It takes a lot of candies to evolve a Pokemon in Pokemon GO, so you need to make sure you're not wasting them. If you're not sure if a Pokemon you caught is worth evolving, this calculator can give you a fairly accurate estimate of how strong they can get. The calculator, from PoGoToolkit.com, has you pick the Pokemon, then enter the CP (combat power) it currently has. As soon as you click "Evolve", you'll see it's maximum potential before powering up. Here's an example:
You catch an Abra with a CP of 400. If you evolve it into a Kadabra, it can potentially have 544-780 CP with a variance of 1.36-1.95. That Kadabra has the potential to evolve into an Alakazam with 761-1287 CP with a variance of 1.4-1.65. An Abra with a CP of 350 would only end being 476-682 and 666-1125, respectively.
This tool only calculates CPs for first-tier Pokemon. For second-tier Pokemon you'll have to take a few extra steps. For example: Enter CPs for a first-tier Pokemon until you get a CP range that your second-tier fits in, then you can see the expected CP for your third-tier evolution.
You'll never be able to know the exact CP a Pokemon will have when it evolves, but this calculator can give you a pretty good idea. As you can see, even a 50 CP gap can make a big difference when it comes to evolving, and save you some Stardust and candies down the line. There are other factors to take into consideration when choosing Pokemon to evolve, but CP is one of the easiest to understand and use to your advantage. Check out the calculator for yourself at the link below.