For around a quarter of married couples and almost half of unmarried persons, Social Security provides more than 90% of income during retirement, according to the Social Security Administration. Social Security benefits are a primary source of income because Americans don't invest enough for retirement and because few employers in the private sector offer guaranteed pension income in today's world. 

Since Social Security may be a key source of support during your golden years, it's important to understand how the program works. In particular, you should understand how your benefits are calculated -- and why it's a good idea to work for at least 35 years before you retire.

Calculator, cup of coffee, and binder with Retirement Savings Plan written on it.

Image source: Getty Images.

Why working for 35 years is important 

Social Security benefits are earned benefits, which means you don't get them until you pay into the system. You need to earn at least 40 work credits to become eligible for Social Security retirement benefits and, as of 2018, you earn one credit for each $1,320 in earnings up to a maximum of four credits annually.

While earning work credits is a prerequisite to qualifying for benefits on your own, working the minimum number of years isn't enough to maximize your benefits. In fact, if you work less than 35 years, your benefits may be much lower than they would be if you worked longer. That's because of the way Social Security benefits are calculated. 

When benefits are calculated, the Social Security Administration calculates averaged indexed monthly earnings during the 35 years when your salary is highest. The Social Security Administration adjusts for wage growth, and applies a formula to determine your basic benefit amount, which is the amount you'd get if you retired at full retirement age

This means if you don't work for a full 35 years, you'll have some years of $0 benefits factored into your calculation. Consider the impact this would have if you earned the equivalent of an inflation adjusted salary of $50,000 throughout a 35-year career versus if you earned the same salary but worked for just 25 years.  If your career ended 10 years early, your average wages would drop down to $35,714 instead of $50,000. In that case, because of the progressive nature of the formula that determines your monthly benefit, what you'd get from Social Security would drop by only around 20%.  

What can you do if you don't work at least 35 years?

While working less than 35 years can make a big impact on the income you receive from Social Security, it's not always feasible to continue working for this long. It's always worth looking into whether your benefits may be higher if you claim on a spouse's work record, but it's especially important if you have to leave the workforce before putting in a full 35 years of work.

If you're at least 62 years old and your spouse is receiving Social Security retirement or disability benefits, you could be eligible to claim your benefits based on your spouse's work history. If you qualify for and apply for benefits on your own work record but are eligible for higher spousal benefits, the Social Security Administration pays a combination of benefits equal to the higher spousal benefit. 

Your spousal benefit could equal between 32.5% to 50% of your spouse's benefits depending upon whether you retire at or before your full retirement age, which is 67 if you were born in 1960 or later, but earlier if you were born sooner. This chart shows the maximum percentage of your spouse's benefits you could be eligible for if you claim on his or her work record, assuming your full retirement age is 67.

Your Age at Retirement % of Your Spouse's Primary Insurance Amount You're Eligible for
62 32.5%
63 35%
64 37.5%
65 41.66%
66 45.83%
67 50%

Source: Social Security Administration

If you're divorced after a marriage of at least 10 years and haven't remarried, you can still qualify for these spousal benefits based on your ex's work record. 

Working longer helps boost your Social Security benefits

If you want to maximize your Social Security benefits, making sure you get 35 years of work in -- or claim on your spouse's record if necessary -- can be essential. If your income is a lot higher at the end of your career than at the start, working even longer than your 35 years could benefit you because the Social Security calculation is based on your highest 35 years of earnings. Dropping that year when you made $2,000 at a summer job in favor of another year of earning $100,000 at the end of your career makes an impact. 

Working longer may also make it possible to delay claiming Social Security benefits, which makes a big difference. If you claim Social Security benefits before full retirement age, your benefits are reduced while if you wait longer than full retirement age, your benefits amount will keep rising until age 70. Find out how to do the math to see whether it makes sense for you to wait to claim benefits.

This increase or decrease based on your age when you claim benefits applies whether you claim on your own work record or a spouse's, so delaying is often the smart move if you want your Social Security benefit to be as high as possible so you'll have more income to live on as a senior.