What's the most efficient way to practice mental math for case interviews while working full-time?

the mistake is thinking mental math requires “study time.” It doesn’t. It requires reps, and those reps fit naturally into dead time during the day.

The most efficient approach is to turn mental math into a habit. On the subway, in the car, walking, or waiting for a coffee.

Do short calculations in your head. Do not use phone/calculators anymore.

Focus on a very small set of operations that dominate case interviews:
percentages, ratios, growth, quick multiplication and division, and rough estimation. 

• During your commute, mentally solve 10–20 quick problems: “What’s 18% of 250?”, “What’s 1.3 × 240?”, “If revenue grows 15% twice, what’s the rough total growth?”
• Say the steps in your head or out loud if you can. This mimics interview conditions.

Two rules make this effective. First, round aggressively unless told otherwise. Second, verbalize the structure before the math. Speed without structure doesn’t help in interviews.

Mental math is one of those things people overcomplicate. You don't need to become a human calculator. You just need to be fast enough that the math does not slow you down during a case.

The best way to practice when you are working full time is in small pockets. Five to ten minutes, not hour long sessions. Do quick math in your head when you are shopping, eating out, or on your commute. What is 15 percent of the bill? If something costs 847 rupees and is 30 percent off, what do you pay? These tiny reps add up fast because you are training your brain to work without paper, which is exactly what a case interview needs.

The real skill is not raw arithmetic. It is learning to round and simplify before you calculate. If the market is 4.8 billion and the share is 12 percent, don't try to do that exact math in your head. Round it to 5 billion times 12 percent, that is 600 million, then adjust slightly down. You get close enough in three seconds. Interviewers don't care if you hit the exact number. They want to see you move through the math without freezing.

Focus on three things. Percentages, division, and rough multiplication. That covers 90 percent of what shows up in cases. Things like "revenue went from 320 million to 400 million, what is the growth rate?" or "15 million customers spending 200 dollars each, what is total revenue?" These exact patterns come up over and over.

Set a daily reminder on your phone. One quick problem a day, sixty seconds. Do that for six to eight weeks and you will see a real difference. Consistency beats intensity every time. Ten minutes a day is way more effective than a three hour weekend session once a week.

One thing people don't talk about. In the actual interview, if you need a moment, just think out loud. Say something like, "So we have 500 million in revenue across five segments, that is about 100 million each." Talking through your math buys you time and shows the interviewer how you think, which is exactly what they want to see.

Don't waste money on fancy courses. The basics done consistently will get you there.

That is the classic challenge—you need the fluency but don't have the hours for rote practice. The key insight here is that case math is less about pure computational speed and more about structured thinking and shortcut identification.

The most efficient practice is the 30-Minute Targeted Drill, executed 3-4 times a week. Don't use scratch paper or a calculator, and structure your practice around the types of operations most common in cases: dealing with millions and billions (zero manipulation), complex division (e.g., $24M / $350K), and multi-step percentage changes. Crucially, as you calculate, you must narrate your steps out loud. This simulates the pressure environment where you have to hold a calculation, communicate your logic, and listen to the interviewer simultaneously. If you can fluently say, "I'm going to drop three zeros from both sides to simplify the ratio..." while performing the calculation, you are building the right skill.

Also, be strategic about when you apply precision. In many difficult case scenarios, the interviewer doesn't need the exact number; they need a quick, justifiable approximation. If you hit a tough division like $18.3M / 31%, verbally pause and propose a simplification: "To move quickly, I will approximate $18.3M to $18M and 31% to $30%. We can refine later if needed." This is an MBB-level maneuver—it shows you recognize the trade-off between speed and precision, and you are taking control of the pace.

Hope it helps you get laser-focused.

Zach, 

Have a slot in your calendar of 20-30 min every day. 

Ideally place it at a time of day when you can focus well and your mind can do proper work. 

For me, for instance, that's the morning, before everything else start happening. 

Then practice then in a focused manner specifically on mental math materials. 

If you need further guidance on this, do reach out and I'm happy to help. 

Also sharing here a guide I've built with the most common formulas that show up in interviews:

• • Cheatsheet: The Must-Know Consulting Terms for Interviews

Best,

Cristian

Hi there,

The most efficient way to practice mental math while working full-time is to build speed through short, daily drills, not long study sessions.

A simple approach that works well:

• 5–10 minutes a day, ideally at the same time (morning or commute)
• Focus on high-frequency case math: percentages, growth rates, breakevens, ratios, and quick multiplication/division with round numbers
• Practice approximations, not exact calculations — interviewers care about direction and logic

What to drill:

• Fraction ↔ percentage conversions (e.g. 1/8 ≈ 12.5%, 2/3 ≈ 67%)
• Percentage change (e.g. 80 → 100 is ~25% increase)
• Splitting big numbers into easy pieces (e.g. 3.6M ÷ 12)
• Back-of-the-envelope market sizing steps

How to practice:

• Do drills out loud to simulate interview pressure
• Time yourself strictly
• Sanity-check every result (“does this feel too big or small?”)

The key is consistency. Ten focused minutes a day for a few weeks usually leads to noticeable improvement.

If helpful, I share a short mental-math cheat sheet with students that’s designed specifically for case interviews.

Best,

Evelina

Hi there,

You should set aside 15-20 min a day to do math drills. You can ask ChatGPT to generate these for you.