Provisional Income

Provisional income is a test figure, not a line on your tax return. Section 86 of the tax code describes it as modified adjusted gross income plus half of your benefits, the IRS applies it through the worksheet in Publication 915 and the Form 1040 instructions, and the Social Security Administration calls the same idea combined income. On a joint return, both spouses’ income counts, even if only one of you receives benefits. The steps are:

The thresholds, which the law calls base amounts, are written into the statute and have never been indexed for inflation. The 50% tier has applied since 1984 and the 85% tier since 1994, so the same dollar limits apply in 2026 even though Social Security benefits have risen with every cost-of-living adjustment, including 2.8% for 2026.

Below the first threshold, none of your benefits is taxable. Between the two thresholds, the taxable amount is the smaller of half your benefits or half of your provisional income over the first threshold. Above the second, it is 85% of the excess over that threshold plus the smaller of the first-tier amount or $4,500 ($6,000 on a joint return), capped at 85% of benefits.

A married couple filing separately who lived together at any time during the year has a base amount of zero, so up to 85% of benefits is taxable from the first dollar, a heavy cost to weigh before filing separately.

The surprises run in both directions. Some income you never pay tax on still counts: municipal bond interest is the best-known example, and above the upper threshold each $1,000 of it can make $850 more of your benefits taxable. Capital gains count in full, even gains taxed at the 0% long-term rate, but only the gain: the part of a sale that returns your cost basis adds nothing. A Roth conversion counts in full in the year you convert, and so do required minimum distributions, which is one reason provisional income often jumps in the early 70s.

Other money does not count at all: qualified Roth IRA withdrawals, qualified charitable distributions sent straight from an IRA, HSA withdrawals for medical costs and Supplemental Security Income. Nor do deductions taken after adjusted gross income. The standard deduction, itemized deductions and the senior deduction for people 65 and older lower taxable income but leave provisional income, and so the taxable share of your benefits, unchanged.

Provisional income sits alongside several similar measures, and each answers a different question.

Adjusted gross income is the line on your return, and it already includes whatever part of your benefits turned out to be taxable, so it is an output of this calculation rather than an input. The MAGI that Medicare uses for IRMAA surcharges is AGI plus tax-exempt interest, read from the return two years earlier. The MAGI for marketplace premium tax credits adds back all non-taxable Social Security, plus tax-exempt interest and untaxed foreign income.

Provisional income is the only one that counts exactly half of your benefits, and its thresholds have stayed frozen for decades. One withdrawal can move all of them at once: it raises provisional income, which raises taxable benefits, which raises AGI and the Medicare MAGI read two years later.

Most provisional-income errors come from treating it like a more familiar number. Because it blends taxable income, untaxed income and half of your benefits, and feeds a formula rather than a bracket, a small misunderstanding can lead to a surprise tax bill or a withdrawal plan that costs more than expected. The way it raises the tax on each extra dollar has its own name, the Social Security tax torpedo. Watch for these mistakes: