Provisional Income
In order to determine the taxability of Social Security benefits, it's first necessary to calculate "provisional income" - a measurement of income used specifically for these purposes.
Provisional income is calculated as your total income (essentially the net amounts included on the front page of your tax return in calculating Adjusted Gross Income), plus any tax-exempt income (e.g., from municipal bonds) and excluded foreign income, plus one half of your Social Security benefits.
If this total exceeds $25,000 for individuals ($32,000 for married couples), then 50% of the excess is the amount of Social Security benefits that must be included in income. If provisional income exceeds $34,000 for individuals ($44,000 for married couples), then 85% of the excess amount is included in income. (Notably, if provisional income exceeds the 85% threshold amount, it must have already surpassed the 50% threshold amount, and there are some additional calculations to coordinate between the two, until a maximum of 85% of all Social Security benefits become taxable.)
Example 1.
Jeremy and Martha have an AGI of $28,000 (and no tax-exempt or foreign income), and receive combined Social Security benefits of $14,000. As a result, their provisional income is $28,000 + $7,000 (half of Social Security benefits) = $35,000, which is $3,000 above the $32,000 threshold. This means that 50% x $3,000 = $1,500 of their Social Security benefits are subject to taxation, which ultimately increases their AGI to $28,000 + $1,500 = $29,500.
Example 2.
Donald and Sarah have an AGI of $44,000 and receive combined Social Security benefits of $24,000. As a result, their provisional income is $44,000 + $12,000 = $56,000, which is $12,000 above the upper $44,000 threshold. This means that $16,200 of benefits are subject to taxation (which is technically 50% of the amount from $32,000 to $44,000 plus 85% of the excess above $44,000), which ultimately increases their AGI to $44,000 + $16,200 = $60,200.
Example 3.
Paul and Megan have an AGI of $58,000 and receive combined Social Security benefits of $24,000. As a result, their provisional income is $58,000 + $12,000 = $70,000, which is $26,000 above the upper $44,000 threshold. This means that $20,400 of benefits are subject to taxation (which is the maximum 85% of the $26,000 excess above the upper threshold, capped out at 85% of their total Social Security benefits), which ultimately increases their AGI to $58,000 + $20,400 = $78,400.
The bottom line
As income rises, more Social Security benefits are subject to taxation, until eventually a maximum of 85% of all benefits are included in income for tax purposes!