How did we arrive at 13 billion pixels?
By Sean Heber

Since we (sadly) don't have a complete archive of all of our work ever, we had to come up with a total pixel count using an approximation technique. We ended up using what is often referred to as a Fermi estimate (https://en.wikipedia.org/wiki/Fermi_problem) to arrive at a total based on multiple estimates of multiple projects and time periods. See also: https://en.wikipedia.org/wiki/Hand-waving

We split the last 20 years up into several pixel-size-based "eras" and then multiplied the number of client projects along with their average sizes during that time period. Here's the raw data:

Old School Mac (16x16 + 32x32 = 1280px):
150 sets
25 icons each
= 4800000px

Pre-Retina Mac Sets (16+32+48+128 = 19968px):
100 sets
15 icons each
= 30336000px

Retina Mac Sets (1398016px):
50 sets
12 icons each
= 838809600px

Old School Client "sets" (16x16 + 32x32 = 1280px):
250 sets
25 icons each
= 8000000px

Pre-Retina Mac Client sets (16+32+48+128 = 19968px):
140 sets
15 icons each
= 41932800px

Pre-Retina Mac MEGA Client sets (16+32+48+128 = 19968px):
10 sets
500 icons each
= 99840000px

Retina Mac Client sets (1398016px):
400 sets
15 icons each
= 8388096000px

Pre-Retina iOS Internal Projects (1107446px per set)
4 "sets"
150 icons each (rough guesses)
= 664467600px

Pre-retina iOS client projects (1107446px per set)
150 sets
10 each
= 1661169000px

Retina iOS client projects (1140061px per set)
75 sets
8 each
= 684036600px

Retina iOS Projects (1140061px per set)
4 sets
30 each
= 136807320px

Windows XP
150 icons
= 11836800px

Windows Vista
100 icons
= 7891200px

Twitter emoji
1000 icons @ 16x16 and 72x72
800 @ 72x72
= 9587200px

Facebook emoji
1200 icons
2 sizes = 512x512, 72x72 = 267328
= 320793600px

The grand total of these numbers is 12,908,403,720px which we rounded up to 13 billion since it's close and we likely missed or forgot entire projects along with large UI-based design projects, background images, etc.