The Warburg diffusion element () is a constant phase element (CPE), with a constant phase of 45° (phase independent of frequency) and with a magnitude inversely proportional to the square root of the frequency by:
where 
* is the Warburg coefficient (or Warburg constant); 
* is the imaginary unit;
* is the angular frequency.
This equation assumes semi-infinite linear diffusion, that is, unrestricted diffusion to a large planar electrode.