We are both retired. Social Security is our sole income. We took several IRA withdrawals in 2018. Had Fidelity withhold 10%, yet we still owe the IRS. Why?

[VolvoGirl · Accepted solution]

The 10% wasn't enough.  And the withdrawals may have made some of your SS taxable.

Up to 85% of Social Security becomes taxable when all your other income plus 1/2 your social security, reaches:
Married Filing Jointly: $32,000
Single or head of household: $25,000
Married Filing Separately: 0

You didn't actually pay the tax.  You had taxes withheld like from your paycheck. You still have to enter the whole amount (before taxes were withheld) with your other income to figure out the total tax (and it may put you into a higher tax bracket)  and then the withholding is subtracted from the total tax to figure your refund or tax due.

[SweetieJean]

Remember state taxes as well.