How the Social Security COLA Is Calculated: Three Months, One Index, and the Rounding Nobody Mentions

Every fall, one percentage gets announced and tens of millions of people hear it as "my raise." It isn't set by a committee, voted on, or negotiated. It falls out of a formula written into the Social Security Act, and the formula is short enough that you can run it on a phone calculator with numbers the government publishes for free.

That's worth doing once, because the formula explains three things the headline number doesn't: why some years pay nothing, why the raise you see is usually smaller than the raise you got, and why people who haven't claimed yet are quietly collecting these increases anyway.

Step 1: pick the right index

The law doesn't use the inflation number you hear on the news. That headline figure is usually the CPI-U, which the Bureau of Labor Statistics says covers "over 90 percent" of the population. The COLA uses the CPI-W, which covers roughly 30 percent — households whose income comes mainly from clerical or hourly wage jobs. The BLS confirms the connection in its own words: the COLA is "the percent increase between the third quarter average of the CPI-W for a given year and the previous peak third-quarter average of the CPI-W" (BLS CPI questions and answers).

Notice who that index describes: working households. Retirees spend a bigger share of their budget on housing and medical care than the typical wage earner does. The BLS publishes an experimental index for Americans 62 and older, the R-CPI-E, but warns its results "should be interpreted with caution," and the law doesn't use it. So the COLA measures the right thing for a slightly different person. That's not a scandal; it's just worth knowing when your own costs rise faster than the announced number.

Step 2: average July, August and September

Twelve months of prices get published every year. The formula ignores nine of them. It takes the CPI-W for July, August and September, adds them, and divides by three.

Here is the computation for the COLA paid in 2026, using the monthly figures on the Social Security Administration's latest COLA page:

Step 3: compare against the right base

The base isn't simply "last year." SSA says it plainly: the law compares against the third quarter of the last year in which a COLA became effective. The BLS phrasing — "the previous peak" — is the more revealing one.

Here's why that matters. If prices fall, or rise too little to round above zero, there is no COLA — and the old base stays in place. The next year's prices then have to climb past that old high before anything is paid. That's how benefits went a full year without an increase in 2016: the adjustment effective December 2015 was 0.0 percent, per SSA's history of automatic COLAs. Benefits never go down because of this formula, but a price drop has to be climbed back out of before they go up.

Step 4: round, then apply to the benefit

The percentage is rounded to the nearest tenth of a percent; if the rounded result is zero, no COLA. That one decimal place is doing real work. 2.765 becomes 2.8, but 2.749 would have become 2.7 — and a tenth of a point on a $2,000 benefit is $2 a month for the rest of someone's life, compounding into every future raise.

Then the new percentage is applied to the PIA, and two more roundings follow, both downward. SSA's own benefit calculation examples show the PIA "truncated to the next lower dime" after the COLAs and the monthly benefit "rounded down to nearest dollar." Tiny amounts, always in the same direction.

Step 5: subtract what's deducted before the money arrives

For 2026, CMS set the standard Part B premium at $202.90 a month, up $17.90 from $185.00 in 2025, according to its 2026 premium fact sheet. Higher-income beneficiaries pay more on top of that. So a person's check goes up by the COLA and then comes partway back down by the premium change.

Worked example: three people, one 2.8% COLA

All three pay the standard Part B premium out of their benefit. Numbers are simplified, but the mechanics and the 2026 figures are real.

Ana's and Luis's raises are the same percentage and wildly different in effect, because the premium increase is a flat dollar amount and eats a bigger bite of a smaller check. Carmen is the interesting one. Her COLA in dollars, about $15, is smaller than the $17.90 premium increase. Without a rule, her deposit would have fallen.

That rule is the hold-harmless provision. SSA's explanation of how hold harmless protects benefits says it covers people who are entitled to benefits for November and December and have Part B premiums for December and January deducted from those benefits. For them, the premium increase is capped at whatever the COLA added, so the net payment can't shrink. It does not protect people enrolling in Part B for the first time, people paying the higher income-related premium, or people whose premium is paid by a state Medicaid program.

Do the division and there's a threshold hiding in 2026's numbers: 2.8 percent of about $640 is $17.90. Anyone protected by hold harmless whose benefit was below roughly $640 saw a flat deposit in January 2026 — a raise that existed on paper and was entirely absorbed by Medicare.

The part almost nobody knows: COLAs start at 62, claimed or not

You might assume the COLA only matters once you're collecting. It starts earlier than that. SSA's worked examples follow a worker first eligible at 62 in 2021 who doesn't retire until 2026: their PIA is increased by the COLAs for 2021 through 2025 — 5.9, 8.7, 3.2, 2.5 and 2.8 percent — before they ever receive a check. In that example a $3,317.47 base PIA becomes $4,152.40.

Checking the newest COLA yourself

Three dates follow the announcement every year. COLA notices appear in the Message Center of my Social Security accounts in late November, with mailed notices too. For people who have Medicare premiums deducted, SSA says the exact new amount arrives in December, once the Part B premium is known. And the first higher payment is the one paid in January.

What the COLA doesn't move

The same fall announcement carries other numbers, and they're easy to lump together. They don't come from the same place. The maximum amount of earnings subject to Social Security tax — $184,500 in 2026, per SSA's 2026 COLA fact sheet — changes each year with the national average wage index, not the CPI-W, according to SSA's page on the contribution and benefit base. So if you're still working, the ceiling on the 6.2 percent that comes out of your paycheck's Social Security line tracks wages, while your future benefit's yearly raise tracks prices.

Taxes are the other thing the COLA doesn't touch directly. A raise in your benefit can change how much of it counts as taxable income, which works on the same marginal logic as every other federal tax bracket: the new dollars are taxed at your top rate, never the old ones retroactively.

The short version

One index built around working households. Three summer months. A comparison against the last peak, not simply last year. A round to one decimal, then two small roundings down. And a Medicare premium that takes its share before the money lands. Every one of those steps is public, which means the fall announcement is less a surprise than the answer to arithmetic anyone can do — and the number worth watching is the one on your own notice, after the deductions.