The 4% rule has long been considered one of the simplest ways to estimate how much you can safely withdraw from your retirement savings over time. Under the rule, you withdraw 4% of your portfolio during your first year of retirement and then increase that dollar amount each year to keep up with inflation.

For example, if you retire with $1 million, under the 4% rule, you'd withdraw $40,000 your first year. If inflation rises 1%, you'd withdraw $40,400 the next year. And repeat.

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It's a pretty straightforward concept, which is one reason the rule has remained popular for decades. But the 4% rule has a big flaw you should know about before you decide to use it to manage your retirement nest egg.

When you don't fit the mold

The 4% rule makes key assumptions about you as a retirement saver. It assumes your portfolio has a fairly even mix of stocks and bonds and that you need your nest egg to last for about 30 years.

If those things are true, then the 4% rule might work well for you. If they're not, then the rule could easily fail you.

A conservative portfolio may not generate enough income to support a 4% withdrawal rate. A more aggressive portfolio with guardrails such as a strong cash cushion might support a 5% withdrawal or higher, allowing you to enjoy your retirement even more.

Similarly, if you're retiring at 57, you may need more than 30 years out of your savings, making a 4% withdrawal rate risky. If you're retiring at 75, you may be able to get away with a higher withdrawal rate.

That's why it's important to make sure your situation is suitable for the 4% rule. If not, then you may want to adopt a different approach to managing your IRA or 401(k).

It's best to customize your strategy to you

Even if you are looking at a pretty even stock/bond split and a roughly 30-year retirement, that doesn't make the 4% rule optimal for you. So rather than follow it just because you've heard of it, work with a financial advisor to figure out a withdrawal rate that allows you to preserve your savings without denying yourself the income to enjoy life.

You may come up with a withdrawal rate that's pretty darn close to 4%. But it's best to customize your strategy rather than follow a rule that gets a lot of good press for its built-in simplicity.