Social Security: Combined Income / Provisional Income and Taxability

Social Security is taxed in a peculiar and arcane manner.

It starts with these two synonyms: combined income and provisional income. They mean the same thing. You’ll see them used interchangeably in the world of financial planning. For today, I’ll use combined income.

Combined income determines how much of your Social Security benefit is subject to federal income taxes.

For some retirees, 0% of their Social Security income is subject to income taxes. For other retirees 85% of their Social Security is subject to taxes. It all depends on those retirees’ combined incomes.

“Subject to Taxes” vs. “Taxed At…”

This brings us to our first important clarification of the day: we need to be very careful about the difference between being “subject to taxes” and being “taxed at” a specific tax rate.

A retiree might have 85% of their Social Security subject to taxes, but it’s (only) being taxed at 12%. The remaining 15% of their Social Security is not subject to taxes at all.

Paying a 85% tax rate would feel obscene.

But having 85% of your income subject to tax? Not a big deal.

The Combined Income Formula

Now, getting back to combined income, which helps us determine how much Social Security is subject to taxes. Combined income has a very peculiar formula.

1. Determine your adjusted gross income (AGI), LESS your Social Security income.
2. Add back any tax-exempt interest
3. Add in 50% of your Social Security benefit

That’s combined income.

For many retirees, their AGI will include:

• wages from any part-time retirement job
• withdrawals from tax-deferred IRAs (including Roth conversions)
• pension income
• interest and dividends
• capital gains
• the taxable portion of Social Security income. (Of course, per Step 1 of the combined income formula, we back-out the Social Security income to determine combined income.)

A Combined Income Example

Let’s look at an example. Roger and Rhonda Retiree had the following situation in 2026:

• No wages.
• $40,000 in IRA withdrawals
• $7,000 from a pension
• $5,000 in interest (all taxable, no tax-exempt) and dividends
• $4,000 in capital gains
• $48,000 in combined Social Security income

Step 1 of the combined income formula gets us to: $56,000 (40 + 7 + 5 + 4)

Step 2 of the formula makes no changes (in this case).

And in Step 3, we add an additional $24,000 (48 / 2), bringing the combine income total to $80,000.

Getting from Combined Income to Social Security Taxability

Here comes the fun part. How do we get from combined income to a specific amount of Social Security that’s subject to taxes?

We’ve completed Step 1. Calculate combined income = $80,000.

Step 2 – Which Tier Applies?

There are three tiers to Social Security taxability.

When combined income is under $32,000, then 0% of the Social Security benefits are taxable.

When combined income is between $32,000 and $44,000, then some of the Social Security benefits are taxable at 50%. The taxable amount of Social Security =…

= 50% * (Combined Income – $32,000)

Then the final tier occurs for combined income above $44,000. The taxable amount in this tier =…

= 85% * (Combined Income – $44,000) + the lesser of [$6,000 vs. 50% of SS benefit]

Yikes. That’s…not simple.

But let’s forge on. Our example, Roger and Rhonda Retiree, have $80,000 in combined income. They are in Tier 3.

Their Social Security taxability =

= 85% * ($80,000 – $44,000) + the lesser of [$6,000 vs. 50% of $48,000]

= 85% * ($36,000) + the less of [$6,000 vs. $24,000]

= 85% * ($36,000) + $6,000

= $36,600

We’re almost done.

Step 3 – Check Against the Cap

No matter what, the upper limit of Social Security taxability is 85%. For Roger and Rhonda, and their $48,000 Social Security benefit, the 85% cap is $40,800 (0.85 * 48,000).

Our calculation above – $36,600 – falls below that cap.

Thus, $36,600 of Roger and Rhonda’s Social Security is subject to taxes.

About ~76% of their Social Security is subject to taxes.

Finalizing Roger and Rhonda’s Taxes

If we now calculate Roger and Rhonda’s full AGI, it’s $92,600. [40 + 7 + 5 + 4 + 36.6]

Let’s assume they’re both over age 65 and thus both receive the OBBBA senior deduction – $6,000 for each.

They also get the additional, longer-standing $1,650 deduction for being 65+.

They also claim the standard deduction – $32,200.

For the sake of conservatism we’ll assume that all of their dividends and interest is treated as ordinary income.

After all deductions are said and done, they have $45,100 of taxable income. $4,000 of this is long-term capital gains. The rest is ordinary income.

The long-term gains are taxed at a 0% tax rate. The ordinary income is split roughly 50/50 between the 10% bracket and the 12% bracket.

Their total federal taxes due is $4,436. Their effective tax rate (on $104,000 in total income) is only 4.3%.

So, even though ~76% of their Social Security income is taxable, it’s taxed at a very low rate.

Why This Matters?

This is important for a few different reasons.

First, it’s good to get clarity that not all of your Social Security is subject to taxes.

Second, it’s important to realize what we specified earlier: being subject to taxes is not the same as being taxed at a specific rate.

Third, this discussion sets us up for more nuanced tax planning topics. This is where the rubber meets the road for retirees.

For example: what would happen if Roger and Rhonda made an additional $10,000 withdrawal out of their IRA?

Social Security Tax Planning

If Roger and Rhonda made an additional $10,000 withdrawal out of their IRA, our default thought might be:

They’re in the 12% marginal tax bracket. So, $10,000 in withdrawals probably leads to another $1200 in taxes?

But wait! What about their combined income?!

We need to re-run the combine income formula, but now for a $90,000 combined income. Their Social Security taxability =

= 85% * ($90,000 – $44,000) + the lesser of [$6,000 vs. 50% of $48,000]

= 85% * ($46,000) + $6,000

= $45,100

Now we check that against the 85% cap. For Roger and Rhonda, and their $48,000 Social Security benefit, the 85% cap is $40,800.

The formula yielded an amount over the 85% cap, so we fall back to the cap itself: $40,800 of their Social Security income is taxable.

If we re-calculate their AGI – with the new $10,000 in IRA withdrawals and the new amount of taxable Social Security – the AGI is now $106,800.

After deductions, their taxable income is now $59,300.

And their total Federal tax bill is $6,140. That’s more than $1,700 more than their previous tax bill.

So, on a $10,000 IRA withdrawal – which could have easily been assumed to be only subject to a 12% income tax – Roger and Rhonda actually owe an additional $1,700 in taxes.

So goes the so-called “tax torpedo” in Social Security.

Again – Social Security is taxed in a peculiar and arcane manner. Make sure you understand how it affects your retirement.

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