A growing percentage of millennials have absolutely nothing saved

A 2018 Bank of America survey found that 1 in 6 millennials — which BoA defined as those between age 23 and 37 — now have $100,000 or more in savings. That's impressive, considering few Americans can cover a $1,000 emergency. But that stat doesn't tell the whole story: While some millennials are increasingly proving to be savvy savers, many other young people are not making progress at holding onto their cash.

A 2017 GoBankingRates survey found that most "young millennials" — which GBR defines as those between 18 and 24 years old — had less than $1,000 in their savings accounts. Nearly half had nothing saved at all.

To make matters worse, the share of millennials with $0 in savings is on the rise. In 2016, 31 percent had $0, compared to 46 percent in 2017.

Note that the two reports look at a slightly different samples: BoA defines millennials as those between 23 and 37, while GBR defines the generation as those between 18 and 34. Regardless, GBR looked at the same population in 2016 and 2017 and found a higher proportion of young people had trouble saving anything in 2017.

Here's the percentage of the GBR survey respondents aged 18 to 24 with:

$0 saved:

In 2016: 31 percent
In 2017: 46 percent

Less than $1,000 saved:

In 2016: 41 percent
In 2017: 21 percent

$1,000 to $4,999 saved:

In 2016: 15 percent
In 2017: 15 percent

$5,000 to $9,999 saved:

In 2016: 4 percent
In 2017: 5 percent

$10,000 or more saved:

In 2016: 8 percent
In 2017: 13 percent

Even "older millennials" — defined by GBR as those between 25 and 34 — struggled to set aside money: 61 percent had less than $1,000 in their savings accounts and 41 percent had nothing at all.

Here too they found that the percentage with $0 in savings had increased.

Here's the percentage of the GBR survey respondents aged 25 to 34 with:

$0 saved:

In 2016: 33 percent
In 2017: 41 percent

Less than $1,000 saved:

In 2016: 34 percent
In 2017: 20 percent

$1,000 to $4,999 saved:

In 2016: 13 percent
In 2017: 13 percent

$5,000 to $9,999 saved:

In 2016: 5 percent
In 2017: 6 percent

$10,000 or more saved:

In 2016: 15 percent
In 2017: 20 percent

How much should you have stashed away?

While the amount you need in savings is highly personal, and specific dollar amounts can be arbitrary, money expert at Intuit Kimmie Greene has a simple formula to help you figure out if you're setting aside enough money.

In your 20s: Aim to save 25 percent of your overall gross pay, Greene tells CNBC Make It. "That 25 percent is the combination of 401(k) withholdings, matching funds from your employer and any cash savings that you have," she notes. "It can also include debt repayment.

"Just make sure your lifestyle expenses don't exceed 75 percent of your gross income."

By age 30: Have the equivalent of your annual salary saved, Greene says. If you earn $50,000 a year, aim to have $50,000 in savings when you hit 30.

Again, this includes any retirement-account contributions, matching funds from your company, cash savings or money you have invested elsewhere, like in index funds or with robo-advisers.

By age 35: Have twice your annual salary saved.

By age 40: Have three times your annual salary saved.

By age 45: Have four times your annual salary saved.

By age 50: Have five times your annual salary saved.

By age 55: Have six times your annual salary saved.

By age 60: Have seven times your annual salary saved.

By age 65: Have eight times your annual salary saved.

Greene's timeline is similar to the one recommended by retirement-plan provider Fidelity Investments, which says a good rule of thumb is to have the equivalent of your salary saved by age 30 and to have 10 times your final salary in savings if you want to retire by age 67.

"While this can sound super daunting today, if you're putting that money to work starting in your 20s, it's not as difficult as it sounds," says Greene.

She also notes that "life is anything but linear" and it's impossible to follow this formula to a tee. You may have to adjust accordingly and save more or less in any given year, depending on major life events, such as having a kid or buying a home.

At the end of the day, the sooner you start saving — for retirement or any other major purchases you hope will be in your future — the better off you'll be.

To start spending less and saving more, check out these helpful resources:

• 8 strategies for saving money from a couple that banked $50,000 last year
• After living on $60 a week for a month, here are my 7 best money-saving tips
• 11 useless things to stop wasting your money on in 2017

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