Differential Pair Impedance Calculator
Edge-coupled microstrip and stripline Zdiff for USB, Ethernet and PCIe pairs — IPC-2141A coupled-line formulas with live coupling factor.
These closed-form fits are good to roughly ±10 % within their validity range (0.1 < w/h < 2, s/h > ~0.2). For USB (90 Ω), Ethernet (100 Ω) or PCIe (85 Ω) production boards, confirm the final stack-up with your fab's field solver — solder mask and glass weave shift the result a few ohms.
Differential Pair Impedance Calculator computes the differential impedance Zdiff of an edge-coupled pair on outer or inner layers — free, instant and private in your browser. Designers routing USB, Ethernet, LVDS, HDMI and PCIe pairs to spec use it to skip the datasheet algebra: type your numbers, read the answer with the substituted formula shown step by step, and share an exact permalink of the calculation.
About Differential Pair Impedance Calculator
Differential Pair Impedance Calculator computes the differential impedance Zdiff of an edge-coupled pair on outer or inner layers using the standard engineering relation: Zdiff = 2·Z₀·(1 − 0.48·e^(−0.96·s/h)) for microstrip, 2·Z₀·(1 − 0.347·e^(−2.9·s/b)) for stripline (IPC-2141A). Worked live: two 0.2 mm traces with a 0.2 mm gap over 0.15 mm FR-4 land close to USB's 90 Ω. The result recalculates on every keystroke, the worked-example panel shows your numbers substituted into the formula, and the Copy permalink button encodes the inputs in the URL so a colleague opens exactly your calculation. Everything runs client-side — nothing you type leaves your device.
How to use Differential Pair Impedance Calculator
- 1Enter your values — the tool starts with realistic defaults for this exact use case, so the worked example is meaningful immediately.
- 2Read the live result and the worked-example panel, which substitutes your numbers into the formula step by step.
- 3Adjust any input to compare scenarios, then use Copy result or Copy permalink to share the calculation.
Why use Differential Pair Impedance Calculator?
- ✓Implements the real formula — Zdiff = 2·Z₀·(1 − 0.48·e^(−0.96·s/h)) for microstrip, 2·Z₀·(1 − 0.347·e^(−2.9·s/b)) for stripline (IPC-2141A) — with the substitution shown, not a black box
- ✓Built for designers routing USB, Ethernet, LVDS, HDMI and PCIe pairs to spec
- ✓Copy result and permalink buttons — share the exact calculation in a README, forum answer or design review
- ✓100% free, no sign-up, runs entirely in your browser (works offline once loaded)
Frequently asked questions
How do you calculate differential pair impedance?+
The differential impedance Zdiff of an edge-coupled pair on outer or inner layers follows Zdiff = 2·Z₀·(1 − 0.48·e^(−0.96·s/h)) for microstrip, 2·Z₀·(1 − 0.347·e^(−2.9·s/b)) for stripline (IPC-2141A). For example, two 0.2 mm traces with a 0.2 mm gap over 0.15 mm FR-4 land close to USB's 90 Ω. The calculator applies the same relation and shows the substituted arithmetic so you can verify every step.
What differential impedance do common interfaces need?+
USB 2.0/3.x: 90 Ω. Ethernet and LVDS: 100 Ω. PCIe: 85 Ω. HDMI: 100 Ω. CAN: 120 Ω (cable). Hitting the target within ±10 % keeps reflections inside the interface's budget; tighter pairs (smaller s) need narrower traces to compensate.
Does the gap between the pair matter more than the width?+
Both matter, but coupling falls off exponentially with s/h. Once the gap exceeds ~2× the dielectric height the traces barely couple and Zdiff ≈ 2×Z₀ — at that point you are really routing two single-ended lines and should control Z₀ instead.
Is the Differential Pair Impedance Calculator free and private?+
Yes — completely free with no sign-up or usage limits, and it runs entirely in your browser: the values you enter are never uploaded or stored on a server.
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