From Roku’s shareholder letter, their annualized revenue from platform alone (not hardware) just hit $672M this latest quarter (multiply $167M by 4). They have about 31 million user accounts, so you simply divide $672M by 31M, and you get about $21 per user per year.
This is not the correct calc ROKU uses for ARPU.
Take the average of subs from this Q and the same Q last year. Take TTM platform revenue divided by that number to get ARPU.
AJ