Once you retire, you'll generally be eligible for some sort of monthly benefit from Social Security. But it's important to have savings beyond those benefits, since they'll probably only replace about 40% of your preretirement income if you earn an average wage. And that's without the potential for benefit cuts.

As such, it's important to do what you can to save for retirement on your own. And if you were to ask BlackRock CEO Larry Fink, he'd tell you that it's imperative that you allocate money for your retirement on a regular basis.

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In fact, one thing it pays to do is set up automatic monthly contributions to a retirement plan, whether it's an IRA you manage yourself or a 401(k) you get access to through your employer. But how much money should you be setting aside on a monthly basis?

The answer truly depends on you. But here's how to arrive at a reasonable estimate.

It's all about what you want retirement to look like

There's no single retirement savings target that guarantees you'll have enough money to cover your bills. It may be that one person retires with $600,000 and does just fine throughout their senior years, while another retires with $2.5 million and runs out of funds in their late 70s.

That's why a good bet is to first figure out what you want your retirement to look like and estimate the cost of that lifestyle. You may not be able to land on an exact number, but you can do your best.

Next, create an account on the Social Security Administration's website to get an estimate of your monthly benefit. This is crucial to calculating how much savings you'll need.

From there, decide what withdrawal rate you'll be comfortable applying to your retirement savings. You may want to go with a rate of 4%, which is what financial experts have long recommended.

So let's say you think your ideal retirement will require $60,000 a year in income, or $5,000 a month. Let's also say that Social Security will pay you a benefit of $2,000 a month. That means you'll need $3,000 a month of retirement income to come from your savings, or $36,000 a year.

If you'll be withdrawing from your savings at a rate of 4% per year, you can multiply $36,000 by 25 to arrive at $900,000. That's your savings target.

But how do you get there?

So we determined that you might need a nest egg worth $900,000 to attain your desired retirement lifestyle. But how much income should you be setting aside each month?

Once again, the answer isn't so simple. It will depend on how many years you have to save for retirement and how your nest egg is invested.

But let's say you're 35 years old, and your goal is to retire at age 65. Let's also assume you'll invest your savings heavily in stocks. From there, it's reasonable to say you'll enjoy an average annual 8% return in your portfolio, as that's actually a bit below the stock market's average.

From there, you can use a tool like this one to determine that it will take about $663 a month in savings to reach your $900,000 goal. That's just under $8,000 a year.

Of course, the nice thing about these types of tools is that you can plug in your own numbers based on your age and savings target. If you want a $1.2 million nest egg but you also have 40 years between now and retirement, your monthly savings requirement will look different.

All told, the sooner you start to figure out how much income you'll need as a retiree, the sooner you can begin saving consistently. And don't worry if your numbers wiggle a bit over time. The key is to start out with a basic idea of how much you need to save monthly so you can get into the habit of allocating those funds to your retirement account. And if you can put that process on autopilot, even better.

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Maurie Backman has positions in BlackRock. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.