Have you heard of this rule? It's a smart little device used by big-time investors, devotees of compound interest, to calculate the time it takes for your money to double, using different interest rates or growth rates. Though not 100% accurate for short periods of time or high rates of growth, it is good enough for quick "top of the head" estimates where you can use a calculator or even mental arithmetic rather than having to resort to a spreadsheet formula.

How does it work? The theory is that the % rate of increase multiplied by the years it takes to double in value will always equal 72. So at 10% per year compound growth rate, your investment will double in value every 7.2 years approximately. Conversely, if it doubles in 6 years, then you have achieved about 12% per year compound growth.

Impress your friends; bamboozle your accountant; with a little practice you may even learn to work out darts scores in your head - though that takes serious dedication!

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