Some 28 years ago when I first came to Canada my cousin gave me a book called Common Sense the magic of compound interest.
In it was this.
The Rule of 72
Have you always wanted to be able to do compound interest problems in your head? Well, let's be honest - probably not.
However, it's a very useful skill to have because it gives you a lightning fast benchmark to determine how good (or not so good) a potential mortgage note (or any investment) is likely to be. And it's surprisingly easy to do in your head... once you know how.
The rule says, that to find the number of years required to double your money at a given interest rate, you simply divide the interest rate into 72. That's why it's called the "Rule of 72"!
For example, if you want to know how long it will take to double your money at 8% interest, you would simply divide 8 into 72 and you'll get 9 years. This is assuming the interest is compounded annually.
As you can see, the "rule" is remarkably accurate, as long as the interest rate is less than about 20%. At higher interest rates the error starts to become significant.
Of course, you can also run it backwards. For example if you want to double your money in 6 years, just divide 6 into 72 to find that it will require an interest rate of about 12%.
In it was this.
The Rule of 72
Have you always wanted to be able to do compound interest problems in your head? Well, let's be honest - probably not.
However, it's a very useful skill to have because it gives you a lightning fast benchmark to determine how good (or not so good) a potential mortgage note (or any investment) is likely to be. And it's surprisingly easy to do in your head... once you know how.
The rule says, that to find the number of years required to double your money at a given interest rate, you simply divide the interest rate into 72. That's why it's called the "Rule of 72"!
For example, if you want to know how long it will take to double your money at 8% interest, you would simply divide 8 into 72 and you'll get 9 years. This is assuming the interest is compounded annually.
As you can see, the "rule" is remarkably accurate, as long as the interest rate is less than about 20%. At higher interest rates the error starts to become significant.
Of course, you can also run it backwards. For example if you want to double your money in 6 years, just divide 6 into 72 to find that it will require an interest rate of about 12%.