Fermi Decomposition Interview Framework
A repeatable Fermi decomposition interview framework for target variables, factor trees, assumptions, ranges, recombination, and explanation.
Candidates who freeze on open-ended estimation prompts.
Start with the target
Write the exact quantity you are estimating, including unit and time period. The target controls every assumption that follows.
Build a factor tree
Break the target into a product or sum of simpler pieces. Good factors are explainable, roughly estimable, and not double-counted.
Concrete framework
Use target = population x eligible share x usage frequency x unit amount. This structure works for many demand, traffic, and sizing prompts.
Recombine and test
After multiplying, compare the result with a lower bound, upper bound, or familiar benchmark. If the answer is implausible, revisit the largest driver.
Common mistakes
Candidates often create overlapping factors or change the unit halfway through. Keep the decomposition written clearly so each factor has one job.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.