Differential pair impedance calculator

The tool first computes the single-ended impedance of one trace with the IPC-2141 microstrip or centred-stripline formulas, then applies the edge-coupled reduction: Zdiff ≈ 2·Z0·(1 − 0.48·exp(−0.96·S/H)) for microstrip and Zdiff ≈ 2·Z0·(1 − 0.347·exp(−2.9·S/B)) for stripline, where S is the edge-to-edge spacing, H the dielectric height to the ground plane (microstrip) or per side (stripline), and B = 2H + T the total plane-to-plane spacing.

USB 2.0 and USB 3.x both target 90 Ω differential (the USB-IF SuperSpeed spec calls out 90 Ω nominal), while HDMI and 100BASE-T Ethernet specify 100 Ω. As a rule of thumb a loosely coupled pair lands near 2× the single-ended impedance, so a 90 Ω pair is designed with roughly 45 Ω per line. The tool flags whether your geometry lands inside a ±10% window of the common targets.

Start from a thin dielectric: a common starting point is W ≈ 0.1–0.2 mm, S ≈ 0.1–0.2 mm on H ≈ 0.1–0.2 mm to the ground plane (microstrip, εr 4.2). Tight spacing couples the lines and pulls Zdiff down; wider spacing approaches the 2× Z0 asymptote. Real impedance depends on the fabricator stack-up, solder mask and etch tolerance, so always confirm with the board house impedance coupons.