VSWR, or Voltage Standing Wave Ratio, is found from the reflection coefficient:

If a transmission line is feeding a DUT and there is an impedance mismatch between the DUT input impedance and the characteristic impedance of the line, there will be a nonzero reflection coefficient Γ.

The existence of a reflection on the transmission line gives rise to constructive and destructive interference along its length and stationary peaks and valleys in the RF voltage will occur. The ratio between the maximum and minimum RF voltage along the length is equal to the VSWR. If there is no variation, then the VSWR = 1 (or often written 1:1), and there is a perfect match with no reflection.

The magnitude of the reflection coefficient may be found from:

See this and more important radio frequency charts and formulas in this comprehensive application note.