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Your retirement may not be as comfortable as you expected if you approach saving for retirement in a haphazard manner. For example, are you simply saving money here and there and hoping for the best? That may not yield great results. It's much more effective to spend some time thinking about how much money you'll actually need in retirement and how you'll amass such a sum. Here are three answers to the "How much should I save?" question.

Paying off your mortgage early can be helpful. Image source: Getty Images.

Still have a mortgage? You'll need more money

Jason Hall: According to the U.S. Census Bureau, the number of homeowners over 65 still paying a mortgage has continued to increase. In 2010, approximately 19 million households with people 65 and older owned a home. Of this total, nearly 6 million, or about 32%, were still paying off a mortgage. By 2014, the most current data available, the percentage of 65-plus homeowners with mortgages was up to 35%. For context, consider that according to a Harvard University study, in 1992, less than 20% of homeowners over 65 had mortgage debt. 

Considering that one of the biggest benefits of homeownership is reducing your housing costs later in life, carrying a mortgage into retirement will mean you'll need more money than you would have otherwise, for mortgage payments. Also, having a mortgage balance further limits your ability to use the equity in your home as a source of income. 

If you have substantial cash savings or the ability to ramp up how much you're paying on your mortgage, it may be a good idea to pay down your mortgage before you retire. This will lower your monthly living expenses in retirement while also giving you more home equity to tap if you need it later. And both of those things mean you'll need less money to retire than you would have otherwise. 

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4%: good rule of thumb

Brian Feroldi: I'm a big fan of keeping things simple, so my advice to future retirees is to follow the "4% retirement rule" that was popularized by William Bengen in the 1990s.

This rule says that a retiree can safely withdraw 4% of their portfolio's value in the first year of retirement and then in subsequent years adjust that initial amount based on inflation. Bergen's research showed that following this formula allowed for at least 30 years of safe withdrawals if the portfolio was composed of 50% stocks and 50% bonds.

In practical terms, this rule suggests that as long as you save at least 25 times your annual retirement spending amount, then you should be in good shape. Thus, if you project that your annual retirement spending will be $50,000, your goal should be to save up $1.25 million before you retire.

There are some shortcomings to using this rule, though. After all, the formula was calculated using historical data, and there's no guarantee that future returns will resemble the past. That's especially true today, given the ridiculously low yield on bonds and frothy stock market valuations. 

Despite all of that, I still think that using the 4% rule is a great starting point. 

Many retirements feature unexpected costs. Image source: Getty Images.

More than you thought

Selena Maranjian: Instead of offering a specific number as to how much money you'll need for retirement, I offer this: You'll probably need more than you think you will. That's because many people underestimate how much money they'll be expected to cough up in retirement. Let us count the ways that happens.

For starters, there's the big-ticket item of healthcare. The folks at Fidelity Investments annually estimate how much a 65-year-old couple will spend out of pocket in retirement on healthcare, and they recently released their latest number: $260,000. Remember, that's just an average. You and your spouse may spend less -- but you may end up spending more, too.

Meanwhile, while you might cheerfully visualize your children helping support you in retirement, the opposite is what often comes to pass. According to a 2015 HSBC survey, about 60% of retirees are providing financial support to at least one other person. Sure, grown kids are supposed to be financially independent, but that's gotten harder and harder in recent years, partly due to student loan debt.

Then there's inflation, which is often overlooked when planning for retirement. Imagine, for example, that you're aiming to amass $750,000 by the time you retire in 25 years, figuring that that sum will be enough to support you. Well, if inflation averages about 3%, its approximate historical average, then that $750,000 will end up having the purchasing power of just $358,000 in today's dollars. Yikes! Be sure to factor inflation into your planning. (Fortunately, Social Security benefits are adjusted for inflation.)