According to the Employee Benefit Research Institute, approximately 46 percent of Americans have tried to calculate how much they'll need for retirement. More than half of all workers and more than a quarter of retirees who participated in the survey said they have attempted to use online tools to determine their retirement needs. Others who have tried to run the numbers admit that they simply guessed. (Source: Retirement Confidence Survey, 2013).

Don't make the same mistake. To make sure you enjoy your retirement without financial worries, you should ensure you have enough money saved. That calculation can be a daunting task. A variety of factors affect your answer and inaccurate estimates for any factor can leave you with way too little in savings. Some of the more significant factors include:

What percentage of your pre-retirement income will you need? You can find various rules of thumb indicating you need anywhere from 60 percent to more than 100 percent of your pre-retirement income. On the surface, it seems like you should need less than that higher amount; After all, you won't have any work-related expenses, such as clothing, lunch, or commuting costs.

But look carefully at your current expenses and how you plan to spend your retirement before deciding how much you'll need. If you pay off your mortgage, stay in good health, live in a city with a low cost of living, and engage in inexpensive hobbies, then you might get by with less than 100 percent of your income. However, if you travel extensively, pay for health insurance, and maintain significant debt levels, even 100 percent of your income may not be enough. You need to take a close look at your expenses and planned retirement activities to come up with a reasonable estimate.

When will you retire? When you retire determines how long you have to save and how long investment returns can compound. Most people would like to retire before age 65, but that typically requires significant personal savings. You want to be sure your retirement savings and other income sources, such as Social Security and pension benefits, will support you for what could be a very lengthy retirement. Even reducing your retirement age by a couple of years can significantly affect the ultimate amount you need.

How long will you live? Most people look at average life expectancies when estimating this. But an average life expectancy means you have a 50 percent chance of living beyond that age and a 50 percent chance of dying before that age. Since you can't be sure which will apply to you, it's typically better to assume you'll live at least a few years past that age. When deciding how many years to add, consider your health as well as how long other family members have lived.

What long-term rate of return do you expect to earn on investments? A few years ago, many retirement plans were calculated using fairly high rates of return. At a minimum, make sure your expectations are based on average returns over a very long period. You might even want to be more conservative, assuming a rate of return lower than long-term averages suggest. Even a small difference in your estimated and actual rate of return can make a big difference in your ultimate savings.

For example, assume you estimate a long-term rate of return of nine percent. If you save $5,000 per year in a tax-deferred account for 25 years at nine percent, you'll have $461,620 before paying any income taxes that may be due. However, if your actual return is eight percent, you'll have $394,772, a difference of $66,848. (This example is provided for illustrative purposes only and is not intended to project the performance of a specific investment vehicle.)

Have you considered inflation? Even modest levels of inflation can significantly impact the purchasing power of your money over long time periods. For instance, after 30 years of just two percent inflation, your portfolio's purchasing power will decline by 45 percent. When estimating an inflation figure, don't just look at the historically low inflation rates from the recent past. Also consider long-term inflation rates, since your retirement could last for decades. For instance, inflation, as measured by the consumer price index, averaged 2.5 percent over a five-year period but averaged closer to five percent over a 30-year period.

What tax rate do you expect to pay during retirement? Especially if you save significant amounts in tax-deferred investments that will be taxable when withdrawn, your tax rate can significantly affect the amount you'll have available for spending. You may find your tax rate is the same or higher after retirement.

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