17.1 Never Calculate Before Defining What You Are Calculating
Before using any numbers, state the quantity you are trying to determine.
For example, if the interviewer gives you the number of customers, purchases per customer, and average price, do not immediately multiply the figures. First establish that you are calculating annual revenue.
Good habit: Say, “I’d like to calculate annual revenue by multiplying the number of customers by purchases per customer and average revenue per purchase.”
This forces you to think about the business logic before the arithmetic.
17.2 Writing the Equation First
Once you know what you are calculating, write the equation before inserting the numbers.
For example:
Revenue = Customers × Purchases per Customer × Average Price
or:
Profit = Revenue − Costs
Writing the equation first reduces the chance of using the wrong inputs or performing the wrong operation.
Practical benefit: If you later make an arithmetic mistake, the interviewer can still see that your analytical setup was correct.
17.3 Explaining Your Approach Before Doing Arithmetic
Briefly explain what you are about to do before calculating.
For example:
“To estimate the annual profit impact, I’ll first calculate incremental revenue, then subtract the additional variable costs and fixed investment required.”
This makes your reasoning easy to follow and gives the interviewer an opportunity to correct a misunderstanding before you spend time on the math.
Keep it concise: One or two sentences are usually enough. Do not give a long explanation for a straightforward calculation.
17.4 Organizing Calculations on the Page
Messy notes create unnecessary errors. Keep calculations visually organized so you can review them quickly.
A useful approach is to write:
- Objective: What you are calculating.
- Equation: The relationship between the variables.
- Inputs: The numbers provided.
- Calculation: The arithmetic.
- Result: The final number and unit.
Leave enough space between calculations so that corrections do not become confusing.
Tip: Keep important intermediate results visible because you may need them later in the case.
17.5 Keeping Track of Units
Units are one of the most common sources of case-math mistakes. A number without a unit is incomplete.
If you calculate:
2 million customers × $50 per customer = $100 million
the units help confirm that the calculation makes sense.
Similarly:
$200 million ÷ 4 million customers = $50 per customer
Best practice: Write units beside your inputs and state the unit when presenting your answer.
17.6 Working with Millions and Billions
Case calculations frequently involve large numbers, so simplify the scale before doing the arithmetic.
For example:
$3 billion = $3,000 million
If the other figures are already expressed in millions, convert everything to millions before calculating.
Similarly:
5 million units × $200 per unit = $1 billion
Common mistake: Getting the core multiplication correct but losing or adding a zero when converting the final result.
Always pause to check the magnitude.
17.7 Choosing Assumptions
Some calculations require information that is not provided. In these cases, you may need to make an assumption.
A strong assumption should be:
- Reasonable: It should be plausible given the context.
- Simple: Prefer numbers that make the calculation manageable when precision is not required.
- Transparent: State the assumption clearly.
- Testable: Recognize that changing the assumption may change the answer.
For example: “Since we do not have the exact conversion rate, I’ll assume 20% for an initial estimate.”
Important: Do not invent assumptions when the interviewer can provide the relevant information. Ask first when the missing input is material.
17.8 Checking Whether an Answer Makes Business Sense
After completing the arithmetic, pause before moving on.
Ask whether the answer is plausible given the client’s size, industry, and objective.
If a company with $500 million in annual revenue appears to generate $5 billion from one small product line, something is probably wrong.
Check: Direction, magnitude, units, and consistency with other facts in the case.
This quick review can catch errors that pure arithmetic checking misses.
17.9 Interpreting the Result
A calculation is not finished when you state the number. Explain what the result means.
Suppose you calculate that a new product could generate $40 million in profit.
A weak response is: “The answer is $40 million.”
A stronger response is: “The product could generate approximately $40 million in annual profit, which would cover about 80% of the client’s $50 million growth target.”
Interpretation: Connect the number directly to the client’s objective.
17.10 Turning a Calculation into an Insight
The strongest candidates go one step beyond interpretation and identify the implication for the case.
If an opportunity contributes only $10 million toward a $100 million target, the insight may be that the client needs additional growth levers. If a capacity expansion costs more than the profit it would generate, the implication may be that the investment is unattractive.
A useful progression is:
Calculation → Result → Meaning → Implication → Next Step
For example: “The price increase would generate approximately $30 million in incremental profit. That exceeds the client’s $20 million target, so pricing appears potentially sufficient on its own. I would next test whether the expected volume decline makes that estimate realistic.”
Final principle: Good case math is structured business reasoning expressed through numbers. Define the question, write the equation, organize the calculation, track the units, check the result, and explain what it means. The arithmetic matters, but the insight you draw from it is what moves the case forward.
