PCB analysis / Free calculator

PCB trace impedance

Estimate characteristic impedance from trace width, copper thickness, dielectric gap and relative permittivity. Choose the cross-section that matches your stack-up.

01 Your inputs

Uncoated surface microstrip: air above the trace, one continuous reference plane below.

Trace geometry
Units for every dimension

Changing units converts width, dielectric gap and copper thickness. 1 mil = 0.0254 mm.

Finished copper width. Supported w/h: 0.1 to 2.

From the lower copper face to the nearest reference-plane face, not total board thickness. Stripline uses this same clear gap above and below.

Use the same unit as width and gap. 35 µm = 0.035 mm ≈ 1.378 mil. Supported t/h: 0 to 0.5, and t must not exceed half the width. Zero is an ideal limit.

Use the stack-up material value at your design frequency. Supported range: 2 to 12.

Calculated on your device. Inputs are not transmitted or stored.

02 Your result

Enter your values and select Calculate.

The method

Match the trace to its reference planes

Use the finished copper dimensions and the dielectric gap from your stack-up. These formulas estimate single-ended characteristic impedance. All lengths in an equation must use the same unit; ln is the natural logarithm.

Microstrip

A surface trace above one plane

Cross-section · looking along the trace

Uncoated microstrip with air above the traceA copper trace of width w and thickness t rests on dielectric above a reference plane. The clear gap h starts at the bottom of the trace and ends at the top of the plane. Air, without solder mask, is above the trace.
Signal copper and reference planesDielectric
w
Finished copper width
t
Copper thickness
h
Clear dielectric gap below the copper
εᵣ
Relative permittivity of the dielectric
h runs from the lower trace face to the upper plane face. The top of the trace is exposed to air; solder mask is excluded. Symbolic model; not to scale.
Z0=87εr+1.41·ln(5.98·h(0.8·w+t))
Symmetric stripline

A trace centred between two planes

Cross-section · looking along the trace

Symmetric stripline with equal dielectric gapsCopper trace width w and thickness t sit between two reference planes in one dielectric. Each clear dielectric gap h starts at a trace face. The inside plane separation b equals two h plus t.
Signal copper and reference planesDielectric
w
Finished copper width
t
Copper thickness
h
Equal clear dielectric gap on each side
b
Inside plane spacing: 2h + t
εᵣ
Relative permittivity of the dielectric
The trace is centred between the planes. h is one clear gap, not half the plane spacing; the copper thickness must also fit between the planes. Symbolic model; not to scale.
Z0=60εr·ln(1.98·(2·h+t)(0.8·w+t))
Z0=f(wh,th,εr)

A worked example

NXP’s microstrip example uses w = 8 mil, h = 5 mil, t = 1.4 mil and εᵣ = 4.1. The estimate is 49.80 Ω, close to the published 50 Ω. These are the default inputs, converted to millimetres. With the same dimensions and equal dielectric gaps above and below, the symmetric-stripline formula gives 31.49 Ω.

Assumptions & limits

Single-ended, straight rectangular copper above wide continuous ideal reference planes. Microstrip is uncoated with air above it. Symmetric stripline is centred in one uniform isotropic nonmagnetic dielectric; the inside plane spacing is b = 2h + t. Supported screening domain: 0.1 ≤ w/h ≤ 2, 0 ≤ t/h ≤ 0.5, t/w ≤ 0.5 and 2 ≤ εᵣ ≤ 12. These product limits are not accuracy bounds. Dimensions have numerical bounds of 10⁻⁶ to 10⁶ in the selected unit; copper thickness may be zero as a mathematical limit. Excludes solder mask, etch shape, roughness, dispersion, losses, anisotropy, differential/coplanar coupling, asymmetric stripline, finite plane edges, vias and connectors. Confirm a controlled-impedance design with your fabricator’s stack-up and a field solver.

Calculation source: Engineering Workbench engine 0.3.0; Texas Instruments DP83816 §6.2.2.2 equations 3–4, with the microstrip example from NXP AN2536 rev. 2, p. 3.

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