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
An external 1 mm wide, 0.035 mm thick trace with a 10 K allowed rise carries about 2.3916 A in the legacy IPC fit. Moving it to an internal layer halves that estimate.
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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.