This is a common issue and it's 100% normal to feel this way. Let me give you my 2 cents ( i did get an offer!).
From my experience, interviews are actually not tricky - they are designed to test you, but the cases are by no means the hardest cases you have probably ever done + they are super predictable...15 mins PEI, a 30 min case, followed by Q time.
It is generally fine to make a mistake in math - i know i did! I said 2 x w was 24 and said 50 goes into 1000, 200 times instead of 20. Interviewers understand you are nervous, and that mistakes happen...nobody is perfect. Of course, try not to make these mistakes but by no means is a math mistake going to stop you from getting an offer.
My advice for building confidence is to take your time and align with the math before going ahead. Take 1 minute to build your approach, explain it to the interviewer and if they say 'this looks good' you can forget about logic and just focus on doing the actual numbers (which BTW, are generally whole, nice numbers in MCK interviews).
Other confidence tips:
- Listen to your favourite movie before going in, do 10 mins of mental math to boost your confidence right before you walk into the building
- Get to the office early, sit there and relax as much as possible. The longer you are in your surroundings the more comfortable you will get.
- THink positive put out positive vibes. Talk to the receptionist, stand up while you wait to display confidence and then be proactive when you talk to the interviewer. Ask how their day is, break the ice....once you realise they are just a person you will feel far more at ease.
- Be confident in your training and prep - You are right, you can't ever be 100% prepared, but if you have done cases and committed yourself, you are probably at your best. This will either be good enough or it won't be...
- Reassure yourself, that they have given this interview for a reason - they see something they NEED. McKinsey also really do want the best from you, they will help you feel at ease and try to allow you to succeed.
- breathing and meditation on the morning - pump yourself up to succeed. Watch 'the rock", WIll Smith or CT Fletcher on youtube for example.
- visualise yourself leading up to the interview doing the math correctly and bonding with the interviewer.
- Stick your shoulders back, make space in the room so that physically you are force to be listened too. Honestly, body language is half the trick here. If your body looks confident, your mind will follow. Dress sharp...look good too as this all helps
- If you make a mistake, pre plan how you will deal with this. EG "sorry, im just a little nervous"...dont say this unless you make more than 1 mistake. Forget it and move on ASAP
- Honestly, just enjoy the experience. You are interviewing with McKinsey. Thousands of people would love this chance...yet they picked you! This is a great opportunity and if you can enjoy the chance to do a case with a McKinsey consultant, you'll do just fine. Hint- Think back to all of those average partners on preplounge and then think about what you're about to do. This is what we train/ed for!!