As a child, my dad always made a point to try to teach me lessons about money, law and practical life skills. He also taught me duct tape can fix just about everything. One lesson that stuck with me, because is constantly repeated it, was the "Rule of 72". I'm not exactly a "rule of thumb" person but the Rule of 72 is knowledge he passed on that I use almost daily. When it comes to saving for retirement, the power of compounding interest should never be underestimated. And as a responsible investor, it can be helpful to know how long it would take to double your investment at a fixed rate of return. The Rule of 72 can be used as a quick rule of thumb to help determine this answer.