Trace voltage drop.
Resistance, IR drop, and power dissipated in a trace — from geometry, current, and temperature. The number that decides whether your far-end rail still regulates.
Single-trace DC model (ρ₂₀ = 1.724×10⁻⁸ Ω·m, α = 0.00393/°C). Real PDN paths include vias, planes, and connectors — for a full IR-drop map across the board, see PDN design and TRM.
The formula
R = ρ(T) · L / (w · t) ρ(T) = ρ₂₀ · (1 + α·(T − 20))
V = I · R P = I² · R
ρ₂₀ = 1.724×10⁻⁸ Ω·m (copper at 20 °C)
α = 0.00393 /°C
t = copper thickness (1 oz ≈ 35 µm)Why it matters
On a low-voltage rail, IR drop eats your tolerance directly. A 0.9 V core at ±5% has only 45 mV of budget — a 100 mm, 10-mil, 1 oz trace at 1 A already drops ~200 mV, four times the budget. That’s why power goes on planes, not traces. This calculator tells you when a trace is fine and when it isn’t.
Temperature coupling
Copper resistance rises with temperature, so a trace that’s self-heating drops more, which heats it more — the coupling TRM solves properly. At 85 °C a trace has ~25% more resistance than at 20 °C.