Followed rigidly, the 4% rule can lead to your running out of money. Image source: Getty Images

Thinking about retirement can be stressful. Most of us need to come to reasonable estimates of how much money we should accumulate before retiring -- and then we have to have plans for withdrawing funds from our nest eggs during our retirement so that we won't run out of money. Many people find the "4% rule" helpful, as it can tell you how much you'll need to retire with and how much to withdraw each year. Unfortunately, it has some drawbacks.

## Meet the 4% rule

The 4% rule was introduced by financial advisor Bill Bengen in 1994 and was made famous in a study by several professors at Trinity University a few years later. It says that you can withdraw 4% of your nest egg in your first year of retirement, adjusting future withdrawals for inflation. This withdrawal strategy assumes a portfolio 60% in stocks and 40% in bonds, and it's designed to make your money last through 30 years of retirement.

Here's an examples of how it works: Imagine that you've saved \$600,000 by the time you retire. In your first year of retirement, you can withdraw 4%, or \$24,000. In year two, you'll need to adjust that rate by inflation. Let's say that inflation over the past year was at its long-term historic rate of 3%. You'll now multiply your \$24,000 withdrawal by 1.03 and you'll get your second year's withdrawal amount: \$24,720. The following year, if inflation is still around 3%, you'll multiply that by 1.03 and get your next withdrawal amount, \$25,462.

The rule can also help you determine how much you'll need to accumulate in the first place once you know how much annual income you'll want in retirement. Let's say, for example, that you'd like total income of \$60,000, and you expect to collect \$25,000 from Social Security. That leaves \$35,000 in income that you'll need to generate on your own. If you assume that \$35,000 is 4% of your nest egg, then you can multiply \$35,000 by 25 in order to arrive at the size the nest egg will need to be: \$875,000. (Why 25? Because one divided by 0.04 is 25.)

So what's the problem with this seemingly perfect rule? Well, a bunch of things.

Image source: Getty Images

Let's start with the fact that the rule was created more than 20 years ago, when interest rates were higher. In such an environment, the bond portion of a portfolio would have been generating more income. We've been in a low-interest rate environment for a long time now, rendering our bonds less able to replenish funds withdrawn each year.

Then there's the rule's assumption that your portfolio will be split 60-40 between stocks and bonds. You might not have or want that allocation. If your portfolio is split 50-50, or you have 75% of it in stocks, then the 4% rule won't work as advertised.

Meanwhile, many people are living longer. Data from a 2015 Centers for Disease Control and Prevention report shows that those born in 2014 can be expected to live, on average, to age 78.8, up from 75.8 in 1995 and 70.8 in 1970. And those are just averages -- many people live much longer (and some much shorter) lives. The 4% rule aims to make your money last for 30 years, but if you retire at 62 and live to 96, you might be quite pinched in your last years.

Finally, remember that every set of 20 or 30 or however many years in the stock market will be at least somewhat different -- some with higher-than-average gains and some with lower-than-average gains. If you plan to follow the rule and withdraw 4% of your nest egg in your first year of retirement, what if the stock market and your portfolio tank by 30% in the year leading up to your retirement? Such things can happen -- the S&P 500 plunged 37% in 2008. If that happens early in your retirement, you'll either be withdrawing far less than you'd planned on or you'll be depleting your nest egg faster.

Image source: Getty Images

## What to do

Fortunately, you don't necessarily have to throw out the 4% rule altogether. You can address the stock market-volatility issue by being flexible. Withdraw less than 4% in bear markets and more than 4% in bull markets.

If you think you stand a decent chance of living more than 30 years in retirement, you can be more conservative, perhaps using a 3% or 3.5% withdrawal rate -- at least in your initial years. That can be helpful during our low-interest rate environment, too. (Many expect rates to inch up in the coming years, though that's far from guaranteed.) You needn't be too rigid about it, though. If the market grows briskly in your first few years, you can reevaluate and perhaps up your withdrawals.

It's a good idea to reassess your financial condition regularly during your retirement, anyway. For example, if when you're 80 you don't think you'll be around in a decade and your coffers are rather full, you could start withdrawing and enjoying more each year -- or just plan to leave more to your loved ones.

Consider buying one or more fixed annuities, too. (Avoid variable or indexed annuities, and focus on ones promising fixed payouts.) With a fixed annuity, you fork over a big chunk of money to an insurer and in exchange, you can receive regular payments -- for the rest of your life. This can be a huge psychological relief as you get older, removing the worry of running out of money and removing the responsibility of having to manage and invest some or much of your money, too. It's also sensible to consult a financial planner about your retirement plans, too -- ideally a fee-only one and not one who benefits from commissions by selling you products.

The 4% rule isn't useless, but if you're going to use it, do so thoughtfully -- and perhaps consider it a rough guide rather than a firm rule.