At whatever your marginal tax rate was for all your other income on your tax return

.......plus a 10% penalty if you are under 59.5 years old and did not roll the $$ into another retirement account or IRA.

.....but whatever tax was withheld does go as a credit against the taxes calculated on ALL your income from the year, in one big pot, including the 1099-R.  (As long as you entered the Federal withholding in box 4 on the software form.

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State tax could be different...depends on the state.