There are a million moving parts to a tax return, and with the many changes to 2018 taxes that complicates things even further.
As a vague, general concept, if you are in the 25% tax bracket, then your income tax would increase by 25% of your income. So in that simplified scenario, converting $10,000 of Traditional IRA to a Roth IRA would result in $2500 of additional income tax.
If you have the CD/downloaded version of TurboTax, they do have a "What If?" worksheet that may be useful.
