How to calculate IRR in head?

How should you approach calculating IRR in your head using figures?

In an interview I was asked something along the lines of: if you acquire a company for $100m using 30% equity and 70% debt and exit in 5 years for $200m assuming no debt repaid / the capital structure remains the same (30%/70%) what would be your IRR?

Then they asked follow up questions such as what if the exit was $300m? Or what if it was 7 years? Or if you paid down debt over the years and your exit structure was 50% equity and 50% debt etc..

It wasn’t word for word like the above - I can’t remember the exact info given / details of the questions but it was something along the lines of this.

I was struggling a lot in calculating it mentally but would appreciated how the correct way to approach the mental math would be!

Thanks

 
Most Helpful

Figure out the MoM then you can get IRR. For a 5 year investment, you can roughly "move the decimal place over and subtract 5%".

For example: 2.5x = ~20% IRR, 3.0x = ~25%

EDIT: I'm seeing a lot of incorrect math below so let me caveat the above by saying this "trick" really only works for MoM returns of 2.0x - 4.0x over a 5 year period, which is where most PE returns lie so it's useful for quick IRR approximations.

Remember that the relationship between IRR and MoM is expontential (IRR = MoM^(1/t) - 1), so you can't just subtract 5% from any MoM (e.g. 7.5x MoM is roughly 50% IRR).

 

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