Coplanar Trace Width Basics
A coplanar trace uses a center conductor and two nearby ground pours. The spacing between them controls impedance. The trace width also matters. Board height, dielectric constant, copper thickness, and mask layers change the field shape. These details make manual estimates slow.
Why This Calculator Helps
This calculator searches for a width that matches a target impedance. It also reports the closest impedance, width in mils, effective dielectric constant, propagation delay, guided wavelength, and electrical length. These outputs help during early layout planning. They also help when comparing fabrication limits. You can test several gaps before sending files to a board shop.
Important Design Inputs
Start with the dielectric constant from your laminate data sheet. Use the real substrate height between the trace layer and the reference plane. Enter the coplanar gap that your manufacturer can hold. Add copper thickness after plating, not only starting foil. Use a realistic width range. A wide search range helps the solver, but a practical range is better for production.
Accuracy Notes
The equation used here is an engineering approximation. Real boards include copper roughness, solder mask variation, etch tolerance, glass weave, plating spread, and nearby objects. High frequency designs may need a field solver. For controlled impedance work, always compare the result with your fabricator’s stackup tool. Treat this page as a planning calculator, not a final guarantee.
Workflow Tips
Run the example values first. Then enter your target impedance. Change only one input at a time. Watch how gap and dielectric constant affect the result. Smaller gaps usually need narrower traces for the same impedance. Higher dielectric constant usually lowers impedance. Export the result as CSV for records. Use the document download when attaching a quick calculation note.
Layout Advice
Keep ground pours continuous near the trace. Add via fences when your design rules allow them. Avoid abrupt neck downs. Keep other nets away from the coplanar field. Use smooth transitions into connectors and components. Match the calculator values with fabrication limits before finishing the board.
Checking Results
Review the impedance error after calculation. If the error stays high, widen the allowed search range. Also test nearby manufacturable widths, because fabrication grids may not match ideal numbers exactly.