No. 2 · Decisions · Equations
Back-of-the-Envelope Calculation
A rough answer in five minutes often beats a precise one next month.
In the Almanac
“Fermi’s scraps of paper”, 1945 · “Warby Parker”, 2010
Both stories in full, three ways to use it and where it misleads.
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What is Back-of-the-Envelope Calculation?
A Back-of-the-Envelope Calculation is a fast, rough estimate using simple assumptions and basic arithmetic. It does not aim for precision, only for the right ballpark, so you can quickly see whether an idea, claim or plan makes sense.
How do you use Back-of-the-Envelope Calculation?
Use it to test bold claims at work, total up the real cost of big purchases, and break personal goals into manageable numbers. When the stakes are high, treat it as a starting point and check your assumptions.
When does Back-of-the-Envelope Calculation matter?
Back-of-the-Envelope Calculation is one of the 35 mental models for decisions, the situations where you ask: Which way should I go? Within that, it belongs to “Weighing the options”, alongside A/B Testing, Cost-Benefit Analysis and Decision Tree.
What is an example of Back-of-the-Envelope Calculation?
Two stories show it at work: Fermi’s scraps of paper (1945) and Warby Parker (2010). Both are told in full in The Mental Models Daily Almanac, which has two stories for each of its 365 models.
Which mental models are related to Back-of-the-Envelope Calculation?
Expected Value, First Principles Thinking and Margin of Safety. Back-of-the-Envelope Calculation itself comes from equations.