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How long will you live? How long will your money last? These questions lie at the heart of retirement planning, and yet most people have unrealistic views of both. One result is that they enter retirement grossly undersaved.

Many people believe they won’t live past 80, says Chip Castille, chief retirement strategist at investment manager BlackRock. But with a 65-year-old couple, one is nearly a lock to make it to 85, and there is an 18% chance one will make it to 95.

Likewise, people tend to overestimate how far their money will go. The average boomer aged 55-65 plans to have $45,500 in annual retirement income, one survey found. Yet the average nest egg of this group is just $132,000. By BlackRock’s estimate, that would generate only $9,129 of guaranteed lifetime annual income.

To address this shortfall, BlackRock is introducing a new online tool called iRetire, which quickly translates savings into the amount of lifetime income that it will produce and, critically, illustrates how simple adjustments to a few key variables can change the equation. With iRetire, BlackRock is moving planners away from a target savings oriented plan to one that targets desired retirement income instead.

“This is how people actually think,” says Castille. “Through your working life you know what you earn and what you need.” Why should it be any different in retirement? Setting an income target, and understanding what it takes to reach that target, is far more digestible than setting a savings goal and then figuring how much you can draw down each year.

The iRetire tool is for financial planners and employs sophisticated analytics to show the interplay between these variables:

  • How long you work;
  • How much you save;
  • How much income you want;
  • How aggressively you invest.

The basic concepts work for everyone, and other firms including Fidelity, T. Rowe Price, and Vanguard have free online tools that can help you start thinking about how, as these variables change, so does your projected income in retirement. You may find the trade-offs illuminating.

Take a 55-year-old with $250,000 in savings and who wants $75,000 a year in income at age 65. With an ultra-conservative investment portfolio, $2,000 a month from Social Security, and another $5,000 a year in savings, this person is on track for only $48,000 of annual retirement income, according to iRetire.

But change those inputs to retire at 67, boost savings to $11,000 a year, and invest in a more aggressive but appropriate portfolio that is 50% stocks and you get to a number that would produce the desired $75,000 of annual income for life. You could get there lots of other ways as well—like retiring at 65 as planned but boosting annual savings to $27,000, or by boosting savings to $13,000 but also having private pension income of $1,000 a month.

As financial planners increasingly focus on income in retirement maybe you should do the same. Targeting a savings level has too many flaws, including the tendency to underestimate longevity and overestimate how far savings will go.

Read next: 3 Strategies for Making Your Money Last Through Retirement