A comfortable retirement requires careful planning. You must know how much money you need to last the rest of your life and where that money will come from. Otherwise, you could struggle to pay your bills in your final years.

If you're nearing retirement or already there, you should be able to answer the following questions easily. If you can't, set aside some time to reevaluate your retirement plan. Your financial security depends on it.

1. How much money do you need to cover your expenses in retirement?

Conventional wisdom says most retirees need 70% to 80% of their pre-retirement income in order to cover their living expenses. But it's different for everyone. If you travel often, you'll probably spend more money than someone who spends their retirement at home, and you may need more than 80% of your pre-retirement income to cover your living expenses.

Senior man with hand on chin thinking

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You can approximate how much money you'll need by adding up the total cost of your expenses in retirement -- mortgage payment (if applicable), utility bills, groceries, etc. Keep in mind that some of your current expenses will disappear. You'll no longer have to save for retirement when you're in retirement, so don't include the monthly amount that you're setting aside for retirement right now. Just focus on living expenses.

Inflation will increase the cost of living over time, and you must account for this. You can't know how quickly inflation will rise, but a good estimate is 3% per year. If your living expenses amount to $40,000 this year, they could be $41,200 this yea,r and $42,436 the next year, and so on. A retirement calculator can help you calculate this.

2. How long will your retirement savings last?

Based on your calculated living expenses, you should have some idea of how many years your current retirement savings will last. Compare this number against your estimated life expectancy. The average 65-year-old man today can expect to live until about 84 while the average 65-year-old woman can expect to live to almost 87. Of course, your life expectancy may be different, depending on your lifestyle and family health history.

If your retirement savings are not enough to see you to your estimated life expectancy, you'll have to make some changes. Consider cutting your expenses in retirement or picking up a part-time job to reduce your withdrawals from your retirement accounts. If you're still working full-time, consider increasing your retirement contributions.

3. How do you maximize your Social Security benefits?

You can begin Social Security at 62, but if you plan to live a reasonably long life, this may not be wise. In order to get your full benefit per check, you must wait until your full retirement age (FRA). This is either 66 or 67, depending on when you were born.

If you start claiming at 62, you'll only get 70% of your scheduled benefit per check if your FRA is 67 or 75% if your FRA is 66. For every month you delay benefits beyond your 62nd birthday, your benefit checks will increase by 2/3 of 1%. You'll hit 100% of your full benefit amount at your FRA and the maximum benefit at age 70. This is 124% for adults with a full retirement age of 67 and 132% for those with a full retirement age of 66.

It's your choice when to start claiming. Delaying benefits will net you more money per check, but you may not be able to wait until your FRA or age 70 if you need Social Security to help pay your bills. A common strategy for couples is for the lower earner to begin benefits at 62 so the couple has money to sustain them while the higher earner delays benefits until he or she qualifies for a larger check. Alternatively, you could both delay benefits as long as possible if you don't need Social Security to get by. Think through all of your options and talk with your spouse, if you have one, to come up with a plan for when to begin Social Security.

In addition to choosing the right age to start taking benefits, you can increase your benefits by increasing your income, if you still have a job. Benefits are based on your average monthly income during the 35 highest-earning years of your life, so anything you do to bring up this average will increase your Social Security checks. You can estimate your current Social Security benefit by creating a my Social Security account.

Arming yourself with this information will help you feel more comfortable with your retirement plan. If the answers to these questions trouble you, make some adjustments. It's better to do this now than when you've run out of savings.