The Rule of 72 Do you know how long it may take for your investments to double in value? The Rule of 72 is a quick way to figure it out. Share | Related Content Saving for Retirement This calculator can help you estimate how much you may need to save for retirement. Where Will Your Retirement Money Come From? Retirement income may come from a variety of sources. Here's an overview of the six main sources. Managing Your Lifestyle Using smart management to get more of what you want and free up assets to invest.