A scale estimator that shows its working. It opens on the book's Twitter example — 3,500 QPS, 55 PB — then change any assumption and watch every line of the arithmetic follow. The point was never the answer; it is the method.
Chapter 2 the five common asks
These four lines are the book's own Twitter estimate. On the default assumptions they reproduce it exactly.
Going further my additions, not the book's
Read load, bandwidth, cache and server count — the rest of the envelope the chapter leaves as an exercise.
86,400 seconds in a day · the ×1000 ladder KB→MB→GB→TB→PB · bytes counted decimally (1 TB = 10¹² B), as the book does.
Circled figures are rounded the back-of-envelope way — to two significant figures, and peak is taken from the rounded QPS, exactly as the book does.
Built by Mhayk Whandson alongside the estimation reference, working through System Design Interview — An Insider's Guide, Chapter 2.