Impedance Calculator
Calculate PCB trace impedance for microstrip, stripline, and coplanar waveguide structures.
Common Target Impedances
| Application | Impedance | Notes |
|---|---|---|
| USB 2.0 | 90 Ω ±10% | Differential pair |
| USB 3.0 | 90 Ω ±10% | Differential pair |
| HDMI | 100 Ω ±10% | Differential pair |
| Ethernet | 100 Ω ±10% | Differential pair |
| PCIe | 85 Ω ±10% | Differential pair |
| RF 50Ω | 50 Ω ±5% | Single-ended |
How It Works
This calculator uses IPC-2141 standard formulas to estimate characteristic impedance based on trace geometry and material properties.
Microstrip
A trace on the outer layer with a ground plane below. Most common for high-speed signals.
- • Lower loss than stripline
- • Easier to manufacture
- • Some EMI radiation
Stripline
A trace buried between two ground planes. Better shielding but higher loss.
- • Excellent EMI shielding
- • More predictable impedance
- • Higher insertion loss
Coplanar
A trace with ground planes on both sides on the same layer. Good for RF and tight spaces.
- • Good for RF transitions
- • Lower dispersion
- • Uses more board space
Design Tips
Impedance Control
- ±10% tolerance is standard for most digital signals
- ±5% tolerance required for RF and sensitive applications
- Always specify impedance requirements in your fab notes
- Request impedance test coupons for verification
Material Considerations
- FR-4 Er varies with frequency (3.8-4.8 typical)
- Higher frequencies need low-loss materials (Rogers, Isola)
- Temperature affects Er slightly
- Resin content affects Er in laminate
Frequently Asked Questions
What dielectric constant should I use for FR-4?
FR-4 typically has an Er of 4.2-4.8 at 1 MHz. At higher frequencies (1 GHz+), use 4.0-4.3. Check your laminate datasheet for exact values.
Why does my calculated impedance differ from the fab house?
PCB manufacturers use 2D/3D field solvers which account for more variables like etch factor, solder mask, and actual material properties. These calculators provide estimates - always verify with your manufacturer.
How do I calculate differential impedance?
Differential impedance is approximately 2× single-ended impedance minus coupling factor. For USB (90Ω differential), target ~45Ω single-ended with appropriate spacing.