The CP range near the bottom of each Pokemon stats page shows what CP a Pokemon would have at minimum IV vs maximum IV. Example:
http://pokemongo.gamepress.gg/pokemon/4
Level 30 Charmander
0% IV 628 cp
100% IV 818 cp
It varies depend on a Pokemon's base stat, but generally a perfect pokemon will have about 20%~30% over 0% one of same level.