Most investors have specific goals like putting kids through college, buying a second home or living comfortably in retirement. Getting there typically takes serious saving over a long period, a good return on your investments, and also accounts for inflation and taxes.

But whether you’ll hit your target by socking away $1,000 a month for 40 years can seem like a crapshoot. That’s why more financial advisors have turned in recent years to a planning device known as a Monte Carlo simulation, to project the odds of meeting a financial goal.

The name comes from the gambling mecca, where odds are everything.

Dustin Javier, a financial planner with Dean Johnson Advisory in Bartlett, Illinois, says he uses these simulators to help clients get a better grip on issues they often underestimate on their own, like damage from inflation and bear markets.

“A Monte Carlo simulator will run thousands of simulations to give you a probability of success rate in being able to fund retirement,” Javier says.

[See: 9 Stocks to Buy for the Aging Baby Boomer Market.]

Investors have several options for figuring how a given investment strategy will work out if all goes as planned. At the simplest level, you can use a spreadsheet, hand calculator or even pencil and paper to see how a sum would grow if you added a set percentage every year to reflect investment return.

The internet is teeming with calculators that go further by accounting for inflation and allowing you to increase the amount saved each year as your income grows.

But most these methods share a basic flaw: they assume inputs like investment return and inflation will be the same every year. We all know that’s not true in real life.

“I think it’s great if an investor takes the time to use a calculator, spreadsheet or piece of paper to plan for their future,” says Derek Hagen, founder of Fireside Financial in Minneapolis. “This gets them well ahead of the game. Most people don’t take this step. The downside to be aware of is that we know these calculations will be wrong. Using a given return for each and every year assumes that the investor gets the same return consistently. Said another way, that would be a world without risk.”

A Monte Carlo simulation addresses this by making thousands of calculations, each with a different pattern of returns and inflation over the years. Often actual patterns from the past are used. Then the calculator looks at how many of the runs produced the outcome desired, such as having a $100,000 annual income for life after retirement begins.

If 90 percent of the runs meet the goal, you can be fairly confident about your strategy. If only 50 percent do, you’d be wise to tighten your belt to invest more, change your asset allocation or plan to retire a few years later to let your nest egg grow larger and reduce the years to fund.

[See: 10 Skills the Best Investors Have.]

So if you consult a financial advisor, ask if he or she uses a Monte Carlo simulator. (Hagen favors MoneyGuidePro, meant for professionals.) Some fund companies and brokerages offer a free simulator on their website, and you can find others with a web search. But not all simulators are the same, so here are a few features a good one will have, and some warnings about taking their results too literally:

Basics. “The most important features are the probability of success, and the ability to change all the inputs,” Hagen says, noting that some simulators give the user more control over assumptions about variables like risk levels and correlations — or how one type of asset moves in relation to others.

“Any good retirement calculator should include, at a minimum, expected return, inflation, desired income, Social Security, retirement age, tax rate and the ability to model any material cash flows that are expected at some future date,” says Matthew S. Eads, portfolio manager at Eads & Heald Investment Counsel in Atlanta.

Details. “Where one program is better or worse than the other comes down to the output,” Hagen says. “Some will keep it very basic and just give you a probability of success. Some let you see each trial (run), some calculate a safety cushion (on average, how much did you have left), and so on. Having the ability to dive into the output can add value if you know what you are looking for. If you are a casual user, this might be too much detail.”

One-time events. Many investors at some point experience good or bad events like a home downsizing, inheritance or stretch out of work, and a good simulator can account for the expected ones rather than just assuming finances plod along on a set trajectory.

“If there are one-time events that are likely to happen, then it’s very important to account for those,” Hagen says. “Inheritances, business sales, settlements, and moving to a smaller home are all part of one’s financial future, and being able to account for your whole financial picture is what makes financial planning so powerful.”

A good program will also allow adjustments for factors like changes in retirement savings rates over the years and significant but temporary expenses like college costs, Eads says.

Limitations. Hagen says that no projection of future results is perfect. Not only can the Monte Carlo simulator create a false sense of security, there’s no guarantee that any of its runs will match real life. Even if one does you won’t know which it will be when you run the program.

The user, of course, can build in a cushion by saving more, keeping open the option of retiring later and creating a fallback plan for a less expensive lifestyle.

Paul Ruedi, CEO of Ruedi Wealth Management in Champaign, Illinois, is a fan of Monte Carlo simulators but cautions that they also do not account for how the user might make those adjustments. A small increase in annual savings, for instance, might dramatically improve the odds of success by nudging some “failed” runs over the line into the “succeed” category.

[See: 8 Things That Matter More Than Money for a Happy Retirement.]

On the other hand, a change like spending a little more than planned each year could significantly increase your odds of running out of money late in life, he adds. So using a Monte Carlo simulator is not license to go on autopilot. You still need to manage your financial life as conditions change.

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How Computer Simulations Help Save Your Nest Egg originally appeared on usnews.com