The estimated tax liability is based on two things, the taxable income generated by the conversion and your subsequent tax rate. To estimate how much will be taxed, you need to know which of two types of contributions you made to the traditional IRAs (all of them, not just the one you're converting). Deductible Contributions are contributions deducted from your gross taxable income in the tax year you made these contributions Nondeductible Contributions, which are contributions you did not deduct from your taxes. These contributions are what is known as basis in the traditional plans and can be referred to as the basis in the conversion process to a Roth IRA.