Do you need a target number for retirement?

How much savings will you need to retire comfortably?

"I have zero idea of the answer to that question," says Mary Wall, a 29-year-old doctoral candidate at Harvard University's Graduate School of Education.

Much of the retirement advice and many of the planning tools out there focus on figuring out a specific target, sometimes called a retirement number, even if you're several decades away from leaving the workforce. For younger workers such as Wall, determining a number might not be realistic. Too much can change between now and when they reach retirement age.

"I have no idea what my lifestyle will be when I'm in my 60s or 70s," Wall says. Instead of obsessing about a specific number, she contributed enough to her retirement account to get the employer match while she was working full time, although she has since hit pause to save up for a down payment on a house.

Uncertainties after you retire, such as health care expenses and how long you're likely to live, also make it difficult to figure out how much you'll need. Even online calculators designed to help workers predict their retirement expenses can't take all the mystery out of retirement planning.

Start saving early, even if it's not much

Many younger workers think they won't have enough money to retire. A recent study by Bank of America and Merrill Lynch shows that two-thirds of Millennials expect to have a financially difficult retirement, based on their current saving habits. Even money-savvy younger workers lack confidence in their ability to predict how much they'll ultimately need.

"I don't have a good sense of how my expenses are going to change," says Allison Quantz, 27, a radio producer who lives in Vermont. "I tend to be kind of obsessed with this stuff, and I still feel like I have no clue what I'm doing."

Many financial advisors caution against becoming so intimidated by an inability to predict future expenses that you don't save anything at all. Dana Twight, a Seattle-based financial planner, says the most important thing is to begin saving at a young age to take advantage of a long time horizon that will allow your money to grow.

"People get frozen because they don't understand what a bargain it is to save now," she says. Twight recommends thinking about things you can control, like the amount you save, rather than all the variables that make it hard to predict your target number.

Starting small is better than not starting at all. Twight is currently providing financial advice to U.S. service members on an Air Force base in Germany, and she sees a lot of younger people who think they don't earn enough to save for retirement. She recently spoke to someone who makes $2,000 a month.

"You can afford 1%," she told him, suggesting that he put away just $20 a month at first and build from there.

Use benchmarks to keep track

Saving enough to capture an employer match as part of a 401(k) or similar plan, if that's available, is a good first step. If you can manage to save more than that, John Sweeney, vice president of retirement and investment strategies for Fidelity Investments, recommends saving 15% of your income consistently in a tax-deferred account. If you do that, he says, "you're going to be in pretty good shape by the time you get to retirement."

Younger workers are more likely to change jobs or even careers, and less likely to retire with a pension, than previous generations. Even if you have many job changes and a fluctuating income during your career, saving a consistent percentage of your income will provide some measure of security, Sweeney says.

Sweeney recommends using benchmarks to make sure you're on track right now. Younger workers should aim to save an amount equivalent to one year's salary by the time they turn 35, with that number rising to three times the annual figure by age 45, and then increasing from there.

But is it still useful to try to calculate a retirement number, even if it's subject to change? That depends on your temperament, Twight says. If you're a planner, you might feel better having a road map. On the other hand, "if you know you don't think very well long-term, then I would say don't look at the number, because $2 million is going to make you nervous," Twight advises.

For now, younger workers would do well to keep it simple:

• Start saving, even if you have to start small.

• Build up until you're saving enough to capture an employer match.

• Increase your contributions until you're saving 15% of your income.

Quantz is optimistic that she'll be OK in the long run. She's currently making about $50,000 a year and is saving 30% of it between her retirement accounts and other savings goals.

Not knowing how much she's going to need for retirement actually spurs Quantz to save more aggressively. That way, she says, "I can operate under the frantic assumption that I have to save everything that I possibly can."

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