A quick Google web search will generate a huge list of online retirement calculators. Look for one that doesn’t require you to register or surrender any personal contact information to use the site. Stick with calculators that are housed within independent websites not related to a financial services company. But if you decide to use the calculator on your broker’s site, check the results against a couple of other calculators to see if the results differ. If the site has an incentive to encourage you to save, it may have assumptions within the calculators that are unreasonably conservative.

Whichever calculator you ultimately decide to rely on, it should only produce final figures that illustrate how much you need to save or how much you need to accumulate to retire; it shouldn’t generate recommendations to buy a specific financial product or invest in a particular investment scheme. Should you find yourself facing a financial sales pitch at the conclusion of the calculator process, close the site and start the process again with another calculator that appears to be more independent.

To begin your online calculation, make a quick dry-run with rough numbers to get an idea of how the calculator works and what information you need. Next, gather any information you were missing during your test run, and repeat the calculation with more accurate numbers. Print the result so you can compare it to other calculators.

Most calculators only allow fixed-rate assumptions for portfolio return and inflation to make the calculator easier to use. This means your calculations are based on the assumption that inflation and investment returns will be the same each year. Because real-life portfolio returns and inflation are never the same every year, the trade-off for calculator simplicity is that your projection is more of a ballpark estimate than bull’s-eye accurate.

The more sophisticated calculators apply Monte Carlo analysis to their results to estimate the likelihood the calculated result will come true. A calculator with Monte Carlo programming runs a formula similar to the ones the basic calculators use and then projects a Monte Carlo estimate of the probability of that outcome. To arrive at this probability, the Monte Carlo simulation runs through the retirement calculation thousands of times, each time relying on a different rate for portfolio return and inflation. The result of the Monte Carlo simulation will be the number of those repeated calculations that were successful—meaning the probability that you had enough money to retire or the probability that saving as much as you’ve calculated will meet your goal, depending on the analysis you were running and the information you put into the calculator.

One popular online Monte Carlo calculator is at www.FlexibleRetirementPlanner.com. This complex calculator lets you distinguish between taxable and tax-deferred retirement assets, as well as tax-free assets such as Roth IRA accounts. You can also change variables such as income tax rate, and you can assume changes in spending during retirement. This calculator generates a measure of the probability that your scenario will result in a financially comfortable retirement based on the retirement assets you will have, the time you’ll probably live, and the economic conditions you’ll encounter. The level of detail in the variables you’ll submit to this calculator gives you a more accurate projection if you put in the right information. Unfortunately, with more detailed input comes a higher likelihood you could make a mistake that will dramatically alter your results. Compare this result with a non-Monte Carlo result to see if you’re in the ballpark.

A web search for retirement calculators will uncover many sites that use the basic calculators developed by KJE Computer Solutions, LLC (www.DinkyTown.net) or CalcXML LLC (www.CalcXML.com). KJE Computer’s calculators incorporate an online technology tool that makes it easy to see how changing a variable, such as portfolio return or inflation, will affect your result. These calculators are fun to use and a good way to learn more about how small adjustments can change a plan. CalcXML’s programs take the input in one table and then present the results with a bar chart and a table. The worksheet stays accessible at the top of the page so you can make changes and rerun it. Try running a version of both of these types of calculators and compare the results. Because of differences in the way programs are written, you’re likely to get different results from each, even though you’re entering the same information. Averaging the results of both programs will give you a rough idea of where your retirement plan stands.