Use this tool to quickly find out what CP your Pokemon will evolve into. The IV Calculator will give a more accurate answer if you want to take the time to input more information. New Battle Simulator
This little tool aims to help trainers decide which Pokemon to evolve. E.g. Would your 300CP Pidgey evolve into something stronger than your 550CP Pidgeotto? Or are you better off evolving the Pidgeotto?
The tool shows what CP a Pokemon is likely to evolve into (and has evolved from) based on samples in PA's database. Not all Pokemon scale up in CP the same during evolution. Some species increase CP by 50%, others by 300%. This tool puts it in one place to help you decide.
Once you've evolved your Pokemon, you might want to check if it has a great Moveset or if it's high on the Best Defender list. If you've got a great Pokemon, have a look at Trainer Gear to see if there's anything to add to your kit to make yourself a better equiped trainer.
Please note that this tool is a fast way of getting an approximation of your Pokemon's future CP. With the IV Calculator, it's now possible to get EXACT CP and HP numbers if you're willing to input a little more information. This tool will be maintained because it's fast and simple, but if you're considering an important evolution like Magikarp to Garydos, Poke Assistant enourages you to use the IV Calculator.
Major credit to trainers nylonee & hyperfocus for gathering the first large colleciton of evolve data, first in the beta, then since launch.