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What it is
"The Rule of 72" is the answer to the question, "What rule tells us how many years it takes to realize we would have had twice as many small, green pieces of paper if we had invested them at a given rate of return instead of stuffing them in our mattress?"More precisely
Suppose you buy a certificate of deposit that is supposed to accumulate 6% of interest compounded annually. To calculate how many years it will take for the value of your certificate to double, divide 6 into 72.
Why?
Don't ask. For more about The Rule of 72 see Investing For Beginners11But they won't explain why the number you have to divide the rate of return into is 72. Some questions are intrinsically unanswerable.