PCB Impedance Calculator

About PCB impedance estimates

Microstrip uses Hammerstad-style approximations; symmetric stripline uses a common closed-form estimate. Embedded microstrip nudges the effective dielectric constant upward for the solder-mask / cover layer. Effective Er also sets the capacitance per length and the propagation delay.

Why controlled impedance matters

At high speed a PCB trace stops behaving like a plain wire and starts behaving like a transmission line with a characteristic impedance. If that impedance doesn't match the driver and receiver — commonly 50 Ω for single-ended lines, 90 or 100 Ω differential for USB and Ethernet — part of the signal reflects back down the trace, causing ringing, overshoot, and data errors. Controlling the impedance means choosing the trace width and dielectric height so the geometry lands on the target value.

What each dimension does

Widen the trace and impedance falls; move it further from its reference plane (more dielectric height) and impedance rises; a higher dielectric constant lowers it. Those three are the knobs you actually turn. Stripline — a trace buried between two planes — runs at lower impedance than a microstrip of the same width because it's coupled to copper on both sides, so it generally needs a narrower trace to hit the same target. The effective dielectric constant also sets how fast the signal travels, which is why the calculator reports propagation delay alongside impedance.

Using the estimate well

Closed-form equations like these are ideal for early stackup planning and for seeing how a change in width or height moves the number. They assume ideal rectangular copper and uniform dielectric, so for a final controlled-impedance spec, confirm the dimensions with your fabricator or a 2-D field solver that accounts for etched copper shape and solder mask.