  The CEPC is a proposed high luminosity Higgs/Z factory, with the potential to
be upgraded to top factory at center-of-mass energy of 360GeV. We perform an
optimization study on the circumference of CEPC. We calculate the instant
luminosity, the construction and operation cost for different circumferences.
With respect to the total cost and average cost per particle, we conclude that
the optimal circumference for the CEPC Higgs operation is 80 km. Taking into
account of the Z pole operation, the potential high-energy upgrade of CEPC (top
factory), the optimal circumference increased to 100 km. The long future
proton-proton upgrade of CEPC (SPPC) also favors a larger circumference, and we
conclude that 100 km is the global optimized circumference for this facility.
