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You won't be able to calculate how much money you'll need for retirement without knowing these 6 answers

By Sarah Schmalbruch March 3, 2015

Saving for retirement can be hard when you don't have a specific dollar amount in mind.

Nicole Lapin, author of "Rich Bitch: A Simple 12 Step Plan for Getting Your Financial Life Together ... Finally," suggests using one of the many retirement calculators available online to estimate the amount you'll need, such as those provided by AARP or FINRA — but first, you need to know a few things in order to get a remotely accurate result.

Before calculating your retirement number, make sure you know:

1. How you want to live in retirement

Lapin points out that "conventional wisdom has long held that you need less money in retirement than you do in your working life." However, the author suggests assuming that you'll spend at least as much money during retirement as you spend while you're still working.

Just how much you save really depends on how large you're planning on living during retirement. If you're okay with being frugal, Lapin advises saving enough to provide nothing less than 60% of your current salary in your retired life. If you want a comfortable lifestyle, shoot for 80%, and if you want what Lapin calls "Betty White status," aim for 100%.

2. How much you expect to get back from your retirement investment

Lapin refers to this question as a "delicate little dance." Assuming too low a return means saving more now, which "lowers your standard of living in your nonretirement years," Lapin writes. She continues, "If however, you assume too high a return, the calculator might say you don't need to save as much now, but you might end up eating cat food when you're 90."

For a number that's neither too crazy nor too conservative, Lapin suggests expecting no more than 7-8%.

3. How much inflation will set you back

Lapin emphasizes the importance of accounting for inflation since "what you really care about is the value your money will have in retirement." She says ignoring inflation will most likely cause you to "show up to the retirement party shortchanged." Lapin suggests assuming a 3% rate of inflation when using a retirement calculator.

4. The age you expect to retire

According to Lapin, the average age to retire is 67. But essentially, this is a personal decision. Lapin says you can play around with this age to see what will work best. The most important thing to keep in mind is staying realistic. "A lot of us would like to retire ASAP, and if you win the lotto or strike it big, that can happen," the author writes. "But for now, don't be unrealistic about the age at which you can realistically retire."

5. How long you're expected to live

No one likes to think about when they're going to die, but when you're considering how much to save for retirement, it's necessary. Lapin recommends using nothing younger than 95, and she says that 100 is better, since people continue to live longer as time goes by.

The author also makes the point that men and women differ on this criteria. "Women do, on average, live longer than men. (Our healthcare costs also tend to be higher than those of men, which is another reason we need to get our retirement situation in check,)" Lapin writes.

6. How much — if anything — you'll get from Social Security

According to Lapin, some calculators have you put in a "Social Security assumption." She says if you receive an annual report from the Social Security Administration, you can use the number from that report. But Lapin also cautions against relying too heavily on Social Security.

"... With Social Security always in danger of being eliminated, don't take it as gospel," Lapin advises. "You might want to put in a lower number or skip it altogether in your calculations." Worst-case scenario, you end up with more money than you expected.