What is the formula for calculating an average rating from ratings originated from different sources?

I want to build a website, where I will show ratings for movies. I want to show there an average rating for a movie and I want to calculate that average from ratings from different sources. The issue is that on some of the desired sources a few people have rated a movie and in other sources thousands of them. E.g. The movie "The Revenant". It has an average rating of 7.9 in IMDB (10000 users have voted) and it also has a 9.9 in "XYZ" website (10 users have voted). How would look like a formula in order to calculate that average?

Initially I thought of just simply assigning weights based on the number of users, but I have the feeling I am missing something. Any ideas?