Q: How will I know whether I have enough money to retire?
A: Some people obviously have more money than they will ever spend, but most retirees are not in that situation and, therefore, need a plan. Some people use spreadsheets, which can give a false sense of security because they assume a consistent return on investments each year. There are too many variables, such as one-time inflows/outflows, inflation, and the variability of investment returns. The best projections are created using financial planning software that can run hundreds or thousands of scenarios, using randomized investment returns. This type of planning will give you a range of outcomes, the median (most likely) outcome, and the probability of success (i.e., not running out of money).
For more details on this topic, read: Retirement Planning: Frequently asked Questions.