This microstrip impedance calculator finds the characteristic impedance of a PCB trace from its width, the dielectric height, and the substrate constant. Enter the values above to design matched lines.
Formula (Hammerstad, W/h ≥ 1)
εeff = (εr+1)/2 + (εr−1)/2 × (1 + 12h/W)−0.5 Z₀ = 120π ÷ (√εeff × (W/h + 1.393 + 0.667·ln(W/h + 1.444)))
Example
εr = 4.4, W = 3 mm, h = 1.6 mm: εeff = 3.33, Z₀ = 50.8 Ω.
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Frequently Asked Questions
How do I calculate microstrip impedance?
Find the effective dielectric constant from the trace width and height, then apply the Hammerstad formula for Z0.
What trace width gives 50 ohms?
It depends on the board: on 1.6 mm FR-4 (er 4.4), about a 3 mm wide trace gives roughly 50 ohms.
What is effective dielectric constant?
It is the average the field sees, between the substrate and air above the trace, always less than the substrate’s er.