Many retirees are startled to learn that Social Security benefits can be taxed — up to 85% of them, in fact. Whether yours are depends on your "provisional income": your other income plus half your Social Security benefits. Cross $25,000 single ($32,000 joint) and up to 50% becomes taxable; cross $34,000 single ($44,000 joint) and up to 85% is taxable. This estimator — named the Social Security Tax Calculator — runs your numbers through those thresholds. It annualizes your Primary SS Benefit (Monthly) and Spouse SS Benefit (Monthly) by Filing Status, adds your Other Annual Income to compute Provisional Income, and derives your Taxable SS Amount plus an Estimated Tax on SS.

Take $2,500 monthly primary plus $1,000 spousal benefits with $30,000 of other income, filing jointly. Annual benefits total $42,000; provisional income lands near $51,000 — above the $44,000 joint threshold, so 85% ($35,700) is taxable. At a 22% marginal rate that is roughly $7,850 of tax on benefits many assumed were tax-free.

Manage provisional income deliberately: Roth withdrawals do not count toward it, while traditional IRA distributions do. The account you tap first changes your tax bill.

With the pre-filled defaults — Primary SS Benefit (Monthly) $2,500, Spouse SS Benefit (Monthly) $1,000, Filing Status Individual, Other Annual Income $30,000 — the Social Security Tax Calculator returns Annual SS Benefits $30,000.00, Provisional Income $45,000.00, Taxable SS Amount $25,500.00, Estimated Tax on SS $5,610.00. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.