Estimation Rules of Thumb

Formulas to quickly size any system in an interview.

Somebody proposes keeping every user's activity feed in memory so it is always fast. The room nods, because fast sounds right.

Twenty seconds of arithmetic: 10 million users, 500 events each, 200 bytes an event. That is a terabyte of memory, which at cloud prices runs to thousands a month, for a feature nobody had costed.

Not one person in that room lacked the knowledge to do that sum. They lacked the habit of doing it out loud before the decision, which is the entire skill and the reason it maps so closely onto real senior work.