PCB Impedance Calculator

About the PCB Impedance Calculator

The PCB Impedance Calculator determines characteristic impedance for printed circuit board traces in microstrip and stripline configurations. Controlled impedance is critical for high-speed digital designs, RF circuits, and any PCB carrying signals above a few hundred MHz where transmission line effects become significant.

This calculator covers the most common PCB geometries: surface microstrip (trace above a ground plane), embedded microstrip (trace with solder mask), stripline (trace between two ground planes), and differential pairs for both configurations. Each calculation uses industry-standard empirical formulas validated against field solvers.

The calculator accepts standard PCB manufacturing parameters—trace width, trace thickness, dielectric height, dielectric constant (Er), and for differential pairs, trace spacing. It outputs impedance in ohms along with effective dielectric constant, propagation delay, and wavelength at your specified frequency, helping you design PCB stackups that meet impedance targets for USB, HDMI, PCIe, DDR, and other high-speed interfaces. It also gives you a fast check on whether a chosen stackup can realistically hit the target before you send the board out.

When This Page Helps

Use this calculator when you need to size PCB traces for a target impedance instead of guessing from a rule of thumb. It is useful for high-speed digital, RF, and differential-pair layout work where stackup details matter. The result depends on geometry and dielectric values, so it is more dependable than a generic trace-width estimate.

How to Use the Inputs

1. Select the trace configuration: microstrip, embedded microstrip, stripline, or differential pair
2. Enter trace width, copper thickness, and dielectric height from your PCB stackup
3. Set the dielectric constant (Er) for your substrate material (FR-4 ≈ 4.2-4.5)
4. For differential pairs, also enter the trace spacing (gap between traces)
5. Optionally enter the signal frequency for wavelength and propagation delay calculations
6. Adjust parameters to meet your target impedance (typically 50Ω single-ended or 90-100Ω differential)

Example Calculation

Tips & Best Practices

• Always get your PCB manufacturer's actual stackup data — nominal values can differ significantly from catalog specs
• For differential pairs, the gap between traces should be consistent along the entire length
• Avoid routing impedance-controlled traces over splits in the ground plane — this changes the reference and impedance
• Use your fab house's impedance calculator on their website — they calibrate to their actual manufacturing process
• Add ±10% impedance tolerance to your requirements — PCB manufacturing varies by that much typically

PCB Stackup Design for Impedance Control

Achieving target impedance starts with stackup design. A typical 4-layer board might use: Layer 1 (signal), Layer 2 (ground), Layer 3 (power), Layer 4 (signal). The dielectric between signal and ground layers determines microstrip impedance. Common prepreg thicknesses are 3-10 mils, with trace widths calculated to hit 50Ω. Work with your PCB manufacturer early—they can recommended trace widths for their standard stackups.

High-Speed Interface Impedance Requirements

Different interfaces have specific impedance targets: USB 2.0/3.x uses 90Ω differential, HDMI uses 100Ω differential, PCIe uses 85Ω differential, DDR4 uses 40Ω single-ended, Ethernet uses 100Ω differential, and SATA uses 50Ω single-ended. Meeting these targets within ±10% ensures compliant signal quality and successful certification testing.

Manufacturing Tolerance and Testing

PCB manufacturers typically guarantee impedance within ±10% of target. They produce test coupons—small trace samples on each panel—and measure with TDR (Time Domain Reflectometry) to verify impedance. For tighter tolerances (±5%), expect higher costs. Always request impedance test reports for controlled-impedance boards and include test coupons in your panel design.

Frequently Asked Questions

• What impedance should I target?

  Most standards specify 50Ω single-ended (USB, PCIe, SATA) or 90-100Ω differential (USB3: 90Ω, HDMI: 100Ω, Ethernet: 100Ω, DDR: 100Ω). RF designs typically use 50Ω (test equipment) or 75Ω (video/antenna).

• What is the dielectric constant of FR-4?

  Standard FR-4 has Er ≈ 4.2-4.5 at 1 GHz. It varies with frequency, temperature, and resin content. For designs above 5 GHz, consider low-loss materials like Rogers or Megtron with Er ≈ 3.0-3.6.

• How accurate are these formulas?

  Empirical formulas are accurate to within 2-5% for typical PCB geometries. For critical designs, use a 2D field solver (e.g., built into Altium, Cadence, or freeware like ATLC). Manufacturing tolerances (±10%) often dominate accuracy.

• What affects impedance most?

  The strongest factors are trace width and dielectric height. Doubling trace width reduces impedance by ~30%. Doubling dielectric height increases impedance by ~30%. Copper thickness and Er have smaller effects.

• What is the difference between microstrip and stripline?

  Microstrip is a trace on the outer layer above a ground plane—it's exposed to air, giving lower effective Er. Stripline is an inner-layer trace between two ground planes—fully enclosed in dielectric, with better shielding but harder to manufacture precisely.

• Why does impedance matter?

  Impedance mismatches cause signal reflections, ringing, and degraded signal integrity. At high frequencies, even a 10% impedance mismatch can cause significant reflections. Controlled impedance ensures clean signal transmission and meets interface specifications.