You've already perfected your curve ball move. You have optimized your Pokemon power up and evolution by using the CP and IV calculators. You have high-powered Pokemon in your bag, ready for battle. You tap on a Pokemon Go gym then you discover the trainers in there have way more CP than yours! Tough luck, right?
Should you run or fight?
Not to worry, if you have done well in your CP and IV calculations, then there's a good chance you will win. It is possible that your Golbat of 810 CP can beat 3 1000+CP Pokemon guarding the gym in Pokemon Go.
But just in case you still don't have the upper-hand, you might want to use this damage calculator. A thread at Reddit has a discussion about a compilation of Pokemon Go damage calculations and posted a work-in-progress excel-based calculator. Sorry, not browser based this time.
Teach me, master
Reddit user 'homu' claims, "Unlike other calculators out there. This one calculates the actual damage dealt by each attack during a gym battle. Make your own copy with this link and try it out!" You can get your copy here.
'homu' also explains, "For input, I highly recommend downloading your Pokemon roster CSV from PokeAdvisor. Simply copy over the entire sheet to the appropriate section in Damage Dealt and Damage Received. Then toggle the target Pokemon's stats, and the calculator will evaluate your entire roster against it." For complete details of the discussion and updates about this Pokemon Go damage calculator, visit this thread at Reddit. Upon further reading, I have learned that Niantic has issues with PokeAdvisor.com, the enrollment for usernames should be back in a week or so to as stated in Reddit. In the mean time, you may use the IV calculator. Once you have done inputting data you can now gauge if you generally have the attack power to conquer Pokemon Go gyms.
This seems a lot of legwork but hey, it's not easy to be the very best! It is recommended that you do your calculations at home where you have proper access to a PC.