The Social Security Administration will happily forecast your future monthly retirement check. Trouble is, it’s often off the mark. Understanding the sometimes-flawed assumptions underlying the estimate can help you make smarter decisions about when to claim your benefit.

First, of course, you should know how to access those estimates. You can find yours online by creating a “My Social Security” account at the Social Security Administration’s site. You also can call 800 772-1213 to request a paper version. (The agency automatically sends paper copies to people 60 and over if they haven’t yet started benefits or created an online account.)

Social Security projects how much you’ll receive if you start benefits at the earliest age, 62, as well as what you’ll get if you start instead at your full retirement age – currently 66 and rising to 67 for people born in 1960 or later – or at 70, when benefits max out.

How benefits are estimated

When you apply for benefits, Social Security uses your 35 highest-earning years to calculate your check. Each of these years is “indexed,” or adjusted to reflect wage and price inflation over time. The dollar amount you earned in 1995, for instance, would be roughly doubled to reflect what the same wage would be worth today.

When estimating your future benefit, however, the agency assumes no future growth in wages or prices, says economist Laurence Kotlikoff, creator of the Maximize My Social Security claiming-strategies site. That often creates “lowball” estimates for younger workers, he says.

“If you are, say, 40, this can produce a 20% underestimate of the actual benefit you’ll receive,” Kotlikoff says.

On the other hand, the agency could be overestimating your benefit if your income has peaked, since the assumption is that you will continue earning roughly the same amount until you apply for Social Security.

Many people in midlife lose their jobs and never make as much again. Illness or disability could knock you out of the workforce prematurely, or you could stop working years before claiming Social Security. Any of those circumstances could result in smaller-than-projected checks.

“You can see why Americans are confused and surprised when they go into the Social Security office with an old statement and learn their benefits will be lower than they thought,” says William Meyer, founder of Social Security Solutions, another claiming-strategies site.

Other factors

Other circumstances can upend the estimates. Some people will qualify for spousal or survivor benefits that are larger than what they earn on their own record. Retirees with minor children can get child benefits that boost their checks.

Nastier surprises may await people who worked for certain government agencies or were employed abroad. If they get pensions from jobs that didn’t pay into Social Security, the “windfall elimination provision” could reduce their Social Security checks significantly.

Lawmakers intended the provision, and the related “government pension offset,” to keep people who didn’t pay much into Social Security from getting more than those who did. But the reductions aren’t always well publicized or explained, and can come as a shock to affected people who were counting on the amounts Social Security promised.

What about shortfall?

Speaking of promises, Social Security’s trustees say the system will have enough revenue to pay only 77% of promised benefits starting in 2035, unless Congress intervenes.

Lawmakers are unlikely to allow benefits to be cut for people in or near retirement. If you’re decades away, though, Social Security’s lowball estimate could turn out to be on target. To be safe, you might want to assume you’ll get even less.

If you’re within 10 years of retirement, on the other hand, getting a more accurate estimate of your benefits can help you plan when to retire. You can start with your My Social Security account, which includes a link to a retirement calculator that allows you to adjust your average future earnings.

The site also has a page of free calculators, including a downloadable detailed calculator that the site accurately describes as “somewhat unwieldy” and “difficult to use.”

You can pay for a more user-friendly option at Maximize My Social Security ($40) or Social Security Solutions ($49.95).

Or consider a session with a fee-only financial planner who has access to similar robust software. This adviser can help you fine-tune your Social Security estimates, advise you on claiming strategies and make sure your retirement isn’t based on false promises.

Liz Weston is a columnist at NerdWallet, a certified financial planner and author of “Your Credit Score.” Email: [email protected]. Twitter: @lizweston.