This equation describes the kinetics such as the decay of radioactive isotopes, the elimination of drugs, and the dissociation of a ligand from a receptor.

X is time.

Y may be concentration, binding, or response. Y starts out equal to SPAN+PLATEAU and decreases to PLATEAU with a rate constant K.

The half-life of the decay is 0.6932/K.

SPAN and PLATEAU are expressed in the same units as the Y axis. K is expressed in the inverse of the units used by the X axis. In many circumstances, the plateau equals zero. When fitting data to this equation, consider fixing the plateau to a constant value of zero.