ARPPU stands for Average Revenue per Paying User. ARPPU is always higher than ARPU (average revenue per user) and usually represents how much revenue you get from a customer. It works best when you calculate it for long periods such as a year to normalize subscriptions of different durations. It's also useful to get ARPPU for each day to see a dynamics.
ARPPU is super easy to calculate:
With Adapty ARPPU is calculated automatically from raw transactions in addition to another subscription metrics. No server code required, works out of the box.