Equivalent circulating density (ecd):

The increase in bottomhole pressure expressed as an increase in pressure that occurs only when mud is being circulated.

Because of friction in the annulus as the mud is pumped, bottomhole pressure is slightly, but significantly, higher than when the mud is not being pumped.

Ecd is calculated by dividing the annular pressure loss by 0.052, dividing that by true vertical depth, and adding the result to the mud weight.