If you wait past your full retirement age to claim Social Security and maximize your monthly payout, you’ll be presented (either in the office or online) with a tempting option when you finally do claim: Would you like a retroactive lump sum payment?

It sounds like a signing bonus. Depending on your benefit amount, the government could write you a check for $10,000, $15,000 or even $20,000 on the spot. For many retirees, seeing a five-figure deposit land in their bank account feels like a victory.

But this money isn’t a bonus. It is a trade-off — and one that often works in the government’s favor, not yours.

Taking that lump sum permanently reduces your monthly check for the rest of your life. If you live a long, healthy life, that immediate cash could cost you tens of thousands of dollars in lost income.

Here’s the math behind the offer so you can decide if the upfront cash is worth the long-term cut.

How the lump sum actually works

The Social Security Administration (SSA) allows you to claim benefits retroactively, but there are strict limits. You can only request retroactive benefits if you have already reached your full retirement age (FRA).

The maximum retroactive period is six months.

When you choose this option, the SSA essentially backdates your claim. If you apply for benefits today at age 70 but ask for the six-month payout, the SSA treats your application as if you filed it at age 69.5.

They’ll pay you those six months of missed checks in one lump sum. However, because your official claim date is now six months earlier, your future monthly payments are recalculated based on that younger age.

(Navigating the agency’s rules can be tricky, especially with recent shifts in service. See “Social Security Is Changing How It Handles Your Case — Why Experts Are Worried.”

The cost: losing your delayed retirement credits

The reason your check shrinks is the loss of delayed retirement credits.

Once you pass your full retirement age (typically between 66 and 67), your benefit grows by 8% for every year you wait to claim, up to age 70. That breaks down to roughly 0.67% per month.

When you accept a six-month retroactive lump sum, you forfeit the delayed retirement credits you earned during those six months.

• The calculation: 6 months x 0.67% growth = 4% permanent reduction.

By taking the cash, you agree to slash your monthly benefit by 4% for the rest of your life. This might not sound like much, but over a 20- or 30-year retirement, the difference adds up fast.

Running the numbers

Let’s look at a realistic scenario to see how the math plays out.

Imagine you’re applying for Social Security at age 70. After waiting this long, your monthly benefit has grown to $3,000.

You decide to take the six-month retroactive lump sum.

• The cut: Your official claim date resets to age 69.5. Because the reduction is based on your primary insurance amount (not your boosted age-70 check), your benefit drops to about $2,903.
• The cash: You receive a check for roughly $17,420 ($2,903 x 6 months).
• The new check: Your monthly payment drops from $3,000 to $2,903.

You now have $17,420 in the bank, but your monthly income is $97 lower forever.

The break-even point

Is the trade worth it? That depends on how long you live.

To find the answer, you divide the lump sum by the monthly loss:

• $17,420 (lump sum) ÷ $97 (monthly loss) = 179.5 months.

That adds up to roughly 15 years.

If you live past age 85, the government comes out ahead. Every month you live beyond that point, you lose money compared to what you would have received if you had just ignored the lump sum and taken the higher monthly check.

If you live to 90, that “free” lump sum will have cost you nearly $6,000 in lost lifetime income.

Making the wrong call on claiming strategies is a common pitfall. See “12 Things That Can Ding Your Social Security Payments.”

When the math says yes

Despite the long-term cost, there are specific situations where grabbing the cash makes perfect sense.

• Poor health: If you have a serious illness and do not expect to live past your early 80s, the break-even math flips in your favor. You’re better off enjoying the cash now than waiting for a higher monthly payout you may not be around to collect.
• High-interest debt: If you are drowning in credit card debt at 20% interest or higher, using the lump sum to wipe out that balance provides an immediate, guaranteed return that outperforms the 8% growth of Social Security.
• Critical immediate need: If you face a foreclosure or a major unexpected expense that cannot be covered any other way, liquidity trumps longevity.

Don’t forget the widow penalty

If you’re married and the higher earner, this decision involves two lives, not one.

When you pass away, your surviving spouse generally steps up to your benefit amount if it is higher than their own. This is known as the survivor benefit.

If you take the lump sum and permanently reduce your check by 4%, you’re also permanently reducing the survivor benefit your spouse will receive after you’re gone. If your spouse is younger or healthier than you, maximizing that monthly check is often the best way to protect their financial future.

Consider your own longevity insurance

Social Security is one of the few income sources that is guaranteed for life and adjusted annually for inflation. This makes it an excellent hedge against the risk of living “too long” and depleting your savings.

A $15,000 or $20,000 check is exciting today, but an extra $100 or $200 hitting your bank account every single month for 25 years offers security that’s hard to buy. Unless you have a specific, urgent use for the cash, the higher monthly paycheck usually provides the better return on investment.