Your money, properly invested, should grow, even if you invest conservatively.<p>So you don&#x27;t need to draw down your principal to pay your living costs.<p>Let&#x27;s say you spend $100K per year, and you can earn 10 percent per year, then you need $1 million.<p>But you don&#x27;t even need that much, because there&#x27;s no reason to die with $1 million in the bank.<p>So it&#x27;s probably some kind of calculus problem, an integral or a derivative, or something.<p>And maybe you can do better than 10 percent! ;-)