Quick quantification in case interviews

Short answer: yes, it gets better with practice, but not with random practice.

1. Memorize a small set of anchors
You do not calculate everything from scratch.

• 1/2 = 50%
• 1/3 ≈ 33%
• 1/4 = 25%
• 1/5 = 20%
• 1/8 ≈ 12.5%
• 1/10 = 10%

Once these are automatic, most percentages become quick adjustments.

2. Round first, refine later
Interview math is about direction and magnitude, not precision.
Example: 47 to 52 is not “5 over 47” — it is roughly +10%.

3. Convert everything to changes, not absolutes
Instead of “what is the exact number,” ask:

• Is this closer to 5%, 10%, or 20%?
  This alone speeds you up massively.

4. Practice in micro-drills, not full cases
5–10 minutes a day of:

• fraction to %
• % change
• growth over 2–3 steps
  beats long case math sessions.

Why some people look “natural”
They are not. They have internalized anchors and patterns, so their brain is recognizing, not computing from scratch

Hi there,

Short answer: yes, it does get better with practice, but not with random practice. The people who look “naturally” fast usually aren’t doing more math, they’re using a few reliable shortcuts and patterns.

The best way to practice is to focus on common conversions and anchors rather than calculating everything from scratch. Memorizing rough fraction to percentage equivalents like 1/2, 1/3, 1/4, 1/5, 1/8, and 1/10 already covers a large share of interview math. Once these are automatic, you can focus on the logic of the case rather than the arithmetic.

For percentage change, approximations matter more than precision. Thinking in deltas and magnitude is usually enough, as interviewers care more about direction and reasoning than exact decimals.

Timed micro-drills are very effective. Five to ten minutes a day of mental math, conversions, and back-of-the-envelope estimates builds speed quickly. Saying the math out loud also helps under interview pressure.

What looks like intuition is really pattern recognition built through repetition.

I also share a quick math cheat sheet with my students when we practice together in coaching sessions, which helps reinforce these shortcuts efficiently.

Best,
Evelina

That math barrier is totally understandable, and it's a critical bottleneck for many candidates. The reality is that the firms don't just test your math ability; they use quick quantification as a proxy for how structured you are under high pressure. When you see someone instantly get 1/8th as 12.5%, they aren't calculating—they are retrieving memorized data.

That perceived "natural intuition" is actually built on two core, learned strategies. First, rote memorization is non-negotiable. You should know every fraction from 1/2 to 1/15 and their decimal/percentage equivalents (e.g., 1/7 is 14.28%, 1/9 is 11.11%). Drill these until they are truly instant, flashcard-style. You should never be calculating these during a case; that time delay is what signals uncomfortableness to the interviewer.

Second, you need to practice structured estimation and magnitude checking. When faced with a complex percentage change (e.g., costs went from $480 million to $610 million), do not jump straight to the exact division. First, check the difference ($130M). Then, use rounding to estimate the magnitude (130/500 = 26%). This confirms your answer should be around 25-30%. This 'chunking' prevents silly errors and saves time because you know exactly where the decimal point needs to go. Dedicate specific practice sessions to this; there are great "M&A Math" drill apps and websites dedicated to this specific style of case math.

Hope this helps you structure your practice! All the best.

hey there :)

Yes I would say, it mostly gets better with practice. Start by memorizing common fractions and their percentages (1/2 to 1/10), practice quick estimation with percentage changes, and do lots of timed mini-calcs outside full cases. Doing pure math drills, even 10–15 min daily, builds speed and intuition. I also run dedicated math practice with my mentees if you want to join.

best,
Alessa :)

It's not natural talent. It's practice. People who seem fast have simply done it many times.

Here's what I recommend. 

First, memorize common fractions and their percentages until you can recall them instantly. 1/8 is 12.5%. 1/6 is about 16.7%. 1/5 is 20%. 1/4 is 25%. 1/3 is 33.3%. 3/8 is 37.5%. These show up all the time. Use flashcards for a week and you'll remember them easily.

Second, practice percentage change calculations until they become automatic. The formula is simple: new number minus old number, divided by old number. But speed comes from doing dozens of these out loud, explaining each step as you go, until you stop thinking about how to do it.

Third, get comfortable with rounding. In case interviews, 47 times 23 becomes 50 times 23, which is 1,150. Then adjust a little if needed. Interviewers don't want perfect answers with lots of decimals. They want to see you get to a reasonable estimate quickly and keep the conversation moving.

The real secret is practicing while talking out loud. Doing math quietly on paper is different from doing math while explaining your thinking to an interviewer. Practice with a timer. Talk through every step. Get used to the uncomfortable feeling of thinking and speaking at the same time.

Ten to fifteen minutes a day for two weeks will make a big difference. There are free mental math apps that can help. Or just take a case book and practice every math problem you find.