Velocity Factor Calculator

This tool calculates the velocity factor from the relative permittivity.


Enter the relative permittivity εr and the tool will provide the velocity factor




VF = 1/(√εr)


  • VF is the velocity factor
  • εr is the relative permittivity or dielectric constant of the medium. It is greater than or equal to 1 (corresponding to a vacuum).

Example Calculation

A relative permittivity of 2.1 corresponds to Teflon material. This gives a velocity factor of 0.69.

???? Velocity Factor of a Transmission Line


The velocity factor (VF), also known as wave propagation speed or velocity of propagation, is a measure of the speed at which an electromagnetic wave travels through a medium compared to the speed of light in a vacuum. It is expressed as a ratio or a percentage. The velocity factor is defined by the equation:

VF = v/c​

where v is the speed of the electromagnetic wave in the medium, and c is the speed of light in a vacuum, approximately 3×108 meters per second.

The velocity factor depends on the properties of the medium, including its permittivity and permeability. In practical terms, the velocity factor is important in the design and analysis of transmission lines, coaxial cables, and waveguides, as it affects the wavelength and impedance of the signals that travel through these media.

As an example for a transmssion line, VF might range from 0.66 to 0.99. This means the electromagnetic wave travels at 66% to 99% of the speed of light. This information is used to calculate the electrical length of the cable for radio frequency applications, ensuring that the signal timing and phase are correctly managed in communication systems.

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