Back-of-the-Envelope Estimation

Subtitle: Get to a defensible number in five minutes

Top-left quadrant - Powers of ten to memorize:

1 thousand = 10^3, 1 million = 10^6 1 billion = 10^9, 1 trillion = 10^12 1 KB = 10^3 bytes, 1 MB = 10^6 bytes 1 GB = 10^9 bytes, 1 TB = 10^12 bytes 86,400 seconds in a day (round to 100k)

Top-right quadrant - The four formulas:

QPS = daily requests / 86,400 Peak QPS = average QPS x 2 to 10 Storage = item size x count x replicas Bandwidth = payload size x QPS Read:write ratio drives the cache plan

Bottom-left quadrant - Latency anchors:

RAM read: 0.1 microseconds SSD read: 0.1 milliseconds Disk seek: 10 milliseconds Same-datacenter round trip: 0.5 ms Cross-continent round trip: 150 ms

Bottom-right quadrant - How to present it:

Round hard: 1M users, not 1.37M Say your assumptions out loud Solve one unit at a time Give a range, not false precision Sanity-check against a known system

Simple difference:

Precision = wrong tool for this job Order of magnitude = the real answer

Sticky note - Common beginner mistake:

Reaching for a calculator and chasing exact digits. Round to one digit; being right about the order of magnitude is the whole skill.