Choose the stack-up before the width
There is no single 50 Ω trace width for every PCB. Width, copper thickness, dielectric spacing, and material permittivity all enter the estimate. Start with the fabricator’s proposed stack-up and the layer carrying the signal.
The tool supports two specific sections: an exposed microstrip above one reference plane, and symmetric stripline centred between two planes. A coated outer trace, an off-centre inner trace, or a trace with nearby coplanar ground needs a different model.
Measure between the copper faces
The input h is the clear dielectric gap from the signal copper face to the facing reference-plane surface. It is not the total board thickness or a centre-to-centre distance. In symmetric stripline, that same clear gap must exist on both sides of the signal.
The inside plane spacing b is therefore 2h + t, where t is signal copper thickness. Enter width, gap, and copper thickness in the same selected unit. Switching millimetres and mils converts all three dimensions together.
Check a published microstrip example
NXP AN2536 gives a microstrip example with an 8 mil trace, a 5 mil dielectric gap, 1.4 mil copper, and relative permittivity 4.1. Entering those values gives about 49.80 Ω with the tool’s closed-form model, near the application note’s 50 Ω.
The same dimensions in the symmetric-stripline model give about 31.49 Ω. This is a different cross-section, not an alternate answer for the original outer-layer trace. The comparison is a useful reminder to select the geometry before interpreting a number.
| Input | Mils | Millimetres |
|---|---|---|
| Trace width w | 8 | 0.2032 |
| Dielectric gap h | 5 | 0.127 |
| Copper thickness t | 1.4 | 0.03556 |
Take the estimate back to the fabricator
Within this model, increasing width generally lowers impedance; increasing the gap generally raises it. Use those trends to understand a proposed stack-up, rather than treating extra decimal places as evidence of accuracy.
The tool rejects geometries outside its supported ratio and material ranges. Passing those checks does not guarantee a particular error bound. Solder mask, etched sidewalls, roughness, frequency dependence, and discontinuities are excluded.
Ask the fabricator to confirm the finished dimensions and material properties with its field solver. If controlled impedance matters, agree the tolerance and verification method with the supplier. This calculator does not replace that process or predict losses, differential impedance, or a via transition.
References & further reading
- NXP AN2536 — microstrip example, page 3
- Texas Instruments DP83816 — microstrip and stripline equations, page 104
Equations describe the stated model. Worked values are rounded for reading; the calculator’s download retains its calculated precision. Read about our checks and their limits.