Stripline Impedance Calculator
Use this quick tool to estimate characteristic impedance (Z0) for a centered stripline trace between two reference planes.
What Is a Stripline Impedance Calculator?
A stripline impedance calculator estimates the characteristic impedance of a PCB transmission line buried between two copper reference planes. Controlled impedance is critical for high-speed interfaces like DDR, PCIe, USB, HDMI, LVDS, and many RF interconnects. If impedance is off-target, reflections increase and signal integrity can degrade.
Unlike a microstrip (on an outer layer), a stripline is embedded in dielectric, which typically gives better EMI performance and tighter field containment. The tradeoff is usually higher loss and more fabrication sensitivity to stackup variation.
Inputs Used in This Calculator
Dielectric Constant (εr)
This value describes how electric fields interact with the dielectric material. FR-4 may be around 3.8 to 4.5 depending on resin content, weave, and frequency.
Plane Spacing (b)
This is the distance between the two reference planes surrounding the stripline layer. It is set by core/prepreg thickness in your stackup.
Trace Width (w)
Wider traces generally reduce impedance. Narrower traces generally increase impedance.
Copper Thickness (t)
Thicker copper lowers impedance because it effectively increases conductor size and changes field distribution.
How to Use This Stripline Calculator
- Set your units (mm or mil).
- Enter dielectric constant from your laminate data.
- Enter plane spacing, trace width, and copper thickness.
- Click Calculate to get estimated Z0.
- Optionally enter a target impedance (such as 50 Ω) to estimate required width.
Design Tips for Better Accuracy
- Use your PCB fabricator’s exact stackup values (not generic FR-4 assumptions).
- Account for etch compensation (finished width can differ from drawn width).
- Keep impedance traces tightly referenced to continuous planes.
- Avoid stubs and abrupt geometry transitions where possible.
- For production, request impedance coupon testing and tolerance reporting.
Microstrip vs Stripline (Quick Comparison)
Microstrip
- External layer trace over one reference plane.
- Easier to probe and often lower dielectric loss.
- More radiated emissions and greater environmental sensitivity.
Stripline
- Internal layer trace between two planes.
- Better shielding and reduced EMI radiation.
- Can require narrower widths for same target impedance in some stackups.
Important Practical Notes
This calculator provides a fast engineering estimate using a common closed-form approximation. Real boards vary due to resin percentage, glass weave, copper roughness, plating, and fabrication tolerances. For final signoff, use a field solver and collaborate directly with your board manufacturer.
Frequently Asked Questions
Is this calculator good for differential stripline pairs?
It calculates single-ended stripline impedance. Differential impedance depends on pair spacing and odd/even mode coupling, so a dedicated differential model is required.
Can I use mils instead of millimeters?
Yes. Just keep all geometry values in the same unit. The formula depends on ratios, so consistency is what matters.
What target impedance should I use?
Common targets are 50 Ω single-ended and 90/100 Ω differential, but your interface standard and silicon vendor documentation should always be the authority.
Final Takeaway
A stripline impedance calculator is a great first step when planning controlled-impedance routing. Use it early, then validate with your CAD tool’s solver and your fabricator’s stackup process to ensure your manufactured board lands on target.