Quant Interview Formula Sheet
A practical quant interview formula sheet that explains when formulas help, when they fail, and how to connect them to reasoning.
Candidates who want formulas tied to interview judgment rather than memorized in isolation.
Formulas follow the model
A formula is useful only after the model is correct. C(n,k) helps with unordered choices, but it is wrong if order matters. Bayes rule helps with base rates, but it is easy to misuse if you swap the conditional direction.
Probability formulas worth knowing
Know complements, multiplication for independent events, conditional probability, Bayes rule, combinations, permutations, and inclusion-exclusion. More important, know the words that trigger each formula and the assumptions that break it.
Expectation formulas worth knowing
Expected value is the weighted average of payoff by probability. Linearity says E[X + Y] = E[X] + E[Y], even when X and Y are dependent. Indicator variables turn many counting expectations into simple sums.
Statistics formulas worth knowing
Variance, covariance, correlation, standard error, and regression coefficients are common enough to review. In interviews, be ready to explain what each formula means and what assumptions make it useful.
Concrete example
For exactly two aces in a five-card hand, C(4,2)C(48,3)/C(52,5) works because the hand is unordered. If the prompt asks for ordered draws, the structure changes. The formula is downstream of that decision.
Common mistakes
Candidates often quote formulas faster than they define variables. A formula sheet should reduce cognitive load, not replace reasoning. If you cannot explain why the formula applies, pause before using it.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.