How Long Will Retirement Savings Last?

It might be the most consequential issue baby boomers will deal with when they retire: did I save enough?

Vanguard’s free online calculator will estimate that for you, using the same sophisticated technique financial advisers charge hundreds of dollars to provide.

The user-friendly calculator uses 100,000 of what are called Monte Carlo simulations of potential future returns to the financial markets to arrive at the probability that a household’s invested savings will last through the end of retirement. To get to this number, older workers enter their information into the calculator – 401(k) account balance, asset allocation, estimated years in retirement, and annual withdrawals – by moving around a sliding scale for each input.

The financial industry recommends aiming for a probability in the 80 percent range – 95 percent is overdoing it. In the end, however, your comfort level is a personal decision.

An important purpose of the calculator is to demonstrate how changes in the inputs can hurt one’s long-term retirement prospects – or improve them. One obvious example is increasing the annual withdrawal amount, which lowers the probability the money will last. To increase your chances, try a later retirement date.

The calculator is a lot of fun, but it has some limitations.

First, it’s no substitute for a detailed pre-retirement financial review. The other issues are primarily mathematical, and they boil down to the difficulty of predicting the future.

The calculator assumes, for simplicity, that a retiree withdraws the same dollar amount from savings every year to supplement Social Security and any pension income. But Anthony Webb, an economist at the New School for Social Research in New York, said this ignores the most important thing retirees should do to preserve their money: adjust the withdrawals every year, depending on how their investments have performed.

“If you encounter icebergs (bear markets), you should cut your spending” and withdrawals, he said.

Another inherent shortcoming are the assumptions about how risky stocks and bonds are. For example, Webb said these calculators assume bonds are a risky investment. This overstates the probability that the household will outlive their assets, he said, because bonds held to maturity are, in fact, a reasonably safe place for long-term investors to park their money.

In contrast, the Vanguard calculator may understate stock risk – and understates the probability of running out of money, Webb said. Economists believe it will be very difficult for stock returns to repeat the extraordinary performance of the past century.

Despite the caveats, Vanguard’s calculator is very useful – just don’t take it as gospel.

Squared Away writer Kim Blanton invites you to follow us on Twitter @SquaredAwayBC. To stay current on our blog, please join our free email list. You’ll receive just one email each week – with links to the two new posts for that week – when you sign up here. This blog is supported by the Center for Retirement Research at Boston College.