Impedance estimate

A first estimate of the characteristic impedance of a track from closed-form formulas. The factory's stack-up decides the real figure.

Inputs

Structure Microstrip (outer layer over one plane) Stripline (inner layer between two planes) Differential microstrip (edge-coupled pair)

Track width

Dielectric height to the plane

Copper thickness

Dielectric constant (Dk)

Use a dot or a comma for decimals.

Results

This is an estimate, good to a few percent at best. The real Dk, dielectric thickness and etched width come from the factory's stack-up, and the factory's engineer adjusts your track width to reach the target. Do not use this page as a final answer.

Single-ended impedance: 48.6Ω

Typical targets: 50 Ω single-ended, 90 Ω differential for USB, 100 Ω differential for Ethernet. Check the standard for your interface.

The formula

Z0 = 87 ÷ √(Dk + 1.41) × ln(5.98 h ÷ (0.8 w + t))

w is the track width, t the copper thickness, h the dielectric height from the track to its reference plane, b the distance between the two planes of a stripline (the track in the middle), s the gap between the two tracks of a pair, Dk the dielectric constant.

Where the formula holds

Microstrip: 0.1 < w/h < 2.0 and 1 < Dk < 15 (IPC-2141). The page also refuses t/h of 0.25 or more, which is its own bound.

Solder mask over the tracks lowers the impedance by a few ohms; the page does not include it. Neither does it model surface roughness or the frequency dependence of Dk.

Source: IPC-2141, controlled impedance circuit boards and high speed logic design. Calculated in your browser. Nothing is sent to a server.