If you use the 4 percent rule or some other withdrawal rubric, it implies a target, or magic number, for how much you should save for retirement.
For example, if you needed $100,000 in annual income, you could simply divide that sum by 4 percent to get a target. In this case, $2.5 million. (Keep in mind that crude figure does not include what benefits you may receive from Social Security or pensions.)
A $2.5 million target may seem daunting for someone making $100,000 annually.
Fidelity Investments, a large provider of retirement accounts, has an easier metric to follow. It recommends people save at least 10 times their ending salary by age 67, which is the age at which most workers qualify for full retirement benefits from Social Security. Under this rule, someone making $100,000 would only need to save $1 million in their retirement accounts.
"A wealth target is problematic because it doesn't take into consideration current interest rates and inflation," Pfau said.
Even if your magic number changes with interest rates and inflation, everyone has a different definition of retirement and spending.
"It is so customized to the individual. Spending can change dramatically depending on what stage of retirement you are in," said Paul Palazzo, a certified financial planner and managing director at Altfest Personal Wealth Management in New York.
Palazzo characterizes the three stages of retirement as go-go (think of early retirees traveling the world), slow-go (hanging out with the grandkids) and no-go (dealing with long-term care).
A magic number may get you in the right territory for retirement, but what you do with your savings will differ from everyone else.