Picture this: You're taking a long vacation. You have some money saved up, but you're not really sure how you'll pass the time or how much you'll spend, or even how long you'll be there. You're just hoping you have enough money to last. Sounds ridiculous, right? Yet people do this with their retirement all the time.

There's a simple way to fix that. Get out a pen and paper and set aside 15 minutes or so to answer the following questions.

Smiling woman holding tablet and writing in notebook

Image source: Getty Images.

1. What do you want your retirement to look like?

This is the fun part of retirement planning, but it's also the necessary starting point. If you don't know how you want to spend your retirement, you won't be able to accurately estimate how much you need to save.

Obviously, you'll need money for the basics, like food, healthcare, and taxes, but depending on your lifestyle, you may also want to budget some money for travel or hobbies or helping family members. List all the things you think you may want to do in retirement and prioritize them so you can figure out which items to cut if necessary to save money.

2. How long do you think your retirement will last?

There are two things you have to figure out here: when you'd like to retire and how long you believe you'll live. Your answers aren't set in stone, so don't worry about getting things exactly right. For now, just choose the age you'd like to retire at. You can always change it later if you realize your initial plan isn't feasible.

For your life expectancy, plan to live into your early to mid-90s, unless you have a medical condition you believe will shorten your life. That could mean a 30-year retirement or longer. You want to overestimate rather than underestimate here, just in case. If you die before you use all your savings, you can leave your extra money to your heirs. But if you run out before you die, life is going to be a lot more challenging.

3. How much do you expect to spend annually in retirement?

Armed with your answers from the two questions above, you should be able to get a rough idea of how much you'll spend in retirement. Do some research to estimate the cost of your travel or hobby plans. For your everyday living expenses, use your current budget as a baseline, but remember that this could change between now and retirement if, for example, you have children living at home right now but don't expect this to be the case forever.

With a rough estimate of your annual expenses, multiply this by the number of years you expect your retirement to last, adding 3% annually for inflation. If that sounds too complicated, you can use a retirement calculator to do this math for you. It should tell you the total estimated cost of your retirement based on the information you entered.

4. How much money do you expect from Social Security and other sources?

You probably won't have to save for the full cost of retirement on your own because you'll get some money from Social Security (no, it's not going to disappear) and possibly a pension or a 401(k) match.

Your company can provide details about the value of any pensions or matches you qualify for, and you can estimate your Social Security benefit by creating a my Social Security account

Subtract the money you expect from these other sources from your retirement expenses to figure out how much you must save on your own. Your retirement calculator may enable you to input some of this information. It should then tell you how much you must save per month to reach your goal.

5. Do you think you need to make any changes to your plan?

Now that you know roughly how much you'll spend and how much you need to save to get there, you can make adjustments. If you're not able to save as much as you need each month, take steps to reduce the cost of your retirement. This could mean working longer, working part time in retirement, or cutting some of your discretionary expenses now and in retirement.

Redo the above calculations with your changes until you find a plan that works for you right now. You can update it if your finances or your expectations for retirement change. In fact, you should look your plan over at least once per year and anytime you experience a major lifestyle change so you can make the necessary small adjustments to keep yourself on track for your goals.