Let's take the customer churn definition from this article, that I have also seen in other sources.
Let’s do the maths with one example: On January 1st you have 1000 customers. 100 of them are not using your service at the end of the month anymore. Furthermore you have acquired throughout January (31st of January) 500 new customers where 125 directly churn out. Simplest calculation looks as follows: (100 + 125)/1000 = 0.225
Why do we divide by 1000? Wouldn't it make more sense to divide by the total amount of users in period, 1500?