How much of your Social Security is taxable? The IRS Publication 915 worksheet, worked out

Up to 85% of your Social Security benefits can be federally taxable -- but never all of it, and often none of it. Whether any of it is taxable doesn't depend on your benefit alone: it depends on a number called provisional income, which blends your other income with half your benefit. The calculation comes from IRC §86 and is spelled out, step by step, as Worksheet 1 in IRS Publication 915. Below is that worksheet applied exactly, with two full worked examples, every figure checked against Publication 915's own two published examples and a set of edge cases before being used here. For your own numbers, use the Social Security taxability calculator.

The base amounts, by filing status

These thresholds are fixed by statute, not adjusted for inflation. The lower threshold ($25,000 / $32,000) has been unchanged since 1983; the upper threshold and the 85% tier ($34,000 / $44,000) were added in 1993 and have never moved since. That's why more retirees owe tax on their benefits every year: ordinary income and benefit amounts rise with inflation and wages, but these lines don't.

Provisional income: the number that decides everything

Provisional income is your AGI excluding Social Security, plus any tax-exempt interest (like municipal bond interest), plus 50% of your annual Social Security benefit. It is not your total income, and it is not your benefit alone -- it's a specific blend of both.

• If provisional income is at or below your lower threshold: 0% of your benefits are taxable.
• If it's between the lower and upper threshold: part of your benefits are taxable, at up to 50 cents of every dollar of provisional income above the lower threshold -- but never more than 50% of the benefit itself.
• If it's above the upper threshold: the 50%-tier amount is locked in, plus 85 cents of every dollar of provisional income above the upper threshold -- capped at 85% of the benefit itself, which is the most that can ever be taxable.

That's the plain-language version. The IRS's own version is Worksheet 1, which runs through specific lines to get the same answer without the "up to" language -- and it's what the worked examples below and the calculator actually compute, not an approximation of it.

Worked example 1: a retiree in the 50%/85% tiers

A single retiree with $20,000 in Social Security benefits and $30,000 of other AGI (pension and part-time income), no tax-exempt interest:

Notice that even though this retiree is over the upper threshold -- technically in the "85%" tier -- only 48% of the benefit ends up taxable, not 85%. The 85% figure is a marginal rate and a hard cap, not the typical outcome; it only approaches 85% at much higher income relative to the benefit.

Worked example 2: hitting the 85% cap

A married couple filing jointly with $30,000 in combined Social Security benefits and $120,000 of other AGI (pensions, a large IRA withdrawal, investment income), no tax-exempt interest:

Here the 85%-of-provisional-income-over-the-threshold math would actually produce a larger number than 85% of the benefit itself, so the flat 85% of total benefits cap takes over instead. Once other income is high enough relative to the benefit, 85% of the benefit is exactly what gets taxed -- it doesn't go higher than that, no matter how much other income there is.

The special case: married filing separately, living with your spouse

If you're married filing separately and lived with your spouse at any point in the year, the $25,000/$34,000 thresholds don't apply to you at all. Instead, Worksheet 1 sends you straight to comparing 85% of your provisional income against 85% of your benefit, and taxes you on whichever is smaller. With no other income, that still works out to less than 85% of the benefit (since provisional income starts at just 50% of the benefit) -- but it takes far less outside income to push it toward the full 85% than it would filing any other way. This is the only filing situation where "up to 85% taxable from close to the first dollar" is accurate.

What this doesn't cover

This is the federal calculation only. Most states don't tax Social Security benefits at all, and the handful that do have their own rules and thresholds -- those aren't modeled here. This also doesn't model the Schedule 1 adjustments (educator expenses, HSA or IRA deductions, student loan interest, and similar) that reduce AGI before Social Security is added back in; if those apply to you, use your actual AGI excluding Social Security as the "other AGI" figure and they're already accounted for.

Why this matters for year-end decisions

Provisional income is driven by ordinary income choices made well before the benefit itself is taxed. A Roth conversion or an RMD taken as cash both add directly to AGI, which raises provisional income dollar for dollar and can push someone from 0% taxable into the 50% or 85% tier -- on top of whatever those moves already do to ordinary tax and Medicare IRMAA. The year-end tax moves guide covers sizing a conversion or an RMD/QCD choice; running the result through the Social Security taxability calculator before finalizing either one shows whether it also moves the tax on your Social Security, not just your bracket.

Every number above comes from applying IRS Publication 915's Worksheet 1 exactly, checked against both of the worksheet's own published examples (a single filer's $2,990 taxable result and the Johnsons' $6,275 MFJ result) plus 17 edge cases -- thresholds hit exactly, the MFS-living-with-spouse rule, and both ways the calculation's internal caps can bind -- in an independent Decimal model, and cross-checked against the calculator's own code in 99 cases with 0 mismatches. Not tax advice; see Publication 915 and a tax professional for your actual return.