thinking-fermi-estimation
Fermi Estimation
Overview
Fermi estimation, named after physicist Enrico Fermi, is the art of making reasonable estimates for quantities that seem impossible to know without direct measurement. By decomposing a question into factors you can estimate, then multiplying, you often get surprisingly accurate order-of-magnitude results.
Core Principle: Break the unknown into known (or estimable) pieces. Even rough estimates combine to reasonable accuracy due to errors canceling out.
When to Use
- Capacity planning ("How much storage will we need?")
- Cost estimation ("What will this infrastructure cost?")
- Market sizing ("How many potential users exist?")
- Feasibility assessment ("Is this even plausible?")
- Sanity checking ("Does this number make sense?")
- Interview questions ("How many piano tuners in Chicago?")
- Quick prioritization ("Is this worth pursuing?")
Decision flow:
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