Microstrip Calculator
Microstrip is the most common planar RF transmission line — a trace on top of a dielectric with a ground plane below. The calculator works in both directions: geometry → impedance, or target impedance → required trace width. Uses Hammerstad-Jensen closed-form equations (accuracy < 0.5%).
Equations & Parameters ▸
\(Z_0 = \dfrac{120\pi}{\sqrt{\varepsilon_{eff}}\!\left[\,w/h + 1.393 + 0.667\ln(w/h+1.444)\right]}\)
| h | Substrate height (mm) — trace to ground plane. FR4 standard: 1.6 mm. |
| w | Trace width (mm). Wider → lower impedance. |
| t | Trace thickness (mm). Standard PCB copper: 35 µm (1 oz), 70 µm (2 oz). |
| εᵣ | Substrate dielectric constant. FR4 ≈ 4.4, Rogers 4003C ≈ 3.55, PTFE ≈ 2.1. |
| εeff | Effective dielectric constant — fields extend partly into air above the trace. |
| vₚ | Phase velocity = c/√εeff. Sets guide wavelength. |
Physical constants used
| c | Speed of light = 2.998×10⁸ m/s |
| µ₀ | Permeability of free space = 4π×10⁻⁷ H/m ≈ 1.2566×10⁻⁶ H/m |
| ε₀ | Permittivity of free space = 8.854×10⁻¹² F/m |
Geometry → Impedance
mm
Trace to ground planemm
mm
35 µm = 1 oz copperFrom Geometry
Characteristic impedance, Z₀—
Effective εᵣ—
Phase velocity, vₚ—
Target Impedance → Width
Ω
mm
From Impedance
Required trace width, w—
Diagram