The idea is to be as precise as possible when helping you estimate the size of your nest egg, but so many are getting lost in the numbers.

Despite the increasing complexity and sophistication of these free retirement calculators, they can all produce different results, varying from dizzyingly confusing to overly simplified. One may make adjustments for disappearing expenses (college tuition or mortgage payments) while another won't.

One site might allow for variances in state income tax rates while another will account for inflation. One site could ask you to list every asset you have, while another just wants the basics.

And if you don't understand the results - or get the right ones - it could put your retirement at risk.

Remember, underestimating how much you should save each month will leave you broke in your retirement. Meanwhile, overestimating can cause major money problems now as you struggle to make mortgage payments or pay your children's college tuition.

Despite all the risks, you can get quite a lot out of simple retirement calculator programs. To learn the basics, read on.

Use More Than One

Relying on a single retirement calculator is retirement suicide. It's called getting a second opinion. So, try out several of these free online tools and compare the results. Pay attention to the questions each asks you and get a feel for how they arrive at their conclusions.

Combine the Results

By looking carefully at the questions each calculator asks you and how it arrives at certain conclusions, you can start to understand it better and even merge and combine the different results. For example, one site might tell you to start saving 60 percent of your income while another will advise you to start saving 30 percent. That's a big difference, but by understanding the results and how they were achieved, you can more likely find the balance.

You Want Features, Just Not a Laundry List

Customization is great as it allows you to fine tune a retirement calculator to your own personal needs and financial situation, but it can also make the process more complicated. Try a calculator like the one offered by the Employee Benefits Research Institute (choosetosave.org).

It allows you to input extra income sources and potential expenses like travel or lifestyle choices that could affect your savings needs. At the same time, it's simple and easy to follow.

Consult With a Professional

Yes, a financial advisor will charge you fees and commissions, but if you're having doubts they can be your best bet. Most people wouldn't leave the fate of a mysterious illness in the hands of WebMD (as helpful as it is), so why would you leave the fate of your entire retirement in the hands of a simple retirement calculator? Take that extra step to build a confident retirement program that will more likely meet your needs to the end of your life.