Perhaps the most useful financial formula is the “Rule of 72”. It is a simple way to “estimate” how long it takes to double your money without using the complex financial formula.
However before you even want to know about this rule of 72, the fist thing you want to know is the compound interest. This is due to the fact that, this rule only applicable for your investment that based on the compound interest (which means you reinvest your interest return into your principal).
72 / Interest Rate (p.a.) = Years to Double Your $Example 1: Investment
How much I can double my investment return of $10k with 8% interest?
72 / 8% = 9 years.
If you’re now 25 years old, you will have $20k at the age of 34 years old.
Example 2: Inflation
I spend $2k per month. How long that my spending will inflate until $4k with my personal inflation rate of 2%?
72 / 2% = 36 years.
If you’re now 25 years old and when you’re 61 years old later, you need to have at least $4k for your monthly spending.
Example 3: Goal Setting
I’m now 30 years old and I have $100k cash now to invest. I want to have $200k in 2 years time. What is investment return that I should get to reach my goal?
72 / 2 years = 36%
So if you want to earn the extra $100k in 2 years time, your investment return is expected to have at least 36% returns.