup.
How do I enter one-ounce copper?
Use the fabricated thickness from your board supplier. One ounce per square foot is nominally about 0.035 mm.
Why is the internal-layer result different?
The legacy fit assigns half the external coefficient to internal traces. Actual heat spreading depends on the surrounding planes.