Google Analytics counts for 1, 7, 14, and 28 day active users don't add up

Question

I am really curious about what '1-day-active-users', '7-day-active-users', '14-day-active-users', and '28-day-active-users' metrics mean in Google Analytics Active users report. The GA documentation is really vague about these terms. According to this post, (Calculate Daily Active users in Google Analytics) 1-day-active-users is the number of unique visitors to the site on a particular day and accordingly, '7-day-active-users` is the number of unique users who initiated sessions on your site or app in the past 7 days.

So for a particular date, say 8th January 2020, shouldn't 7-day-active-users be the sum of 1-day-active-users from January 1 to January 7? .

This is a report drawn from GA :

1/1/2020    14241   56483   85966   132875
2/1/2020    18134   55620   84800   132922
3/1/2020    17318   54804   83822   132434
4/1/2020    9766    54151   84288   132666
5/1/2020    13798   53987   83892   132736
6/1/2020    18134   53754   83705   132759
7/1/2020    17784   53544   84428   131947
8/1/2020    18087   54687   84917   131807
9/1/2020    18437   55503   84684   131389
10/1/2020   17038   55083   83868   131064 

Therefore the 7-day-active-users for 8 should be = 109175 but it is actually 54687. Where am I going wrong?

My ultimate goal is to find out the daily active users and monthly active users for the complete website.

Answer 1

The key is that "uniqueness" is within the time period: if someone visits 3 times during the week, they will count 3 times toward 1-day active user totals, but only 1 time toward the 7-day active users total. Over all your web traffic, summing the 1-day totals will lead to a lot of overcounting compared to the 7-day total.

The date ranges include their final date, as well; for 8 January you would be comparing to 2-8 January rather than 1-7. Summing the 1-day active user values and comparing that to the 7-day active user value will give you a sense of how much your visitors return to the site within a week.