Differential Pair Impedance Calculator
Calculate odd-mode and differential impedance of edge-coupled microstrip or stripline differential pairs (USB, HDMI, PCIe, DDR).
How to Use
- Pick topology: microstrip (outer layer) or stripline (inner layer).
- Enter trace width W, gap S (spacing between the two traces), dielectric height H, thickness T, εr.
- Result: single-ended Z₀, odd-mode Z_odd, differential Z_diff = 2 × Z_odd.
- Common targets: USB 2.0 = 90Ω, HDMI/PCIe = 100Ω, DDR4 = 80-100Ω.
Show Work
Formulas
History of Differential Signaling
Differential signaling dates to Alexander Graham Bell's 1881 twisted-pair telephone patent — using a balanced signal pair instead of single-ended-over-ground to cancel out electromagnetic interference. The same principle governs every modern high-speed digital interface: LVDS (1994), USB 2.0 (2000), HDMI (2002), PCIe (2003), DDR2+ (2003+), and USB-C/USB4 (2016+).
The move from single-ended to differential signaling in PCs accelerated in the 2000s as clock speeds exceeded a few hundred MHz and single-ended ground bounce became intolerable. A differential pair with tight coupling presents equal-and-opposite current to the return plane, cancelling the EMI that a single-ended signal would radiate. The receiver needs only to detect the voltage difference, making it immune to common-mode noise pickup.
The differential-impedance target depends on the interface spec: USB 2.0 is 90Ω nominal, HDMI and PCIe are 100Ω, LVDS is 100Ω, DDR4 is 80-100Ω. Matching this on the PCB requires controlling the trace width, spacing, and stackup — all three parameters interact, which is why modern PCB design relies on impedance-controlled stackups from the fab house.
About This Calculator
Pick microstrip or stripline topology and enter trace width W, intra-pair gap S, dielectric height H (or B for stripline plane-plane), copper thickness T, and εr. The tool computes single-ended Z₀, odd-mode impedance (one trace with the other driven opposite), and differential impedance Z_diff = 2 · Z_odd.
For tight-tolerance impedance (±5-10%), specify the stackup with your PCB fab and request impedance-controlled routing. Controlled-impedance fabs will run TDR verification coupons on each panel. For casual/hobbyist use, these calculated values are adequate. Everything runs client-side.
About the Differential Pair Impedance Calculator
The Differential Pair Impedance Calculator is a free tool for electronics and circuit design. It runs right in your web browser, so there is nothing to download. Calculate odd-mode and differential impedance of edge-coupled microstrip or stripline differential pairs (USB, HDMI, PCIe, DDR).
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What is differential impedance?
The impedance seen by the differential signal (voltage difference between the two traces). Z_diff = 2 × Z_odd, where Z_odd is one trace's impedance when the other carries an equal-and-opposite signal.
Why tightly couple?
Close trace spacing increases mutual coupling, which REDUCES common-mode susceptibility and EMI radiation — the two signals' fields cancel outside the pair. Loose coupling behaves like two independent 50Ω lines.
Gap tolerance?
±10% spacing variation shifts Z_diff by ~5%. For tight ±5% impedance, specify ±0.025mm on the fab drawing and include impedance-controlled stackup callout.
How do I use the Differential Pair Impedance Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
USB 2.0 (90Ω diff)
Typical FR4: W=0.5mm, S=0.125mm, H=0.25mm for 90Ω differential microstrip.
HDMI / PCIe (100Ω)
Inner-layer stripline: W=0.18mm, S=0.15mm on 0.3mm between planes.
Ethernet 1000BASE-T
100Ω diff, CAT6-equivalent PCB routing with tight length matching.
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