Case Interview Math – horror stories from friends and contacts (or perhaps your own experience) are probably going through your head just at reading the term. We hear them all the time too. As one of the top reasons for rejection in a consulting interview, case interview math presents a unique barrier to entry.
Why is case interview math even a thing? On a job, consultants are often – and suddenly – asked questions such as “how would a 20% salary cut in the sales division affect my bottom line?” Consulting case interview math exists to filter candidates who can confidently answer such questions – and provide the business insight to boot.
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Computational Errors
Candidates interviewing at McKinsey, BCG and Bain commonly report getting rejected due to computational errors. This is not because the math is complex. The issue is the added layers of complexity present in case interviews:
 Extracting the relevant information from the data
 Working through the math while simultaneously explaining what you are doing
 Quickly computing the answer
 Contextualizing the numerical answer within the case (i.e. providing the business insight)
Doing all the above is a daunting task. Doing them in front of an interviewer who decides if you land one of the most coveted jobs on the planet is even tougher.
Case interview math is a major barrier that candidates come up against when casing. We have seen candidate growth plateau due to case interview math. As such, Management Consulted has developed a process that is proven to work.
Our drillbased method relies on three major focus areas. These have been proven, in both our proprietary MC Mental Math Course and targeted MC Interview Coaching, to improve candidate outcomes. The focus areas are the following:
 Case Interview Mental Math: drilling down on what you need to know (market sizing, verbal math, chart math, and graph math)
 Case Interview Math Tricks: incorporating easy and replicable methods
 Case Interview Math Drills: targeted practice using case interview math questions
Case Interview Mental Math
Fortunately, there is a lot of math you DON’T have to know for the case interview. Consulting case interview math relies almost exclusively on addition, subtraction, multiplication and division. These concepts must be applied quickly and correctly. Generally, it should take you 10 seconds to compute problems such as “$225,000 times 22”. It is important to know that while being quick is impressive, being wrong is unimpressive. Never sacrifice accuracy for speed.
Consulting case interview math is tough because you must simultaneously come up with and explain your approach. Further, you must walk the interviewer through the approach to ensure they are up to speed.
The major concepts that you must know to succeed at case interview mental math include:
 Profit Calculation
 Return on Investment
 Breakeven Period
 Rule of 70
 Profit Margin
Case Interview Math Tricks
This is the step where candidates see the fastest growth. After learning required concepts, it is important to learn how to make the traditional approach quicker. The best thing about case interview mental math is that it can be learnt. Slow candidates can easily become fast by learning tricks and practicing. That’s why the focus first should be on accuracy – speed can come afterward.
Learning the right case interview math tricks will make utilizing our free case interview math drills in the next section all the more effective. The following are the top tricks that will help make you quicker:

Reduce Numerical Complexity
In cases you will be working with numbers in the billions and millions. This makes the math tougher – but it doesn’t have to. We recommend removing extra zeros and adding them in only later. For example: 14,000,000,000/5,000 = 14,000K/5K = 2,800K

Approximate to Round Numbers
One of the best things about case interview math is that it is permissible to approximate numbers. Always ask the interviewer before rounding numbers. An example of rounding is the following: 23% of 490,000 = 25% of 500,000 = 125,000 (real answer = 112,700)

The Halve and Double Method
When multiplying two numbers that are of similar magnitude, it can be easier to halve the smaller one and double the larger one. For example: 220 X 450 = 110 X 900 = 99,000

Learning the Rule of 70
One of the most complex parts of case interview math is growth rates. While tested only in the hardest of math cases, it is impressive to be comfortable with this concept. The rule of 70 estimates the doubling period for an investment growing at X%. If an investment grows at 10%, it takes 70/10 = 7 years to double.
You can find more tricks in our article on fast math and through our interview coaching service. And don’t forget to practice with our free math drills!
Case Interview Math Drills
Case interview math drills are the most important part of building your computational ability. With more practice, math tricks become second nature. Case interview math drills allow you to progress to a point where you can seamlessly solve problems.
We recommend using our case interview math tool, which contains several free math drills. The case interview math tool exposes you to case style math problems so that you become familiar with question types.
The aim with drilling should be to do the math quickly enough that you can simultaneously walk someone through your process. Further, you should be so comfortable that you are able to contextualize the answer right after computing it.
Conclusion
None of these “steps” to succeed at case interview math are successful in isolation. Doing just one part will get you marginally ahead. Going through all three parts and following through with rigorous practice will unlock your casing potential. The best candidates are both quick, correct and able to provide effective insights. The key to this is practice and rigor, as well as a seasoned casing partner. With Management Consulted Interview Coaching and our Case Interview Math Tool, you can fully utilize these insights and many more.
Additional Reading: