Fermi Estimation Interview Prep
Fermi estimation interview prep for quant candidates: assumptions, decomposition, sanity checks, and clear communication.
Candidates practicing estimation rounds and open-ended sizing questions.
What Fermi questions test
Fermi questions test decomposition and calibration. The interviewer wants to see whether you can turn an unknown quantity into a chain of reasonable assumptions and then pressure-test the final scale.
Concrete example
For "how many piano tuners are in London," start with population, households, piano ownership, tuning frequency, and tunings per worker. The exact answer matters less than whether the assumptions are transparent and the result is plausible.
How to practice
Practice writing assumption trees. Use round numbers, state uncertainty, and revise when a branch dominates the answer. Compare your estimate to a rough external fact after the session to improve calibration over time.
Answer structure
A strong Fermi answer names the target quantity, breaks it into drivers, assigns ranges, computes a midpoint, and then checks whether the order of magnitude feels plausible.
Common mistakes
Candidates hide assumptions or choose fake precision. Saying "about 2 million" after three rough assumptions is less credible than giving a range and explaining which assumption drives it.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.