Millions of retired seniors depend on Social Security to pay their expenses, so much so that it's their single greatest source of income. As such, getting less money out of the program than anticipated is something that could really hurt you in retirement. Here are a few reasons your benefits might not be as high as you'd hoped they would be.

1. You don't know your full retirement age

Your Social Security benefits are based on your 35 highest years of earnings on record, but the age at which you initially file for those benefits could affect your ultimate payment each month. If you file at full retirement age, you'll get the exact monthly benefit your earnings record entitles you to. Here's what full retirement age looks like, depending on the year you were born:

Year of Birth

Full Retirement Age

1943-1954

66

1955

66 and 2 months

1956

66 and 4 months

1957

66 and 6 months

1958

66 and 8 months

1959

66 and 10 months

1960

67

DATA SOURCE: SOCIAL SECURITY ADMINISTRATION.

That said, you're allowed to file for Social Security before or after full retirement age, and if you opt for the former, your benefits will be reduced by a certain percentage for each month you file early. The youngest age you're eligible to collect benefits is 62, so if you file then but have a full retirement age of 67, you'll cut your benefits by 30%.

Closeup of older man

IMAGE SOURCE: GETTY IMAGES.

In a Fidelity survey last year, 74% of Americans admitted to not knowing their full retirement age. If you're one of them, commit that age to memory so you don't inadvertently file for benefits ahead of schedule and reduce them in the process.

2. You didn't work a full 35 years

Your Social Security benefits will be based on your 35 highest years of earnings. But if you didn't work a full 35 years, you'll have a $0 factored into your personal benefits equation for each year you're missing an income, which could result in a lower benefit payment each month.

The solution? Work past full retirement age and hold off on filing for Social Security during that time. This will achieve two purposes. First, you'll get to replace some zero-earning years with an actual income. Secondly, for each year you delay benefits past full retirement age up until age 70, you'll grow your benefits by 8%. This means if your full retirement age is 67 and you wait until 70 to file, you'll get a 24% increase in benefits right off the bat.

3. You're still earning income from another job

The Social Security Administration allows seniors to work and collect benefits at the same time. But if you're doing so and haven't yet reached full retirement age, you'll risk having a portion of your benefits withheld if your income exceeds a certain level, which changes from year to year. For the current year, your first $17,640 is exempt from what's known as the Social Security earnings test, but once you start making more than that, you'll have $1 in benefits withheld for each $2 you earn elsewhere. If you're reaching full retirement age this year, that threshold increases to $46,920. After that, you'll have $1 in Social Security withheld for every $3 you earn.

Keep in mind that the amount you have withheld from your benefits in these situations won't be lost forever (whereas when you file for benefits early, you do indeed risk reducing them for life). Rather, that money will be added back into your benefits once you reach full retirement age. Initially, though, your Social Security payments might be lower than you'd like them to be.

If you're expecting to enter retirement with a limited amount of personal savings, then you'll no doubt come to rely heavily on Social Security to cover your expenses. And if that's the case, you'll want to know how much income to expect from it. Rather than guess at that figure, create an account on the Social Security Administration's website and see what your actual benefits will look depending on when you claim them. From there, you'll know whether you should hold off on filing to boost those payments, or whether you're OK to start collecting them immediately.

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