Deciding when to start receiving Social Security retirement benefits is one of the trickiest financial decisions most people will ever make, with lots of moving parts and considerable uncertainty. It's also one of the biggest.

"In many cases Social Security represents up to half their retirement income," says David Freitag, a financial planning consultant for MassMutual in Springfield, Mass. "For most people from a cash equivalent perspective, this is the largest asset they have in their retirement portfolio. And a bad decision about their largest asset could be fatal to the success of their retirement plan."

The most popular way people handle this decision is to start taking benefits at the earliest possible age, which is 62, according to an analysis of Social Security Administration data by Boston College's Center for Retirement Research. This is despite the fact that many advisors suggest waiting until full retirement age, typically 66, or the maximum of 70, because the amount of the monthly benefits rises significantly the longer you wait to file.

However, experts also say that not everyone should wait until 70. Spousal benefits, personal health and other factors, including some with no clearly assignable value such as longevity expectations, make this decision as unique as a fingerprint. To help out, a number of calculators have been developed to help crunch the data.

Available tools include free online calculators from the Social Security Administration and AARP. A commercial offering, Quicken Social Security Optimizer, requires an annual $50 subscription. Professional financial advisors have their own, such as Social Security Pro from Impact Technologies, which costs $295 for a year's subscription.

One way they differ is in the level of detail, says Martin Cowley, executive vice president of product development for LifeYield, a Boston financial software company that developed Quicken's product. For instance, some ask only for your age instead of your birth date, he notes. That matters because benefit size can vary significantly depending on how many months, not just years, before full retirement age a claimant files.

Crowley also says tools too often recommend people wait until full retirement age or later. "Often what you'll see from any Social Security optimization tool is a suggestion to delay," he says. "But if somebody's life expectancy is short enough, that's not a smart thing to do. A shorter life expectancy might mean you need to take the money earlier."

Other issues, such as the possibility of claiming an ex-spouse's benefits, complicate matters further. Thoroughly assessing these requires using a number of different tools on the Social Security website, taking a short-cut with a free tool like the AARP offering, or using one of the commercial tools.

Whether using the Quicken optimizer or working with an advisor like Freitag, people will provide information by answering a series of questions. These will cover matters as straightforward as their birth date and as ephemeral as how long they might continue working.

At the end, a user won't always get a simple answer, such as, "Wait until age 66," because personal preferences can change things. That's why one of the best ways to use these tools, Cowley says, is to do what-if scenarios to test how different decisions might play out until one of them feels comfortable.

Further muddying the water is the fact that the decision isn't purely personal. Family members also have to be accounted for. The timing of a claim can make an annual difference of several thousand dollars in how much a retired couple will have to spend, notes Bruce Tannahill, MassMutual's director of estate and business planning. "It can lead to some fun decisions with individuals and between spouses," he says.