Sanity Checks in Case Interviews: Catch Math and Logic Errors Fast

Strong candidates rarely fail case interviews because they cannot calculate. They fail because they confidently state numbers or conclusions that do not make sense. Sanity checks in case interviews help you catch obvious math and logic errors before you say them out loud, protecting both your credibility and your recommendation. Whether you are checking case interview calculations, validating assumptions, or applying plausibility checks in case interviews, these quick reality tests are a core consulting skill. 

TL;DR – What You Need to Know

Sanity checks in case interviews are quick reality tests that help candidates validate math, assumptions, and logic so results remain credible and business realistic.

• Interviewers expect candidates to use sanity checks to identify unrealistic numbers, weak assumptions, and flawed conclusions before communicating results.
• Orders of magnitude estimation and back of the envelope calculations help verify numerical scale without repeating full math.
• Plausibility checks in case interviews confirm alignment with business context using unit checks, directional logic, and realistic boundaries.
• Benchmarks and assumptions anchor case interview math checks in real world reference points to prevent fragile or misleading conclusions.

What sanity checks in case interviews actually mean

Sanity checks in case interviews are quick reality checks candidates use to confirm that numbers, assumptions, and conclusions make business sense before communicating them. They ensure results are directionally correct, logically consistent, and aligned with real world constraints without requiring full recalculation.

At a high level, sanity checks act as a safeguard between analysis and communication. Before you state a result, you briefly assess whether it passes a basic common sense test.

Sanity checks typically apply across three areas:

• Numbers, where you sense check scale using orders of magnitude estimation
• Assumptions, where you validate inputs through benchmarking assumptions and reality checking assumptions
• Logic, where you confirm logical consistency in case interviews by validating cause and effect

For example, if a profitability estimate implies margins far above industry norms, a business plausibility check should immediately trigger reassessment. These checks form the foundation of reliable case interview math checks.

Why interviewers expect sanity checks during case math

Sanity checks help interviewers evaluate judgment during case math by revealing whether candidates understand scale, feasibility, and business realism beyond pure calculation. A candidate who calculates accurately but misses unrealistic outputs signals weak consulting intuition.

During a case, interviewers observe whether you:

• Recognize when results are too large or too small to be realistic
• Question assumptions that drive extreme outcomes
• Adjust conclusions when numbers conflict with business context

For instance, confidently stating revenue that exceeds the entire market size is more damaging than making a small arithmetic error. Sanity checks demonstrate awareness of constraints and decision making maturity.

How to sanity check numbers in case interviews quickly

Sanity checks in case interviews can be applied quickly by validating scale, direction, and realism rather than recalculating exact figures. These checks help catch errors under time pressure without slowing down the interview.

Effective quick checks include:

• Rounding inputs to confirm orders of magnitude estimation
• Using back of the envelope calculations to estimate outputs
• Comparing results against everyday reference points

For example, if your math implies each customer buys an unrealistic quantity of a high value product, a quick sense check should raise concern. Knowing how to sanity check numbers in case interviews helps prioritize decision relevance over numerical precision.

Common plausibility checks in case interview math

Plausibility checks in case interviews evaluate whether calculated results are realistic given the business context. They are especially useful when calculations involve multiple steps or layered assumptions.

Common plausibility checks include:

• Unit checks to ensure consistency between inputs and outputs
• Directional checks to confirm results move logically with assumptions
• Boundary checks to verify outputs fall within reasonable ranges

If increasing prices unexpectedly leads to higher demand in your math, a plausibility check should prompt immediate review. These checks ensure case interview math reflects how businesses actually operate.

Using benchmarks and assumptions to check case calculations

Case interview math checks rely on benchmarks and assumptions to test whether results make sense at a practical level. Sanity checks using benchmarks help prevent overconfidence in fragile calculations.

Helpful benchmark based checks include:

• Comparing market size estimates to known industries or regions
• Estimating per customer spend based on common purchasing behavior
• Adjusting assumptions slightly to test whether conclusions remain stable

If a small assumption change reverses your conclusion entirely, the analysis is likely too sensitive. Benchmarks reinforce logical consistency in case interviews and strengthen credibility.

Catching logic errors before stating conclusions out loud

Sanity checks also apply to logic, not just numbers. Logical errors occur when conclusions do not clearly follow from the analysis presented.

Before stating a conclusion, ask:

• Does the recommendation directly answer the original objective
• Are cause and effect relationships clearly supported
• Have constraints or tradeoffs been ignored

For example, recommending rapid expansion while acknowledging severe capacity limits creates a logical inconsistency. Logic checks ensure recommendations remain coherent and defensible.

How sanity checks improve accuracy and confidence in cases

Sanity checks in case interviews improve accuracy by preventing avoidable errors and improve confidence by reinforcing trust in your own analysis. They act as a final quality filter before communication.

Consistent use of sanity checks leads to:

• Fewer obvious math and logic mistakes
• Stronger alignment between analysis and recommendations
• Clearer communication under time pressure

Over time, sanity checking becomes instinctive. Candidates who apply it consistently sound more credible, more structured, and more consultant like throughout the case interview.

Frequently Asked Questions

Q: How to avoid math mistakes in case interviews?
A: To avoid math mistakes in case interviews, use sanity checks to pause and sense check numbers, assumptions, and final outputs before speaking. Simple reality checks help catch unrealistic results early.

Q: How to sanity check numbers in case interviews?
A: You should sanity check numbers in case interviews immediately after completing a calculation and again before stating conclusions to ensure results remain realistic and decision-relevant.

Q: How do interviewers evaluate case interview math accuracy?

A: Interviewers evaluate case interview math accuracy by assessing whether calculations are directionally correct, assumptions are reasonable, and results make business sense.

Q: Do consultants use a lot of math in interviews?
A: Consultants do not use complex math in interviews, but they rely heavily on basic arithmetic, estimation, and business judgment rather than advanced calculations.

Q: Can you use estimation instead of exact math in case interviews?
A: Yes, candidates are expected to use estimation instead of exact math in case interviews through orders of magnitude estimation and back of the envelope calculations.