How do you figure out what you need to save in order to retire?

First, you need to estimate your expenses in retirement, as well as identify your income sources. You also need to predict how long you’ll live and whether you’ll have any costly health issues as you get older. (Hint: We’re afraid that’s likely.)

Just thinking about calculating all this makes me exhausted!

At Ellevest, we do a lot of this initial work for you. We project your salary, taxes and savings right up until retirement and use that amount as the starting point for your desired retirement income. We also assess whether the income sources and savings you’ve told us about will be enough to support your retirement. And we use the most recent life expectancy tables to determine your longevity. In general, women tend to live more than five years longer than most men, and that extra time means greater costs that you need to save more for.

Here are a few guidelines to keep in mind as you work out your specific number:

Aim for 90% replacement of your pre-retirement take-home pay.

This is a great place to start because you already know whether you make enough to cover your living expenses and support your desired lifestyle. You might have lower expenses in retirement since you may not have some costs like mortgage payments or work expenses. However, you might also incur higher health care costs or wish to spend more on travel and other leisure activities.

Multiply your ending salary by 10 to 15 to estimate the sum that will produce that level of income.

Because inflation will chip away at the value of your money, this number may be larger than you might think. Based upon the retirement income you think you’ll need, Ellevest determines the sum you should aim for in future dollars, which are adjusted for the impact of inflation.

Some checkpoints to make sure you’re on the right track to hit that goal: You should aim to have saved the equivalent of your salary by age 35, three times your salary by 45 and five times by 55.

Each year, shoot to stash at least 10% to 15% of your annual income in your retirement accounts.

Ellevest recommends saving even more, at least 20% of your annual income, for retirement and other financial goals. That percentage includes your employer's 401(k) match, if you have one. So if your company contributes 4% of your pay to the account, you only need to save another 6% to 16% yourself.

Of course, guidelines are only meant to give you ballpark figures and lead you to save more. Your specific situation may vary from these guidelines ... and it's bound to change many times as you grow older.

For example, let’s say you’re 35 years old, making $80,000 a year and have $100,000 already saved for retirement. After looking at your total financial situation and goals, we might recommend putting away 12% of your pay to retire by age 67. But if you anticipate having high-cost health issues as you age, we might suggest bumping your savings up to 16% of your salary. Or if you want to retire earlier, say, when you’re 62 years old, we'd suggest something like saving 20% of your pay and planning for a more conservative lifestyle in retirement.

No matter your circumstances, one piece of advice that holds true for everyone: You need to start thinking seriously about saving for retirement as soon as possible—and having actual numbers to aim for can make doing so a whole lot easier.