Maths and details in Case interview

Question

I've been preparing for consulting alone for 5 months now and get very familiar with the flow. I have no problem with structuring but when it comes to maths, I mess up mostly everytime. I just can not grasp the term, do math quickly then deliver it verbally to the interviewer all at once. Remembering details is another big problem. That's for the context, here are my questions:

1/ What are the concepts I need to know in order to do maths in case interview? How to calculate quickly, accurately and communicate at the same time?
2/ How can I manage the information I have and use it later on given I keep forgetting?
3/ Should I practice with peer given this problem? Im afraid I might mess up during the case and can not learn much.

Thanks so much, any perspective would be a great help :)

Answer 1

Hi there — a lot of candidates struggle with this, so you're definitely not alone. Math and information management are very common pain points in case interviews.

1. What math concepts should you focus on?

Case interview math is usually not complicated, but it requires being structured, calm, and communicative. The most useful concepts to master are:

Core calculations (non-exhaustive)

Percentages (e.g., 20% of 250, % growth/decline)
Ratios and proportions
Multiplication and division with round numbers
Weighted averages
Breakeven calculations
Market sizing arithmetic
Growth rates (rough YoY changes)

Techniques to make math easier

Round numbers strategically (e.g., 49 → 50)
Break calculations into steps
Example: 24 × 35 → (24 × 30) + (24 × 5)
Say your approach before calculating
“I'll first multiply revenue per unit by volume to get total revenue.”

Communicating while calculating
You don't need to do everything simultaneously. A simple structure works well:

State the formula
Plug in the numbers
Calculate step by step
Give the final answer with interpretation

Example: “Profit equals revenue minus costs. Revenue is $50 per unit × 200 units, so that's $10,000. Costs are $6,000. That leaves us with a profit of $4,000.”

Interviewers care more about clear thinking than speed.

2. How to manage information and avoid forgetting things

This is usually a note-taking structure problem, not a memory problem.

A simple approach:

Divide your page into sections

Left side:

Case objective
Key question

Middle:

Structure/framework

Right side:

Numbers and data

Bottom:

Insights/conclusions

Also:

Circle key numbers
Write units clearly ($M, %, units)
When new data appears, quickly summarize it out loud
(“So we have 3M customers and $20 average spend.”)

This reduces the need to memorize because everything is visible on paper.

3. Should you practice with peers?

Yes, this is actually important to do.

Practicing alone helps with understanding the flow, but case interviews are interactive. You need practice with:

Explaining your math verbally
Thinking under pressure
Structuring out loud
Handling interviewer questions

Messing up during peer practice is exactly how you improve. In fact, candidates who only practice alone often struggle more in real interviews.

Even if the math goes wrong, you’ll still learn:

How to communicate your logic
How to recover from mistakes
How to slow down and structure calculations

One extra tip for math mistakes

Many candidates rush because they think speed is expected. In reality:

Accuracy + clear communication > speed.

It’s perfectly fine to say: “Let me take a few seconds to calculate this.”

That small pause often prevents most mistakes.

If you need help working through these challenges, feel free to reach out — happy to help with more specific coaching.