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A PCB trace carrying a high-speed signal needs a controlled characteristic impedance (commonly 50Ω or 75Ω) to avoid signal reflections. This calculator estimates the impedance of a microstrip trace (a trace running over a ground plane) using a standard, widely-used approximation formula.
How it works
Enter the board's dielectric constant (εr — commonly around 4.5 for standard FR-4), the trace's height above the ground plane, its width, and its thickness, all in mils. The calculator applies the IPC-2141-derived microstrip approximation to estimate the characteristic impedance.
- Enter dielectric constant (εr) — e.g. 4.5 for FR-4.
- Enter height above ground plane, h (mils).
- Enter trace width, w (mils).
- Enter trace thickness, t (mils) — e.g. ~1.4 for 1oz copper.
- Click Calculate to see your results.
Examples
A typical FR-4 microstrip trace
A trace with εr = 4.5, 10 mil height above the ground plane, 10 mil width, and 1.4 mil thickness (standard 1oz copper) works out to roughly 66 Ω — close to, but not exactly, the common 50Ω target, illustrating why real designs iterate on width to hit a specific target impedance.
Who should use it
- Early-stage PCB design for impedance-controlled traces.
- Electronics coursework on transmission-line basics.
Industry applications
- PCB design and electronics manufacturing
- High-speed digital and RF circuit design
Advantages
- Fast, simple estimate without needing full field-solver software.
- Widely used, well-established approximation in PCB design.
Limitations
- An approximation, not an exact field-solver result — precision-critical designs need dedicated software verification.
Common mistakes to avoid
- Mixing units (e.g. entering height in millimeters and width in mils) — keep all three dimensions in the same unit.
- Treating the result as an exact, guaranteed value rather than a design-stage approximation.
Best practices
- Use this as a starting point for trace width, then verify with your PCB fabricator's own impedance calculator or field-solver, since manufacturing tolerances and exact stack-up affect the real result.
- Confirm your board's actual dielectric constant from its datasheet rather than assuming a generic FR-4 value.
Tips
- To hit a specific target impedance (like 50Ω), try a few different trace widths and see which one lands closest — width has the most direct, intuitive effect on the result.