Formula (IPC-2221, external): I = 0.048 × ΔT^0.44 × A^0.725 — (internal): I = 0.024 × ΔT^0.44 × A^0.725, where A is cross-section area in mils² (1 mil = 0.001 in). Solved for area: A = (I / (k×ΔT^0.44))^1/0.725, then width = A / copper-thickness. Trace resistance uses copper resistivity ρ = 1.68×10⁻⁸ Ω·m.
This is the standard simplified IPC-2221 model used by most online trace-width tools; for high-current power planes or dense multilayer boards, verify against your fab's capabilities and, for critical designs, IPC-2152.