How much money is needed for a comfortable retirement? It's a question only you can answer.

The amount depends on factors such as your retirement goals, living situation, general health and expected lifespan. None of these are easy to predict, so it's essential to store away a substantial nest egg -- and be financially prepared for almost anything.

An online retirement calculator can help. These web-based tools enable you to put concrete figures around your retirement expectations. Many online calculators are free, and some are more complex than others. But they all operate with the same general purpose: to help you financially plan for a secure future.

Simply plug numbers into specific variables -- such as your current age, annual income and retirement savings -- and you can determine if you are financially on target to meet your retirement goals. If you're off course, you can use the calculator to obtain a better sense of how to fine-tune your future investment strategy.

Here are three steps to finding the best retirement calculator for your financial situation:

Step 1. Spend More Time, Get More Clarity

The more time it takes to fill out the question fields on the retirement calculator, the more thorough, and therefore, more accurate the calculation is likely to be. It can take you anywhere from about three to 30 minutes to fill one out online.

If you want just a quick gauge of where you think you're headed, a simple retirement calculator (you will see some examples later in this article) should be just fine. However, if you want a more complete analysis, it is worth dedicating the time to filling out a longer, more intensive retirement questionnaire online or through a trusted financial advisor.

To get the most out of your retirement calculation session, you should be prepared with all of your financial documents in front of you. You will likely need to fill in information about your pension contributions, your registered savings plans and any insurance or annuity plans you currently pay into.

Step 2. Compare Calculators for the Best Fit

Not all calculators are created equal. A good online retirement calculator is built to factor in changing hypothetical variables, such as inflation and tax rates. Look for calculators that take into account inflation, inflation adjustment, tax rates and savings plan rates. Without calculating these elements, your long-term picture isn't going to be accurate.

Still, it's important to realize the numbers you plug in are estimates and could substantially increase or decrease by the time you actually retire. Therefore, don't put all your faith into a retirement calculator and don't base your entire retirement plan on just one calculation. Try several calculators to see if the figures you get are close.

Step 3. Be Realistic

Some calculators may ask you to plug in numbers such as your expected life-span or the projected rate of return on your investments. These numbers are nearly impossible to determine. But, you can use your own personal history, combined with nationwide trends, to help you better predict the future.

To ensure you have enough money past 80 (the average US life expectancy), it's suggested you play it safe and put age 90 as your default lifespan setting. Similarly, a projected rate of return higher than what's risk-appropriate for your age (i.e., don't expect 8% or 10% returns if you plan on retiring in the next decade) can throw your projections way out of whack, so work with a trusted advisor to get a realistic rate of return to plug into your retirement calculations.

You should also take into account where you plan on living, especially if you ever plan to retire abroad. Where you live in the world will have a substantial impact on your taxes and the government-based pension programs you may pay into. Both will affect your retirement outcome. Find a calculator that's right for you, based on the country in which you live.

Calculators We Like

Based on the factors above, we've scoured the Internet to find what we believe are the best free retirement calculators out there.

U.S. Retirement Calculator: Employee Benefit Research Institute's Ballpark E$timate is a free, simple-yet-effective retirement calculator. It consists of only 16 questions, but it takes into account projections such as expected lifespan and inflation. The results summarize how much you'll need to save annually in order to meet your retirement goals.

Canadian Retirement Calculator: A federal government agency, Service Canada, offers a simple-but-very-thorough free tool to help you calculate your financial needs as you enter retirement. You're able to input your company pension and registered retirement savings plans (RRSPs), plus expected rates of return. The results summarize your expected before-tax annual income.

InvestingAnswers' Free Calculators:

If your goal is to retire a millionaire, try these two financial calculators:

If you simply want to figure out how much you could save over time by investing or saving a portion of your income, try the Simple Savings Calculator.

Keep in mind, online retirement calculators are simply one tool to help in the retirement planning process. No matter how good the calculator -- free or not -- each will have its own strengths and limitations.

Try number crunching with more than one calculator before committing to specific retirement savings plan. And, always realize that your plans may change; these changes could impact your life savings. While a retirement calculator can't do the planning for you, it can help you more accurately assess your finances so you can see gold in your golden years. We invite you to try all of the InvestingAnswers financial calculators.

Here are Three More Ideas to Help Reach Your Financial Independence:

  1. Free yourself from credit card debt this year. Learn how to pay 0% on your balances for up to 21 months in The 4 Best Credit Cards for Balance Transfers.
  2. Pay off your mortgage to free up an extra $1,000 to $2,000 every month. Check out 3 Ways to Pay Off Your Mortgage 15 Years Early.
  3. Shrink and eliminate your car payments.If you're paying 6% APR or more, it's time to know The Top 3 Reasons to Refinance Your Auto Loan.