Differential Pair Impedance Calculator
Edge-coupled differential pairs carry balanced signals. The differential impedance Z_diff = 2·Z_odd is what high-speed differential lines (USB, HDMI, LVDS) must be controlled to. Odd-mode signals travel in opposite directions; even-mode in the same direction.
Equations & Parameters ▸
\(Z_{diff}=2Z_{odd},\quad Z_{odd}\approx Z_0\!\left(1-0.347\,e^{-2.655s/h}\right),\quad Z_{even}\approx Z_0\!\left(1+0.347\,e^{-2.655s/h}\right)\)
| w | Trace width (mm). |
| s | Edge-to-edge spacing between the two traces (mm). |
| h | Substrate height (mm). Trace to nearest ground plane. |
| t | Trace thickness (mm). Standard 1 oz = 0.035 mm. |
| εᵣ | Substrate dielectric constant. |
| Z_odd | Odd-mode impedance — the impedance each trace sees when signals are opposite. |
| Z_diff | Differential impedance = 2·Z_odd. The target specification, typically 100 Ω. |
| Z_even | Even-mode impedance — both traces driven together. Z_cm = Z_even/2. |
Physical constants used
| c | Speed of light = 2.998×10⁸ m/s |
| µ₀ | Permeability of free space = 4π×10⁻⁷ H/m ≈ 1.2566×10⁻⁶ H/m |
| ε₀ | Permittivity of free space = 8.854×10⁻¹² F/m |
Inputs
mm
mm
trace edge to trace edgemm
trace to nearest groundmm
1 oz Cu = 0.035 mmFR4≈4.4, Rogers4003≈3.55, PTFE≈2.1
Results
Differential
Differential impedance, Z_diff—
Odd-mode impedance, Z_odd—
Common-mode
Even-mode impedance, Z_even—
Common-mode impedance, Z_cm—
Single-trace Z₀—
Diagram