Units and model
The legacy IPC-2221 empirical equation is I = k × ΔT^0.44 × A^0.725, with A in square mils, k = 0.048 for external traces or 0.024 for internal traces. Width and temperature modes invert that same equation. One mil is 0.0254 mm and the reference copper conversion is 1 oz/ft² ≈ 0.0347 mm. See the equation in Texas Instruments’ layout guideline.
Resistance uses the computed final width and full estimated operating temperature, ambient plus rise. Copper resistivity at 20°C is 1.68×10⁻⁸ Ωm and its assumed coefficient is 0.00393/°C. Percentage drop requires a supply voltage; the 1 V advanced default makes the reference’s implicit denominator explicit. The voltage-limit current uses resistance at the modelled temperature and is a separate electrical check, not a self-consistent thermal limit.
Why your number may differ
The reference’s IPC-2152 option substitutes an unsupported approximation using mm, producing about 0.01 A at the defaults. We do not treat it as a valid IPC-2152 equation. Selecting it retains a clear limitation notice and uses the sourced legacy equation for screening. A genuine IPC-2152 assessment depends on board construction, nearby copper, environment and thermal measurements; this calculator does not establish compliance.
The legacy default estimate is about 2.4 A, rather than 0.01 A. Our resistance uses final computed width and ambient plus full rise, so it changes correctly in width and temperature modes. The reference computes resistance before updating those values and uses only half the rise. Geometry and materials can differ from nominal values. Validate the actual board and its allowed temperature; avoid interpreting these estimates as a safe operating rating.