Here's how much you have to save every month to retire with $1 million

The definitive guide to retirement savings plans

Half of Americans think $1 million is enough to retire on, according to personal finance site NerdWallet.

Depending on when you stop working and what you want your lifestyle to look like in your golden years, you may need more or less than that. But if you're aiming for $1 million, NerdWallet has broken down exactly how much you have to save per month to reach that goal by age 67.

The chart assumes a 6 percent average annual investment return. The green bars represent how much you need to save and the blue bars represent your investment return.

Hover over the bars to see the exact numbers.

Here's the full breakdown of how much you would have to save per month to have $1 million by age 67 if you start at age:

20: $319 per month
25: $440 per month
30: $613 per month
$864 per month
$1,240 per month
$1,831 per month
$2,831 per month

As the chart shows, the amount you have to save depends a lot on how early you start. If you begin setting aside money at age 20, you have to save $319 per month to reach $1 million. If you start at age 30, though, that number nearly doubles: You need to save $613 per month.

"Someone who starts early can save less overall and rely on compound interest to pick up the slack," Arielle O'Shea, NerdWallet's investing and retirement specialist, tells CNBC Make It. Compound interest makes a sum grow at a faster rate than simple interest, because in addition to earning returns on the money you invest, you also earn returns on those returns over time.

If you haven't already started investing, the simplest way to start is to contribute to your employer's 401(k) plan, a tax-advantaged retirement savings account, or other retirement savings accounts, such as a Roth IRA or traditional IRA.

You can also research low-cost index funds, which Warren Buffett recommends, and online investment platforms known as robo-advisers.

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