Trace Impedance & Electrical Performance Calculator
Characteristic impedance, propagation delay, L and C per length for microstrip and stripline.
What it does
Estimates the characteristic impedance of a trace — microstrip on an outer layer over a plane, or stripline between two planes — plus propagation delay and inductance and capacitance per length.
Where it's used
Routing 50 Ω RF lines, USB / Ethernet / HDMI differential pairs and fast clock lines.
How to use it
- Choose microstrip or stripline.
- Enter the trace width and copper weight.
- Enter the dielectric height and εr from your fab’s stack-up.
- Adjust the width until Z₀ reaches your target.
The formula explained
- Z₀
- characteristic impedance, Ω
- εr
- dielectric constant of the board material
- εeff
- effective εr seen by a microstrip (part of the field is in air)
- tpd
- propagation delay per length
- W, H, T
- trace width, dielectric height, copper thickness
Worked example
A 0.5 mm microstrip over a 0.3 mm dielectric.
Inputs:
- Geometry: Microstrip (outer layer)
- Trace width W: 0.5 mm
- Copper weight (thickness T): 1 oz (35 µm)
- Dielectric height H (to plane): 0.3 mm
- Dielectric constant εr: 4.5
- Trace length: 50 mm
The calculator gives:
- Characteristic impedance Z₀: 50.72 Ω
- Effective εr: 3.361
- Propagation delay: 6.115 ps/mm
Tips and common mistakes
- Use the εr and heights from your manufacturer’s stack-up — FR-4 varies roughly from 3.8 to 4.6.
- These closed-form equations are accurate to within a few per cent; use a field solver for critical lines.
- A wider trace or a thinner dielectric lowers Z₀.
Questions people ask
How wide should a 50 Ω trace be?
On FR-4 with εr around 4.5, a 50 Ω microstrip is roughly 1.7–2 times as wide as the dielectric is thick. The example 0.5 mm trace over 0.3 mm works out at about 50.7 Ω.
What about differential pairs?
This calculator gives single-ended impedance. Differential standards have their own targets — about 90 Ω for USB and 100 Ω for Ethernet and HDMI — which depend on the gap between the two traces too.
When does a trace need controlled impedance?
When it is long compared with the distance a signal edge travels during its rise time. A common rule of thumb is more than about one sixth of that distance: at about 6.1 ps/mm, a 1 ns edge makes traces longer than about 27 mm critical.
Reference: IPC-2141
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Enter your values to get the result with the formula, substituted values and an explanation of each step.