Dear Liz: I am confused about “safe withdrawal rates” from retirement accounts. I’ve read that withdrawing 4% of savings each year is the gold standard that financial planners utilize to ensure that life savings are preserved in retirement.

However, if the Standard & Poor’s 500 index returns on average 8% a year, and if the life savings are locked down in a mutual fund that is indexed to the S&P 500, then shouldn’t the annual withdrawal amount, to preserve those savings, be 8%? Limiting my withdrawals to 4% means my retirement would be pushed several years down the road. Can you clarify?

Answer: It’s good you asked this question before you retired, rather than afterward when it might have been too late.

You’re right that on average, the S&P 500 has returned at least 8% annualized returns in every rolling 30-year period since 1926. (“Rolling” means each 30-year period starting in 1926, then 1927, then 1928, and so on.)

But the market doesn’t return 8% each and every year. Some years are up a lot more. And some are down — way down. In 2008, for example, the S&P 500 lost about 37% of its value in a single year.

Such big downturns are especially risky for retirees, because retirees are drawing money from a shrinking pool of assets. The money they withdraw doesn’t have the chance to benefit from the inevitable rebound when stock prices recover. Bad markets, particularly at the beginning of someone’s retirement, can dramatically increase the odds of running out of money.

Inflation also can vary, as can returns on cash and bonds. All these factors play a role in how long a pot of money can be expected to last. The “4% rule” resulted from research by financial planner William Bengen, who in the 1990s examined historical returns from 1926 to 1976. Bengen found there was no period when an initial 4% withdrawal, adjusted each year afterward for inflation, would have exhausted a diversified investment portfolio of stocks and bonds in less than 33 years.

Some subsequent research has suggested a 3% initial withdrawal rate might be better, especially for early retirees or those with more conservative, bond-heavy portfolios.

Free online calculators can give you some idea of whether you’re on track to retire. A good one to check out is T. Rowe Price’s retirement income calculator. But you’d be smart to run your findings past a fee-only financial planner as well. The decisions you make in the years around retirement are often irreversible, and what you don’t know can hurt you.